The Ten Most Habitable Exoplanets
The purpose of NASA’s Kepler mission is to estimate the number of habitable planets in our region of the Milky Way galaxy. Of particular interest are planets that are less than twice the size of Earth and orbit in their sun’s “habitable zone,” a region where temperatures are moderate and where the surface of a planet may have liquid water. Planet candidates require additional follow-up observations for confirmation of their existence. Scientists have confirmed a total of 859 exoplanets to date. So far the Kepler mission has confirmed 817 planets around 642 Stars, 2,740 possible planet candidates and 18,406 planet-like detection events of which 250 are in the habitable zone. The complete list of Kepler planet candidates is available in an interactive table at the NASA Exoplanet Archive.
The Ten Most Habitable ExoPlanets
1a. Gleise 581g
Topping the Habitable Planets Catologue is Gleise 581g. In 2008, Dr Ragbir Bhathal, a scientist and member of SETI at the University of Western Sydney reported a strange light signal from a region of space about 20 light years away from Earth in the Libra constellation. Unfortunately, a second signal is required to confirm and none has been reported. Two years later, a planet was identified around a red dwarf star named Gliese 581 in the Libra constellation which is currently considered to be the most habitable planet identified to date. Gleise 581g has a diameter – 1.2 to 1.4 times that of Earth and circles its Sun every 39 days in a tidally locked position. That is, one side always faces the Sun so that one side of the planet is always dark and one side is always lighted. The planet is believed to have a rocky surface and is three billion years older than Earth so there has been plenty of time for life to develop. This planet has the second highest recorded Earth Similarity Index (ESI) rating (92%). The ESI combines information from the planet’s radius, density, escape velocity, and surface temperature and compares it to Earth’s measurements.
In response to Dr. Ragbir Bhathal’s signal, SETI sent a signal back to Gliese in 2008 which should arrive in 2029.
Some scientists continue to dispute it’s existence. However, these scientists haven’t conducted any actual research or presented their arguments in peer reviewed scientific journals. Another problem for this planet is it’s Red Dwarf star ( see section below entitled Problems for Planets That Surround Small Stars ).
1b. Gliese 581 d
Gliese 581 may have a second habitable planet. Gliese 581 d is 3.1 times the size of Earth and 5.6-7 times as massive. It may have an eccentric orbit which may take it out of the habitable zone at times. It’s rotation is also debated. It’s average day may be as long as 67 Earth days. The planet is cold but a greenhouse effect would make it sufficiently warm to harbor life as we know it.
2. Gliese 667 Cc
Gliese 667 Cc is a red dwarf star 22 light years from Earth and scientists believe this planet has the highest potential of any known extraterestial planet for being a water world. It’s also a hot world with an average estimated temperature of 86 °F if the world has a cloud cover similar to ours. More clouds could increase the temperature. The planet orbits Gliese 667 once every 28 days. Gliese 667 Cc has the highest rating for sustainability of vegetation of any planet yet discovered. By all time high we really mean that. Earth only gets a rating of 72% on this index! The mass of the planet GJ667Cc is estimated to be (at least) 4.5 times that of the Earth which means if the planet is rocky like ours things things would weigh 60% more and the air pressure could be much higher. The planet also would have fewer metals.
Gliese’s planets probably has a nice view as it’s part of a triple star system. This is a relatively new star system that is less than 2 billion years old so life may have not had time to develop. As part of a trinary star system, the two other stars would also make for a beautiful skyline. However, recent research suggests that planets from binary and trinary star systems may have unstable orbits. Computer simulations suggest that in most cases the binary star system will eventually radically change the orbits of the planets.
3. Kepler-22 b
Kepler-22 b is 536 light years from Earth. It’s mass has not been determined but it is not greater than 6.4 times the mass of Earth and its diameter is 2.4 times as great. It’s estimated average temperature is 72 to 88 °F and it takes 290 days to circle its sun-like star. The planet’s ESI has the highest rating on this list (81%). The shape of its orbit is unknown but it’s believed to have long days. It probably has a liquid or gaseous shell with no greenhouse effect. It may have habitable moons. The age of the star system is unknown. As in the case of Gliese 667 Cc it’s sustainability may be hampered by an unstable orbit caused by it’s trinary star system.
