THE SIRENS OF MARS THE SMALLEST LIGHTS IN THE UNIVERSE In 2013, the House Committee on Science, Space and Technology asked three prominent scientists whether life — aliens, if you wish — exists beyond Earth. “Do the math,” said Sara Seager, an astrophysicist from M.I.T. Ralph Hall, a 90-year-old congressman from Texas, said he couldn’t do the math; that was the problem. The math actually isn’t all that complicated. The universe brims with galaxies, a couple hundred billion at least, and each of those galaxies brims with billions of stars. Planets — called exoplanets — orbit most of those stars. As of this writing, we have confirmed the existence of 4,197, but astronomers detect another exoplanet seemingly every few days. Some are gigantic and insanely hot; some have big puffy atmospheres; some climb up over and slip under their stars in polar orbits; some have “years” that last only days. The universe overflows with worlds. In our galaxy alone, there are probably about 40 billion planets that could support life. To assume that ours is the only one that hosts living things seems a tad self-absorbed, doesn’t it? Congressman Hall asked, “Is there life out there?” “Yes,” said Mary Voytek, an astrobiologist at NASA. “Yes,” said Steven Dick, a science historian at the Library of Congress. “Yes,” said Seager. Aliens might not be little green people, of course: They might be based on silicon or ammonia, rather than carbon; they might use cosmic rays as an energy source; they might be ghostly microbes paddling around sulfuric pools. As Seager urges in her stark, bewitching new memoir, “The Smallest Lights in the Universe,” we have to shake loose “of our Earth-based biases — our ‘terracentrism,’ … that peculiar blindness born of being human.” Or as the Georgetown planetary scientist Sarah Stewart Johnson puts it in her own lovely memoir, “The Sirens of Mars: Searching for Life on Another World,” “It’s one of our biggest intellectual and practical challenges — like trying to imagine a color we’ve never seen.” In “The Smallest Lights,” Seager begins with her almost feral childhood in Toronto, roving streets and tending younger siblings, often alone, by her own admission, “small and silent” and “wired a little differently.” At 15 she wandered into a University of Toronto presentation about a supernova and “sat enthralled in the pin-drop quiet, ravished by an amazing tale of discovery.” She promptly threw herself into advanced math and canoeing with equal gusto, fell in love with a paddling partner named Mike, attended graduate school at Harvard, was awarded a position at the Institute for Advanced Study in Princeton, N.J. and developed a new technique for detecting exoplanets and analyzing their atmospheres. Tenured at M.I.T. by the time she was 36, she commenced a lifelong hunt for a second Earth. Then Mike got sick and died, and Seager, now a widowed mother of two, came unglued. The merciless seesaw of her grief makes for harrowing reading. “Hour by hour,” she writes, “I felt either broken or bulletproof.” In time, a group of six other widows, meeting every other Friday, “moving from house to house like emotional squatters,” gradually helped Seager return to the world. The second half of her story gleams with insights into what it means to lose a partner in midlife, and just as the widows helped Seager feel less alone, her story is sure to help any readers grappling with a similar loss. “When you lose someone,” she writes, “you don’t lose them all at once, and their dying doesn’t stop with their death. You lose them a thousand times in a thousand ways. You say a thousand goodbyes. You hold a thousand funerals.” Johnson’s “The Sirens of Mars” oscillates between a history of Mars science and an account of the author’s own journey as a planetary scientist seeking sparks of life in the immensity. She presents efficient thumbnails of astronomers like Percival Lowell, who popularized the idea of visible “canals” on Mars as evidence of an alien civilization; Carl Sagan, who suggested that big, turtlelike organisms “are not only possible on Mars; they may be favored”; and Maria Zuber, the only woman among the 87 investigators on the 1996 Mars Global Surveyor science team. Along the way, you come to appreciate the astonishing ingenuity required to safely send rovers the size of Mini Coopers several hundred million kilometers through a frozen vacuum, land them on another planet and drive them around by remote control. Most compelling are Johnson’s memories of formative moments, as a young girl prowling roadcuts for fossils with her father, as a wide-eyed graduate student entering the Jet Propulsion Laboratory for the first time (“It felt holy to be in those rooms”) and as an awed young scientist in Copenhagen holding in her hands bacterial cells 20,000 times older than she is. Johnson remembers feeling odd when, as a college sophomore, she attended a lecture by Zuber. “My back straightened as it became clear what I was responding to,” she writes. “It was the first time I’d ever heard a woman give a planetary science talk.” If Johnson’s prose swirls with lyrical wonder, as varied and multihued as the apricot deserts, butterscotch skies and blue sunsets of Mars, Seager’s is rawer and starker, full of blues and blacks, written in the ink of grief, suffering, healing and — ultimately — clarity. In Seager’s hands you’re as apt to learn about “a special body bag that’s designed to slide down stairs” as about storm-wracked rogue exoplanets where it rains molten iron. Both books beautifully dramatize the emotional precarity of having one’s career pinned to the fate of space hardware. Both address the challenges of being female physical scientists in a male-dominated field, and both convey the struggle of operating in the vast scales of the universe at work, then commuting home to operate in the humbler scales of the domestic sphere. “I was constantly torn in two, always some form of distracted,” Seager writes. Johnson, after spending her workday tending to a Mars rover millions of kilometers away, hurries to pick up her kids at preschool. “It feels almost impossible to leave,” she says, “one love tearing me from another.” Spoiler alert: Neither book ends with the exultant discovery of extraterrestrial life. In Johnson’s final pages you find yourself spiraling through Herodotus and Euclid; in Seager’s you watch her learn how to love again. But these are not disappointments — on the contrary, their testimonies are reminders that we are all just part of a continuum of investigation that extends back through the Enlightenment to Ibn al-Haytham, Aryabhata, and Aristotle, human links in centuries-long chains of questions. Both writers exemplify the humanity of science: Seager and Johnson laugh, grieve, hope, fail, try, fail and try again. “We started from almost nothing,” Johnson writes about Mars, though she could be talking about pretty much every human endeavor. “We’ve gone careening down blind alleys and taken countless wrong turns, yet somehow, miraculously, the passion, ingenuity and persistence we have brought to the enterprise have moved us toward a truer understanding of another world.” Why keep searching for life elsewhere when we sometimes seem to have a hard time appreciating it in our own backyard? What does it say about us? “It says we’re curious,” Seager writes. “It says we’re hopeful. It says we’re capable of wonder and wonderful things.” However fleeting our individual lives might be, it’s comforting to imagine that we Earthlings might not be utterly alone in the immensity of time and space. Do the math. Life may be more resilient and pervasive than we think. How lucky we are to be here long enough to appreciate that. An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such.[1] The first confirmation of detection occurred in 1992. A different planet, initially detected in 1988, was confirmed in 2003. As of 1 June 2022, there are 5,059 confirmed exoplanets in 3,733 planetary systems, with 824 systems having more than one planet.[2][3] There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star; thus, 85% of the exoplanets detected are inside the tidal locking zone.[4] In several cases, multiple planets have been observed around a star.[5] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[c][6][7] Assuming there are 200 billion stars in the Milky Way,[d] it can be hypothesized that there are 11 billion potentially habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included.[8] The least massive planet known is Draugr (also known as PSR B1257+12 A or PSR B1257+12 b), which is about twice the mass of the Moon. The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b,[9][10] about 30 times the mass of Jupiter. However, according to some definitions of a planet (based on the nuclear fusion of deuterium[11]), it is too massive to be a planet and might be a brown dwarf instead. Known orbital times for exoplanets vary from a few hours (for those closest to their star) to thousands of years. Some exoplanets are so far away from the star that it is difficult to tell whether they are gravitationally bound to it. Almost all of the planets detected so far are within the Milky Way. However, there is evidence that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist.[12][13] The nearest exoplanets are located 4.2 light-years (1.3 parsecs) from Earth and orbit Proxima Centauri, the closest star to the Sun.[14] The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone (or sometimes called "goldilocks zone"), where it is possible for liquid water, a prerequisite for life as we know it, to exist on the surface. However, the study of planetary habitability also considers a wide range of other factors in determining the suitability of a planet for hosting life.[15] Rogue planets are those that do not orbit any star. Such objects are considered a separate category of planets, especially if they are gas giants, often counted as sub-brown dwarfs.[16] The rogue planets in the Milky Way possibly number in the billions or more.[17][18] The official definition of the term planet used by the International Astronomical Union (IAU) only covers the Solar System and thus does not apply to exoplanets.[19][20] The IAU Working Group on Extrasolar Planets issued a position statement containing a working definition of "planet" in 2001 and which was modified in 2003.[21] An exoplanet was defined by the following criteria:
This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018.[22][23] The official working definition of an exoplanet is now as follows:
The IAU noted that this definition could be expected to evolve as knowledge improves. AlternativesThe IAU's working definition is not always used. One alternate suggestion is that planets should be distinguished from brown dwarfs on the basis of formation. It is widely thought that giant planets form through core accretion, which may sometimes produce planets with masses above the deuterium fusion threshold;[24][25][11] massive planets of that sort may have already been observed.[26] Brown dwarfs form like stars from the direct gravitational collapse of clouds of gas and this formation mechanism also produces objects that are below the 13 MJup limit and can be as low as 1 MJup.[27] Objects in this mass range that orbit their stars with wide separations of hundreds or thousands of AU and have large star/object mass ratios likely formed as brown dwarfs; their atmospheres would likely have a composition more similar to their host star than accretion-formed planets which would contain increased abundances of heavier elements. Most directly imaged planets as of April 2014 are massive and have wide orbits so probably represent the low-mass end of brown dwarf formation.[28] One study suggests that objects above 10 MJup formed through gravitational instability and should not be thought of as planets.[29] Also, the 13-Jupiter-mass cutoff does not have precise physical significance. Deuterium fusion can occur in some objects with a mass below that cutoff.[11] The amount of deuterium fused depends to some extent on the composition of the object.[30] As of 2011 the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 MJup in the observed mass spectrum reinforces the choice to forget this mass limit".[31] As of 2016 this limit was increased to 60 Jupiter masses[32] based on a study of mass–density relationships.[33] The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity."[34] The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.