4a. Tau Ceti e
In December, University of California researchers reported that Tau Ceti e, a planet 11.9 light years away from our sun with a mass 4.3 times that of Earth that is in the habitable zone. It orbits Tau Ceti e once every 168 days. Tau Ceti is a very sun-like star both in terms of brightness and age. The Sun is part of the Cetus constellation.
There are several problem with this planet’s potential habitability. It’s estimated surface temperatures are 155 °F and the possibility of a greenhouse effect on such a planet could make temperatures too high to support life on anything but a moon. At those temperatures, one would hope that Tau Ceti e has a strong tilt that could make some regions much cooler. This solar system has an asteroid belt that is ten times as large as Earth’s which might make for some hellish meteor storms. Given it’s proximity to Earth, Tau Ceti was targeted by SETI researchers in the 1960s. No one found any radio signals coming from the star system.
4b. Tau Ceti f
This high gravity planet has seven times the mass of Earth and is 2.3 times the size. With estimated average temperatures of -31 °F, this planet probably won’t make your list of best exoplanet vacation spots. However, with a greenhouse effect the average temperatures could be as high as 122 °F but that would make the atmosphere very stuff. The planet is in a slow orbit of 642 days. Given the proximity of these two planets to Earth, next generation telescopes may be able to visually examine the atmospheres of these neighboring planets.
5. HD 85512 b
HD 85512 b is at minimum 3.6 times the mass of Earth and is not tidally locked so there would be days and nights. The planet circles it’s sun every 39 days. It has temperatures similar to Earth (76.73 °F) and orbits its star once every 58 days. With 50% cloud cover the planet would have liquid water. It’s atmospheric rating is ideal. It’s star is orange colored and approximately as old as the Sun. It is 39 light years from Earth and has an ESI of 77%.
6. Gliese 163 c
Gliese 163 orbits a red dwarf star 36 light years away from Earth every 26 days. It may be rocky or gaseous. It has a mass that is at least 6.9 times greater than Earth so its atmosphere may be ten times as dense as Earth’s and it is a very hot world 140 °F. However, it’s moons might be cooler and quite favorable to life. This is a relatively new star system that is less than 1 billion years old so life may not have had time to develop.
7. HD 40307 g
Every 198 days, HD 40307 g orbits a dwarf star that is 42 light years from Earth. It has a mass that is at least seven times greater than Earth’s and it is 2.4 times as big. The estimated temperature is 43 °F. There has been speculation that HD 40307 g migrated into its current orbit as have the other planets in this solar system (see discussion below entitled Problems for Planets That Surround Small Stars).
Kepler-47c orbits the binary system of Kepler-47 once every 303 days. Kepler-47 is 4,900 light years from Earth. Kepler-47A is the larger and brighter of Kepler 47c’s two stars and is the same size as our Sun. The second star is 36% the size of the Sun with only 1% the luminosity. It is probably a gas (water vapor) giant planet that may have moons capable of supporting life. Kepler-47c is 4.6 times as large as Earth.
9. 55 Cancri f
55 Cancri f is a super metal rich planet that resides 41 light years from Earth where it’s in a circular orbit around a yellow dwarf star every 260 days. Scientists suspect it may be covered in water clouds. It is a very large planet with a mass about 0.14 times as large as Jupiter. Such a large planet could have Earth like moons. Given the new methods used to discover this planet, some organizations still categorize 55 Cancri f as just a planet candidate.
10. GJ 1214 b
GJ 1214 b orbits a dim red dwarf star 40 light years away from Earth every 38 hours. The planet is as hot as an oven (248–540°F) and may be slowly boiling off its atmosphere. However, scientists believe GJ 1214 b is an ocean planet. Therefore, there is potential for sea life.
Planet Candidates (The Best Is Yet To Come!)
At a distance of 1,004 light years, Kepler KOI-1646.01 isn’t that close to Earth but it might be worth the trip. It’s the planet with the most Earth-like characteristics of any exoplanet to date. In fact, on the ESI it rates an astonishing 93%. It’s suitability for vegetation again rates a 3rd all time high at 88% – higher than Earth’s rating! It’s iron-rock to water mixtures are virtually identical to Earth. It’s also a class M planet which means it’s size is roughly the same as Earth’s (see picture above).
KOI-1876.01 is more than twice the diameter and six times as large as Earth but it has a vegetation rating of 96%! It’s 915 light years from Earth and a bit on the cold side. However, some experts believe this planet may be gaseous but such a large planet has a high potential for having moons with similar characteristics.