[35] Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by coulomb pressure or electron degeneracy pressure with the dividing line at around 5 Jupiter masses.[36][37] Exoplanet HIP 65426b is the first discovered planet around star HIP 65426.[38] The convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union (IAU). For exoplanets orbiting a single star, the IAU designation is formed by taking the designated or proper name of its parent star, and adding a lower case letter.[39] Letters are given in order of each planet's discovery around the parent star, so that the first planet discovered in a system is designated "b" (the parent star is considered to be "a") and later planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist. For centuries scientists, philosophers, and science fiction writers suspected that extrasolar planets existed, but there was no way of knowing whether they existed, how common they were, or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers. The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but was not recognized as such. The astronomer Walter Sydney Adams, who later became director of the Mount Wilson Observatory, produced a spectrum of the star using Mount Wilson's 60-inch telescope. He interpreted the spectrum to be of an F-type main-sequence star, but it is now thought that such a spectrum could be caused by the residue of a nearby exoplanet that had been pulverized into dust by the gravity of the star, the resulting dust then falling onto the star.[1] The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12.[40] The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as (relatively) small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not tightly bound to stars, so they're actually wandering through space or loosely orbiting between stars. We can estimate that the number of planets in this [faraway] galaxy is more than a trillion.[41] On 21st March 2022, the 5000th exoplanet beyond our solar system was confirmed.[42] Early speculations
In the sixteenth century, the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun (heliocentrism), put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. In the eighteenth century, the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[44] In 1952, more than 40 years before the first hot Jupiter was discovered, Otto Struve wrote that there is no compelling reason why planets could not be much closer to their parent star than is the case in the Solar System, and proposed that Doppler spectroscopy and the transit method could detect super-Jupiters in short orbits.[45] Discredited claimsClaims of exoplanet detections have been made since the nineteenth century. Some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Company's Madras Observatory reported that orbital anomalies made it "highly probable" that there was a "planetary body" in this system.[46] In the 1890s, Thomas J. J. See of the University of Chicago and the United States Naval Observatory stated that the orbital anomalies proved the existence of a dark body in the 70 Ophiuchi system with a 36-year period around one of the stars.[47] However, Forest Ray Moulton published a paper proving that a three-body system with those orbital parameters would be highly unstable.[48] During the 1950s and 1960s, Peter van de Kamp of Swarthmore College made another prominent series of detection claims, this time for planets orbiting Barnard's Star.[49] Astronomers now generally regard all the early reports of detection as erroneous.[50] In 1991 Andrew Lyne, M. Bailes and S. L. Shemar claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing variations.[51] The claim briefly received intense attention, but Lyne and his team soon retracted it.[52] Confirmed discoveries
As of 1 June 2022, a total of 5,059 confirmed exoplanets are listed in the Extrasolar Planets Encyclopedia, including a few that were confirmations of controversial claims from the late 1980s.[2] The first published discovery to receive subsequent confirmation was made in 1988 by the Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang of the University of Victoria and the University of British Columbia.[53] Although they were cautious about claiming a planetary detection, their radial-velocity observations suggested that a planet orbits the star Gamma Cephei. Partly because the observations were at the very limits of instrumental capabilities at the time, astronomers remained skeptical for several years about this and other similar observations. It was thought some of the apparent planets might instead have been brown dwarfs, objects intermediate in mass between planets and stars. In 1990, additional observations were published that supported the existence of the planet orbiting Gamma Cephei,[54] but subsequent work in 1992 again raised serious doubts.[55] Finally, in 2003, improved techniques allowed the planet's existence to be confirmed.[56] Coronagraphic image of AB Pictoris showing a companion (bottom left), which is either a brown dwarf or a massive planet. The data was obtained on 16 March 2003 with NACO on the VLT, using a 1.4 arcsec occulting mask on top of AB Pictoris. On 9 January 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[40] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Follow-up observations solidified these results, and confirmation of a third planet in 1994 revived the topic in the popular press.[57] These pulsar planets are thought to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of gas giants that somehow survived the supernova and then decayed into their current orbits. As pulsars are aggressive stars, it was considered unlikely at the time that a planet may be able to be formed in their orbit.[58] In the early 1990s, a group of astronomers led by Donald Backer, who were studying what they thought was a binary pulsar (PSR B1620−26 b), determined that a third object was needed to explain the observed Doppler shifts. Within a few years, the gravitational effects of the planet on the orbit of the pulsar and white dwarf had been measured, giving an estimate of the mass of the third object that was too small for it to be a star. The conclusion that the third object was a planet was announced by Stephen Thorsett and his collaborators in 1993.