Another class M planet KOI-701.03 is 740 light years from Earth. It has an ESI of 87% and a vegetation rating of 87% but it’s a hot world that is about twice the size of Earth. Also, if this planet does have any inhabitants they have already been offended by Gawker.
KOI-854.01 has an ESI of 86%. and a 44% rating for suitability for vegetation. It is 1,030 light years away. It would have somewhat colder temperatures than Earth and is twice as large with a thicker atmosphere that may increase surface temperatures.
Kepler candidate KOI-172.01 has a radius around 1.5 times that of the Earth and orbits in the habitable zone around a Sun-like star.
The citizen group Planet Hunters includes a list of planet candidates. PH2 b is Jupiter-sized and is believed to orbit its sun every 282.5 days, at a distance of about .83 AU. 1 AU equals the distance between the sun and Earth. Such a planet could have moons capable of supporting life (see next section for why this planet has such a strong potential for harboring life).
Large Gaseous Planets May Have Habitable Moons Even Outside the Habitable Zone
The size of the planets may not matter that much just so long as the orbit is circular. Pull a Saturn or Jupiter into a warmer Earth orbit, and these gas giants both have Earth-size moons which if warmed might be habitable. For example, a warmer Europa’s icy surface would thaw if Jupiter’s orbit was closer to the Sun. Similarly, Jupiter’s Callisto and Ganymede also may have vast underground oceans beneath 60 miles of rock. Saturn’s Titan has been used as a frozen model of what Earth may have been like four billion years ago. That suggests, that large gaseous planets in the habitable zone may have moons that can sustain life. Saturn’s Enceladus is believed to have reservoirs of liquid water that lie beneath a frozen surface. Dark stripes that appear each summer at Horowitz crater on Mars may be caused by salty meltwater. Move it into a near Earth orbit and you might have a pretty nice vacation spot. At 850 °F Venus might not seem like a likely place for life but some scientists have speculated that atmospheric sulfur dioxide and carbon monoxide might serve as food for floating microbes.
Even beyond the habitable range, life may exist. Jupiter’s Europa has more warm water than Earth which is heated by Jupiter’s effect on the planet’s core. Unfortunately, no one knows what if anything is swimming in those pitch black warm seas because they are covered by ten miles of ice. We only know that on Earth similar waters have an abundance of life. Europa’s water gets warmer as you move toward the planet’s core. Scientists have even speculated that Titan’s icy methane seas may harbor some bizarre form of life that is not water but methane based.
Problems for Planets That Surround Small Stars
M-dwarf planetary systems are the most common and are quite different from our own solar system. M-dwarfs are cooler and much smaller than the sun. They account for about three-quarters of all stars in the Milky Way. Although Earth size planets have been found around such dwarfs, these planets tend to orbit very close to the sun. A recent survey found five such planets orbiting around a single star. Because M dwarfs are small and cool, their temperate zone—also known as the “habitable zone,” the region where liquid water might exist—is also further inward. Even though only the outermost of Kepler-32’s five planets lies in its temperate zone, many other M-dwarf systems have more planets that sit right in their temperate zones
New research by the Harvard-Smithsonian Center for Astrophysics (CfA) suggests that there are 17 billion earth-like planets in the Milky Way in an orbit closer than Mercury. In other words, one in six stars has a Earth-like planet. The researchers found that 17 percent of stars have a planet 0.8 – 1.25 times the size of Earth in an orbit of 85 days or less. About one-fourth of stars have a super-Earth (1.25 – 2 times the size of Earth) in an orbit of 150 days or less (Larger planets can be detected at greater distances more easily.). The same fraction of stars has a mini-Neptune (2 – 4 times Earth) in orbits up to 250 days long.
Red dwarf planet emit less light so the planet must be closer to it’s sun. The sun would appear to be red in the sky and much larger than our sun. Flares on M-dwarfs can be a thousand times stronger than compared to our Sun. Such mega-flares can double the brightness of the star in minutes. Life on the surface of GJ667Cc would have to find a solution for this problem, especially since the planet is close to its flaring host star.
Many red dwarfs may be often covered by starspots (the analogues of sunspots) that could reduce the energy output of the star by as much as 40% for periods that may last months. Together with the fact that the red dwarf star emits almost no ultraviolet light, these varying light conditions could be a potential problem for the formation of life as we know it.