[59] On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting a main-sequence star, nearby G-type star 51 Pegasi.[60][61][62] This discovery, made at the Observatoire de Haute-Provence, ushered in the modern era of exoplanetary discovery, and was recognized by a share of the 2019 Nobel Prize in Physics. Technological advances, most notably in high-resolution spectroscopy, led to the rapid detection of many new exoplanets: astronomers could detect exoplanets indirectly by measuring their gravitational influence on the motion of their host stars. More extrasolar planets were later detected by observing the variation in a star's apparent luminosity as an orbiting planet transited in front of it.[60] Initially, most known exoplanets were massive planets that orbited very close to their parent stars. Astronomers were surprised by these "hot Jupiters", because theories of planetary formation had indicated that giant planets should only form at large distances from stars. But eventually more planets of other sorts were found, and it is now clear that hot Jupiters make up the minority of exoplanets.[60] In 1999, Upsilon Andromedae became the first main-sequence star known to have multiple planets.[63] Kepler-16 contains the first discovered planet that orbits around a binary main-sequence star system.[64] On 26 February 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These exoplanets were checked using a statistical technique called "verification by multiplicity".[65][66][67] Before these results, most confirmed planets were gas giants comparable in size to Jupiter or larger because they are more easily detected, but the Kepler planets are mostly between the size of Neptune and the size of Earth.[65] On 23 July 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star.[68] On 6 September 2018, NASA discovered an exoplanet about 145 light years away from Earth in the constellation Virgo.[69] This exoplanet, Wolf 503b, is twice the size of Earth and was discovered orbiting a type of star known as an "Orange Dwarf". Wolf 503b completes one orbit in as few as six days because it is very close to the star. Wolf 503b is the only exoplanet that large that can be found near the so-called Fulton gap. The Fulton gap, first noticed in 2017, is the observation that it is unusual to find planets within a certain mass range.[69] Under the Fulton gap studies, this opens up a new field for astronomers, who are still studying whether planets found in the Fulton gap are gaseous or rocky.[69] In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS.[70] Candidate discoveriesAs of January 2020, NASA's Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed,[71] several of them being nearly Earth-sized and located in the habitable zone, some around Sun-like stars.[72][73][74] Exoplanet populations – June 2017[75][76]
In September 2020, astronomers reported evidence, for the first time, of an extragalactic planet, M51-ULS-1b, detected by eclipsing a bright X-ray source (XRS), in the Whirlpool Galaxy (M51a).[77][78] Also in September 2020, astronomers using microlensing techniques reported the detection, for the first time, of an earth-mass rogue planet unbounded by any star, and free floating in the Milky Way galaxy.[79][80] Directly imaged planet Beta Pictoris b Planets are extremely faint compared with their parent stars. For example, a Sun-like star is about a billion times brighter than the reflected light from any exoplanet orbiting it. It is difficult to detect such a faint light source, and furthermore the parent star causes a glare that tends to wash it out. It is necessary to block the light from the parent star in order to reduce the glare while leaving the light from the planet detectable; doing so is a major technical challenge which requires extreme optothermal stability.[81] All exoplanets that have been directly imaged are both large (more massive than Jupiter) and widely separated from their parent star. Specially designed direct-imaging instruments such as Gemini Planet Imager, VLT-SPHERE, and SCExAO will image dozens of gas giants, but the vast majority of known extrasolar planets have only been detected through indirect methods. The following are the indirect methods that have proven useful: Indirect methods
When the star is behind a planet, its brightness will seem to dimIf a planet crosses (or transits) in front of its parent star's disk, then the observed brightness of the star drops by a small amount. The amount by which the star dims depends on its size and on the size of the planet, among other factors. Because the transit method requires that the planet's orbit intersect a line-of-sight between the host star and Earth, the probability that an exoplanet in a randomly oriented orbit will be observed to transit the star is somewhat small. The Kepler telescope used this method. Discovered extrasolar planets per year and by detection method (as of September 2014):
Animation showing difference between planet transit timing of one-planet and two-planet systems When a planet orbits multiple stars or if the planet has moons, its transit time can significantly vary per transit. Although no new planets or moons have been discovered with this method, it is used to successfully confirm many transiting circumbinary planets.[85]
Planets may form within a few to tens (or more) of millions of years of their star forming.[92][93][94][95][96] The planets of the Solar System can only be observed in their current state, but observations of different planetary systems of varying ages allows us to observe planets at different stages of evolution. Available observations range from young proto-planetary disks where planets are still forming[97] to planetary systems of over 10 Gyr old.[98] When planets form in a gaseous protoplanetary disk,[99] they accrete hydrogen/helium envelopes.[100][101] These envelopes cool and contract over time and, depending on the mass of the planet, some or all of the hydrogen/helium is eventually lost to space.[99] This means that even terrestrial planets may start off with large radii if they form early enough.[102][103][104] An example is Kepler-51b which has only about twice the mass of Earth but is almost the size of Saturn which is a hundred times the mass of Earth. Kepler-51b is quite young at a few hundred million years old.[105] The Morgan-Keenan spectral classification Artist's impression of exoplanet orbiting two stars.[106] There is at least one planet on average per star.[5] About 1 in 5 Sun-like stars[a] have an "Earth-sized"[b] planet in the habitable zone.[107] Most known exoplanets orbit stars roughly similar to the Sun, i.e. main-sequence stars of spectral categories F, G, or K. Lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[108][109] Despite this, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft, which uses the transit method to detect smaller planets. Using data from Kepler, a correlation has been found between the metallicity of a star and the probability that the star hosts a giant planet, similar to the size of Jupiter. Stars with higher metallicity are more likely to have planets, especially giant planets, than stars with lower metallicity.[110] Some planets orbit one member of a binary star system,[111] and several circumbinary planets have been discovered which orbit around both members of binary star. A few planets in triple star systems are known[112] and one in the quadruple system Kepler-64. This color–color diagram compares the colors of planets in the Solar System to exoplanet HD 189733b. The exoplanet's deep blue color is produced by silicate droplets, which scatter blue light in its atmosphere. In 2013 the color of an exoplanet was determined for the first time. The best-fit albedo measurements of HD 189733b suggest that it is deep dark blue.[113][114] Later that same year, the colors of several other exoplanets were determined, including GJ 504 b which visually has a magenta color,[115] and Kappa Andromedae b, which if seen up close would appear reddish in color.[116] Helium planets are expected to be white or grey in appearance.[117] The apparent brightness (apparent magnitude) of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with high albedo that is far from the star.[118] The darkest known planet in terms of geometric albedo is TrES-2b, a hot Jupiter that reflects less than 1% of the light from its star, making it less reflective than coal or black acrylic paint. Hot Jupiters are expected to be quite dark due to sodium and potassium in their atmospheres but it is not known why TrES-2b is so dark—it could be due to an unknown chemical compound.[119][120][121] For gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths. Also, elliptical orbits can cause major fluctuations in atmospheric composition, which can have a significant effect.[122] There is more thermal emission than reflection at some near-infrared wavelengths for massive and/or young gas giants. So, although optical brightness is fully phase-dependent, this is not always the case in the near infrared.[122] Temperatures of gas giants reduce over time and with distance from their star. Lowering the temperature increases optical albedo even without clouds. At a sufficiently low temperature, water clouds form, which further increase optical albedo. At even lower temperatures ammonia clouds form, resulting in the highest albedos at most optical and near-infrared wavelengths.[122] Magnetic fieldIn 2014, a magnetic field around HD 209458 b was inferred from the way hydrogen was evaporating from the planet. It is the first (indirect) detection of a magnetic field on an exoplanet. The magnetic field is estimated to be about one tenth as strong as Jupiter's.[123][124] Exoplanets magnetic fields may be detectable by their auroral radio emissions with sensitive enough radio telescopes such as LOFAR.[125][126] The radio emissions could enable determination of the rotation rate of the interior of an exoplanet, and may yield a more accurate way to measure exoplanet rotation than by examining the motion of clouds.[127] Earth's magnetic field results from its flowing liquid metallic core, but in massive super-Earths with high pressure, different compounds may form which do not match those created under terrestrial conditions. Compounds may form with greater viscosities and high melting temperatures which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Forms of magnesium oxide such as MgSi3O12 could be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[128][129] Hot Jupiters have been observed to have a larger radius than expected. This could be caused by the interaction between the stellar wind and the planet's magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.[130] In August 2018, scientists announced the transformation of gaseous deuterium into a liquid metallic form. This may help researchers better understand giant gas planets, such as Jupiter, Saturn and related exoplanets, since such planets are thought to contain a lot of liquid metallic hydrogen, which may be responsible for their observed powerful magnetic fields.[131][132] Although scientists previously announced that the magnetic fields of close-in exoplanets may cause increased stellar flares and starspots on their host stars, in 2019 this claim was demonstrated to be false in the HD 189733 system. The failure to detect "star-planet interactions" in the well-studied HD 189733 system calls other related claims of the effect into question.[133] In 2019 the strength of the surface magnetic fields of 4 hot Jupiters were estimated and ranged between 20 and 120 gauss compared to Jupiter's surface magnetic field of 4.3 gauss.[134][135] Plate tectonicsIn 2007, two independent teams of researchers came to opposing conclusions about the likelihood of plate tectonics on larger super-Earths[136][137] with one team saying that plate tectonics would be episodic or stagnant[138] and the other team saying that plate tectonics is very likely on super-Earths even if the planet is dry.[139] If super-Earths have more than 80 times as much water as Earth then they become ocean planets with all land completely submerged. However, if there is less water than this limit, then the deep water cycle will move enough water between the oceans and mantle to allow continents to exist.[140][141] VolcanismLarge surface temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[142][143] RingsThe star 1SWASP J140747.93-394542.6 is orbited by an object that is circled by a ring system much larger than Saturn's rings. However, the mass of the object is not known; it could be a brown dwarf or low-mass star instead of a planet.