However, a recent study posed even more problems for low mass stars.
1. Habitable planets that are close to their sun are probably tidally locked so one side of the planet is constantly in daylight while the other side would be very cold and in constant darkness. Tidally bound planets may not be habitable. The dark of these planets will always be too cold to support life as we know it. However, the always lighted sides are likely to become exposed to too much radiation and the temperature differences would cause extreme weather variations.
2. Similarly, tides can cause the planet`s rotation to become perpendicular to its orbit eliminating seasonal variation and again creating dark and light sides with similar problems to 1. In addition, the equator of such a planet could become so hot that it would burn off the atmosphere.
3. Tidal heating causes massive vulcanism. Planets where there are constant lava flows are obviously not habitable.
All of the problems would seem to make it unlikely that low mass stars have habitable worlds. However, it’s possible that in rare cases none of these problems would develop and a habitable world could develop around a loss mass star.
The Estimated Frequency of Habitable ExoPlanets in the Milky Way
The Keppler research thus far strongly suggests that the galaxy has an abundance of planets. The Kepler Mission is designed to
Determine the abundance of terrestrial and larger planets in or near the habitable zone of a wide variety of stars;
Determine the distribution of sizes and shapes of the orbits of these planets;
Estimate how many planets there are in multiple-star systems;
Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets;
Determine the properties of those stars that harbor planetary systems.
“The initial discoveries of the Kepler mission indicate that at least a third of the stars have planets and that the number of planets in our galaxy must number in the billions,”
said William Borucki, Kepler principal investigator at NASA’s Ames Research Center in Moffett Field, Calif.
“There are at least 100 billion planets in the galaxy, just our galaxy,”
said John Johnson, assistant professor of planetary astronomy at Caltech and coauthor of the study, which was recently accepted for publication in the Astrophysical Journal.
“The Earth isn’t unique, nor the center of the universe. The diversity of other worlds is greater than depicted in all the science fiction novels and movies,”
said Geoff Marcy, professor of astronomy at the University of California, Berkeley.
Kepler data suggests that larger planets are much less common which is unfortunate because as we’ve just noted the moons surrounding large gaseous planets in the habitable zone seem like ideal places to harbor life. Only about 3 percent of stars have a large Neptune (4 – 6 times Earth), and only 5 percent of stars have a gas giant (6 – 22 times Earth) in an orbit of 400 days or less. For other planet sizes, all stars seem to have the same types of planets surrounding them. For example, Neptunes are found just as frequently around red dwarfs as they are around sun-like stars. The same is true for smaller worlds. These Kepler findings contradict previous research.
Steve Howell, Kepler mission project scientist at Ames said
“It is no longer a question of will we find a true Earth analogue, but a question of when.”
Statistical Aberrations in Our Current Data
Large planets with close orbits influence Suns more. Planets who circle their suns quickly have more transits to detect. Our region of the Milky Way may be quite different than other regions. At this point it is probably premature to estimate how many planets of any size exist in the entire Galaxy, let alone other galaxies.
You Can Join in the Search For New Planets
Planet Hunters is a citizen run group that helps find planets. Anyone can help. Just go to the website to learn more.
How The Kepler Orbiting Space Telescope Works
The Kepler space telescope identifies planet candidates by repeatedly measuring the change in brightness of more than 150,000 stars in search of planets that pass in front, or “transit,” their host star. At least three transits are required to verify a signal as a potential planet. When a planet crosses in front of its star as viewed by an observer, the event is called a transit. Transits by terrestrial planets produce a small change in a star’s brightness of about 1/10,000 (100 parts per million, ppm), lasting for 1 to 16 hours. This change must be periodic if it is caused by a planet. In addition, all transits produced by the same planet must be of the same change in brightness and last the same amount of time, thus providing a highly repeatable signal and robust detection method. Once detected, the planet’s orbital size can be calculated from the period (how long it takes the planet to orbit once around the star) and the mass of the star using Kepler’s Third Law of planetary motion. The size of the planet is found from the depth of the transit (how much the brightness of the star drops) and the size of the star. From the orbital size and the temperature of the star, the planet’s characteristic temperature can be calculated. Knowing the temperature of a planet is key to whether or not the planet is habitable (not necessarily inhabited). Scientists need to observe at least two and preferably more cycles to confirm a planet.
by Todd Miller
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