[144][145] The brightness of optical images of Fomalhaut b could be due to starlight reflecting off a circumplanetary ring system with a radius between 20 and 40 times that of Jupiter's radius, about the size of the orbits of the Galilean moons.[146] The rings of the Solar System's gas giants are aligned with their planet's equator. However, for exoplanets that orbit close to their star, tidal forces from the star would lead to the outermost rings of a planet being aligned with the planet's orbital plane around the star. A planet's innermost rings would still be aligned with the planet's equator so that if the planet has a tilted rotational axis, then the different alignments between the inner and outer rings would create a warped ring system.[147] MoonsIn December 2013 a candidate exomoon of a rogue planet was announced.[148] On 3 October 2018, evidence suggesting a large exomoon orbiting Kepler-1625b was reported.[149] AtmospheresClear versus cloudy atmospheres on two exoplanets.[150] Atmospheres have been detected around several exoplanets. The first to be observed was HD 209458 b in 2001.[151] Sunset studies on Titan by Cassini help understand exoplanet atmospheres (artist's concept). As of February 2014, more than fifty transiting and five directly imaged exoplanet atmospheres have been observed,[152] resulting in detection of molecular spectral features; observation of day–night temperature gradients; and constraints on vertical atmospheric structure.[153] Also, an atmosphere has been detected on the non-transiting hot Jupiter Tau Boötis b.[154][155] In May 2017, glints of light from Earth, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere.[156][157] The technology used to determine this may be useful in studying the atmospheres of distant worlds, including those of exoplanets. Comet-like tailsKIC 12557548 b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet.[158] The dust could be ash erupting from volcanos and escaping due to the small planet's low surface-gravity, or it could be from metals that are vaporized by the high temperatures of being so close to the star with the metal vapor then condensing into dust.[159] In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.[160] Insolation patternTidally locked planets in a 1:1 spin-orbit resonance would have their star always shining directly overhead on one spot which would be hot with the opposite hemisphere receiving no light and being freezing cold. Such a planet could resemble an eyeball with the hotspot being the pupil.[161] Planets with an eccentric orbit could be locked in other resonances. 3:2 and 5:2 resonances would result in a double-eyeball pattern with hotspots in both eastern and western hemispheres.[162] Planets with both an eccentric orbit and a tilted axis of rotation would have more complicated insolation patterns.[163] Surface features can be distinguished from atmospheric features by comparing emission and reflection spectroscopy with transmission spectroscopy. Mid-infrared spectroscopy of exoplanets may detect rocky surfaces, and near-infrared may identify magma oceans or high-temperature lavas, hydrated silicate surfaces and water ice, giving an unambiguous method to distinguish between rocky and gaseous exoplanets.[164] Surface temperatureArtist's illustration of temperature inversion in exoplanet's atmosphere.[165] The temperature of an exoplanet can be estimated by measuring the intensity of the light it receives from its parent star. For example, the planet OGLE-2005-BLG-390Lb is estimated to have a surface temperature of roughly −220 °C (50 K). However, such estimates may be substantially in error because they depend on the planet's usually unknown albedo, and because factors such as the greenhouse effect may introduce unknown complications. A few planets have had their temperature measured by observing the variation in infrared radiation as the planet moves around in its orbit and is eclipsed by its parent star. For example, the planet HD 189733b has been estimated to have an average temperature of 1,205 K (932 °C) on its dayside and 973 K (700 °C) on its nightside.[166] As more planets are discovered, the field of exoplanetology continues to grow into a deeper study of extrasolar worlds, and will ultimately tackle the prospect of life on planets beyond the Solar System.[167] At cosmic distances, life can only be detected if it is developed at a planetary scale and strongly modified the planetary environment, in such a way that the modifications cannot be explained by classical physico-chemical processes (out of equilibrium processes).[167] For example, molecular oxygen (O Habitable zoneThe habitable zone around a star is the region where the temperature is just right to allow liquid water to exist on the surface of planet; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star, so that the habitable zone can be at different distances for different stars. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet: desert planets (also known as dry planets), with very little water, will have less water vapor in the atmosphere than Earth and so have a reduced greenhouse effect, meaning that a desert planet could maintain oases of water closer to its star than Earth is to the Sun. The lack of water also means there is less ice to reflect heat into space, so the outer edge of desert-planet habitable zones is further out.[171][172] Rocky planets with a thick hydrogen atmosphere could maintain surface water much further out than the Earth–Sun distance.[173] Planets with larger mass have wider habitable zones because the gravity reduces the water cloud column depth which reduces the greenhouse effect of water vapor, thus moving the inner edge of the habitable zone closer to the star.[174] Planetary rotation rate is one of the major factors determining the circulation of the atmosphere and hence the pattern of clouds: slowly rotating planets create thick clouds that reflect more and so can be habitable much closer to their star. Earth with its current atmosphere would be habitable in Venus's orbit, if it had Venus's slow rotation. If Venus lost its water ocean due to a runaway greenhouse effect, it is likely to have had a higher rotation rate in the past. Alternatively, Venus never had an ocean because water vapor was lost to space during its formation [175] and could have had its slow rotation throughout its history.[176] Tidally locked planets (a.k.a. "eyeball" planets[177]) can be habitable closer to their star than previously thought due to the effect of clouds: at high stellar flux, strong convection produces thick water clouds near the substellar point that greatly increase the planetary albedo and reduce surface temperatures.[178] Habitable zones have usually been defined in terms of surface temperature, however over half of Earth's biomass is from subsurface microbes,[179] and the temperature increases with depth, so the subsurface can be conducive for microbial life when the surface is frozen and if this is considered, the habitable zone extends much further from the star,[180] even rogue planets could have liquid water at sufficient depths underground.[181] In an earlier era of the universe the temperature of the cosmic microwave background would have allowed any rocky planets that existed to have liquid water on their surface regardless of their distance from a star.[182] Jupiter-like planets might not be habitable, but they could have habitable moons.[183] Ice ages and snowball statesThe outer edge of the habitable zone is where planets are completely frozen, but planets well inside the habitable zone can periodically become frozen. If orbital fluctuations or other causes produce cooling then this creates more ice, but ice reflects sunlight causing even more cooling, creating a feedback loop until the planet is completely or nearly completely frozen. When the surface is frozen, this stops carbon dioxide weathering, resulting in a build-up of carbon dioxide in the atmosphere from volcanic emissions. This creates a greenhouse effect which thaws the planet again. Planets with a large axial tilt[184] are less likely to enter snowball states and can retain liquid water further from their star. Large fluctuations of axial tilt can have even more of a warming effect than a fixed large tilt.[185][186] Paradoxically, planets orbiting cooler stars, such as red dwarfs, are less likely to enter snowball states because the infrared radiation emitted by cooler stars is mostly at wavelengths that are absorbed by ice which heats it up.[187][188] Tidal heatingIf a planet has an eccentric orbit, then tidal heating can provide another source of energy besides stellar radiation. This means that eccentric planets in the radiative habitable zone can be too hot for liquid water. Tides also circularize orbits over time so there could be planets in the habitable zone with circular orbits that have no water because they used to have eccentric orbits.[189] Eccentric planets further out than the habitable zone would still have frozen surfaces but the tidal heating could create a subsurface ocean similar to Europa's.[190] In some planetary systems, such as in the Upsilon Andromedae system, the eccentricity of orbits is maintained or even periodically varied by perturbations from other planets in the system. Tidal heating can cause outgassing from the mantle, contributing to the formation and replenishment of an atmosphere.[191] Potentially habitable planetsA review in 2015 identified exoplanets Kepler-62f, Kepler-186f and Kepler-442b as the best candidates for being potentially habitable.[192] These are at a distance of 1200, 490 and 1,120 light-years away, respectively. Of these, Kepler-186f is in similar size to Earth with its 1.2-Earth-radius measure, and it is located towards the outer edge of the habitable zone around its red dwarf star. When looking at the nearest terrestrial exoplanet candidates, Proxima Centauri b is about 4.2 light-years away. Its equilibrium temperature is estimated to be −39 °C (234 K).[193] Earth-size planets
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55 Cancri e (abbreviated 55 Cnc e, formally named Janssen /ˈdʒænsən/) is an exoplanet in the orbit of its Sun-like host star 55 Cancri A. The mass of the exoplanet is about 8.63 Earth masses and its diameter is about twice that of the Earth,[4] thus classifying it as the first super-Earth discovered around a main sequence star, predating Gliese 876 d by a year. It takes less than 18 hours to complete an orbit and is the innermost-known planet in its planetary system. 55 Cancri e was discovered on 30 August 2004. However, until the 2010 observations and recalculations, this planet had been thought to take about 2.8 days to orbit the star.[3] In October 2012, it was announced that 55 Cancri e could be a carbon planet.[5][6] 55 Cancri e DiscoveryDiscovered byMcArthur et al.Discovery siteTexas, United StatesDiscovery date30 August 2004Detection method Radial velocityOrbital characteristicsApastron0.01617 AU (2,419,000 km)Periastron0.01464 AU (2,190,000 km)Semi-major axis 0.01544 ± 0.00005 AU (2,309,800 ± 7,500 km)[1]Eccentricity0.05 ± 0.03[2]Orbital period (sidereal) 0.7365474 (± 0.0000014)[2] d17.677 hInclination83.59 +0.47 −0.44[2] Time of periastron 2,449,999.83643 ± 0.0001[3]Argument of periastron 86.0 +30.7−33.4[2]Semi-amplitude6.02 +0.24 −0.23[2]Star55 Cancri APhysical characteristics Mean radius 1.875 ± 0.029[2] REarthMass7.99 +0.32−0.33[2] MEarth Mean density 6.66+0.43−0.40[2] g cm−3 Surface gravity 2.273 gTemperature2,709 K (2,436 °C; 4,417 °F) (average maximum)1,613 K (1,340 °C; 2,444 °F) (average minimum) 2,573 K (2,300 °C; 4,172 °F) (avg day side) ~1,644 K (1,371 °C; 2,500 °F) (avg night side)
In February 2016, it was announced that NASA's Hubble Space Telescope had detected hydrogen and helium (and suggestions of hydrogen cyanide), but no water vapor, in the atmosphere of 55 Cancri e, the first time the atmosphere of a super-Earth exoplanet was analyzed successfully.[7] In July 2014 the International Astronomical Union (IAU) launched NameExoWorlds, a process for giving proper names to certain exoplanets and their host stars.[8] The process involved public nomination and voting for the new names.[9] In December 2015, the IAU announced the winning name was Janssen for this planet.[10] The winning name was submitted by the Royal Netherlands Association for Meteorology and Astronomy of the Netherlands. It honors the spectacle maker and telescope pioneer Zacharias Janssen.[11] Transit of 55 Cancri e 55 Cancri e PIA20068 Like the majority of extrasolar planets found prior to the Kepler mission, 55 Cancri e was discovered by detecting variations in its star's radial velocity. This was achieved by making sensitive measurements of the Doppler shift of the spectrum of 55 Cancri A. At the time of its discovery, three other planets were known orbiting the star. After accounting for these planets, a signal at around 2.8 days remained, which could be explained by a planet of at least 14.2 Earth masses in a very close orbit.[12] The same measurements were used to confirm the existence of the uncertain planet 55 Cancri c. 55 Cancri e was one of the first extrasolar planets with a mass comparable to that of Neptune to be discovered. It was announced at the same time as another "hot Neptune" orbiting the red dwarf star Gliese 436 named Gliese 436 b. Planet challengedIn 2005, the existence of planet e was questioned by Jack Wisdom in a reanalysis of the data. He suggested that the 2.8-day planet was an alias and, separately, that there was a 260-day planet in orbit around 55 Cancri. In 2008, Fischer et al. published a new analysis[3] that appeared to confirm the existence of the 2.8-day planet and the 260-day planet. However, the 2.8-day planet was shown to be an alias by Dawson and Fabrycky in 2010;[1] its true period was 0.7365 days. TransitThe planet's transit of its host star was announced on 27 April 2011, based on two weeks of nearly continuous photometric monitoring with the MOST space telescope.[13] The transits occur with the period (0.74 days) and phase that had been predicted by Dawson and Fabrycky. This is one of the few planetary transits to be confirmed around a well-known star, and allowed investigations into the planet's composition. The radial velocity method used to detect 55 Cancri e obtains the minimum mass of 7.8 times that of Earth,[4] or 48% of the mass of Neptune. The transit shows that its inclination is about 83.4 ± 1.7, so the real mass is close to the minimum. 55 Cancri e is also coplanar with b. The planet is extremely likely to be tidally locked, meaning that there is a permanent day side and a permanent night side.[14] 55 Cancri e receives more radiation than Gliese 436 b.[15] The side of the planet facing its star has temperatures more than 2,000 kelvin (approximately 1,700 degrees Celsius or 3,100 Fahrenheit), hot enough to melt iron.[16] Infrared mapping with the Spitzer Space Telescope indicated an average front-side temperature of 2,573 K (2,300 °C; 4,172 °F) and an average back-side temperature of around 1,644 K (1,371 °C; 2,500 °F).[citation needed] Exoplanet 55 Cancri e orbiting its host star (artist concept) It was initially unknown whether 55 Cancri e was a small gas giant like Neptune or a large rocky terrestrial planet. In 2011, a transit of the planet was confirmed, allowing scientists to calculate its density. At first it was suspected to be a water planet.[13][4] As initial observations showed no hydrogen in its Lyman-alpha signature during transit,[17] Ehrenreich speculated that its volatile materials might be carbon dioxide instead of water or hydrogen.[17] An alternative possibility is that 55 Cancri e is a solid planet made of carbon-rich material rather than the oxygen-rich material that makes up the terrestrial planets in the Solar System.[18] In this case, roughly a third of the planet's mass would be carbon, much of which may be in the form of diamond as a result of the temperatures and pressures in the planet's interior. Further observations are necessary to confirm the nature of the planet.[5][6] A third argument is that the tidal forces, together with the orbital and rotational centrifugal forces, can partially confine a hydrogen-rich atmosphere on the nightside.[19] Assuming an atmosphere dominated by volcanic species and a large hydrogen component, the heavier molecules could be confined within latitudes < 80° while the volatile hydrogen is not. Because of this disparity, the hydrogen would have to slowly diffuse out into the dayside where X-ray and ultraviolet irradiation would destroy it. In order for this mechanism to have taken effect, it is necessary for 55 Cancri e to have become tidally locked before losing the totality of its hydrogen envelope. This model is consistent with spectroscopic measurements claiming to have discovered the presence of hydrogen[20][21] and with other studies which were unable to discover a significant hydrogen-destruction rate.[17][22] In February 2016, it was announced that NASA's Hubble Space Telescope had detected hydrogen cyanide, but no water vapor, in the atmosphere of 55 Cancri e, which is only possible if the atmosphere is predominantly hydrogen or helium. This is the first time the atmosphere of a super-Earth exoplanet was analyzed successfully.[7][23] In November 2017, it was announced that infrared observations with the Spitzer Space Telescope indicated the presence of a global lava ocean obscured by an atmosphere with a pressure of about 1.4 bar, slightly thicker than that of Earth. The atmosphere may contain similar chemicals in Earth's atmosphere, such as nitrogen and possibly oxygen, in order to cause the infrared data observed by Spitzer.[24][25] In contradiction to the February 2016 findings, a spectroscopic study in 2012 failed to detect escaping hydrogen from the atmosphere,[17] and a spectroscopic study in 2020 failed to detect escaping helium, indicating that the planet probably has no primordial atmosphere.[26] Atmospheres made of heavier molecules such as oxygen and nitrogen are not ruled out by these data. VolcanismNASA "Exoplanet Travel Bureau" poster for 55 Cancri e Large surface-temperature variations on 55 Cancri e have been attributed to possible volcanic activity releasing large clouds of dust which blanket the planet and block thermal emissions.[27][28] By 2022, the observation had shown a large variability in the planetary transit depths, which can be attributed to large-scale volcanism or presence of the variable gas torus co-orbital with the planet.[29]
Coordinates: 08h 52m 35.8s, +28° 19′ 51″ |