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Kepler-22b: A ‘Super-Earth’ in the Habitable Zone

It’s fun to see Kepler-22b — an intriguing new world that lies 600 light years from us toward Lyra and Cygnus — being referred to as the ‘Christmas planet’ in the newspapers this morning, the latter a nod to Kepler chief scientist William Borucki, who said he thought of the planet that way, as a seasonal gift to the team. Borucki’s enthusiasm is understandable, and it’s echoed by Geoff Marcy (UC-Berkeley), who called the Kepler-22b work a ‘phenomenal discovery in the course of human history.’ I can’t argue with scientists of this calibre — with a surface temperature not so different from an April afternoon where I live, Kepler-22b can lay claim to being the smallest planet we’ve found orbiting in the habitable zone of a star like our Sun.

The host star is, in fact, a G5-class object with mass and radius only slightly less than that of our Sun, which is a G2, and the planet in question orbits it with a period of 289 days, some 15 percent closer to its star than we are to ours. Liquid water could surely exist on this object, and the excitement grows from that fact as well as the fact that this is the smallest-radius planet discovered in any habitable zone thus far. At 2.4 times the size of the Earth, it falls into the ‘super-Earth’ category about which we need so much more information. So far, we can say about its mass only that it is less than 36 times that of Earth (this is based on the absence of a measurable radial velocity wobble in the host star in follow-up observations). The mass of other ‘super-Earths’ has been measured at five to ten times that of Earth.

Image: This diagram compares our own solar system to Kepler-22, a star system containing the first “habitable zone” planet discovered by NASA’s Kepler mission. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. Liquid water is essential for life on Earth. Credit: NASA/Ames/JPL-Caltech.

A second Earth? Hardly, but habitable conditions could exist here even if, as seems likely, Kepler-22b is more of a Neptune than an Earth, perhaps one with a planet-encircling ocean. But we have so much to learn — is this actually a rocky world, or an ocean planet or a kind of cross between Neptune and the Earth, with gas, liquid and plenty of rock? Whatever the case, Kepler-22 has been a long time coming, with the first transit captured just three days after Kepler became operationally ready. The all important third transit was acquired just about a year ago.

The Kepler science conference at NASA Ames is ongoing (it runs from the 5th to the 9th), and we now have an 89 percent increase in the number of planet candidates identified by the hard-working instrument, the total reaching 2,326. A NASA news release on the latest findings says that 207 of these candidates are approximately Earth-sized, while 680 fit the ‘super-Earth’ category, 1,181 are Neptune-size, 203 are similar to Jupiter in size and 55 are larger than Jupiter. The main trend here is a dramatic increase in the number of smaller planet candidates.

The new data show 48 planet candidates in their star’s habitable zone, a decrease from the 54 reported in February that is related to a slightly changing definition of ‘habitable zone’ in the new Kepler catalog. Natalie Batalha (San Jose State University) is Kepler deputy science team lead:

“The tremendous growth in the number of Earth-size candidates tells us that we’re honing in on the planets Kepler was designed to detect: those that are not only Earth-size, but also are potentially habitable. The more data we collect, the keener our eye for finding the smallest planets out at longer orbital periods.”

All true, and exciting in every respect. Still, I can’t help thinking how we keep finding new causes for celebration, each one with a slightly smaller world, or one just a bit more in the habitable zone than the previous. In my household we have a birthday tradition that stretches what would be a one-day event into a week-long affair. On my actual birthday, I’ll get presents from the kids. But maybe one of them couldn’t be there, so that sets up a second ‘birthday’ the next day. And then a day or two later we’ll go have a birthday dinner out. You get the point: It’s fun to drag out festivities, and we can expect more of this phenomenon as Kepler continues its work. Because one of these days it’s going to tag a planet of definitely Earth-mass in the habitable zone of a G-class star, and that’s going to be the true ‘second Earth’ we’ve all been hoping to find.


Comments on this entry are closed.

  • djlactin December 6, 2011, 11:28
  • jkittlejr December 6, 2011, 12:32

    The longer this experiment runs the better results we can get! It is simply a matter of having accumulated sufficient signal to noise and 2) sufficient time to collect many transients. But think about all those systems (the majority) where the orbital plane is of the planets are out of alignment with observation from earth. We simply have not collected enough data yet, in the sample size we have, to find the best matches to earth… but time will tell, as long as the funding continues. We are obviously just getting to the good part, if we double the mission length there will be a bountiful harvest of data on the smaller words as well as ones with larger orbits. These points may all be obvious to most of the readers, but exciting none the less!
    Now how do we do a meaningful survey of nearby stars (all those within 30 light years) that may be reachable with multi-generational star ships? Kepler technology is great for large samples of stars in a concentrated area but misses too many worlds in a small sample size scttered across the sky, – I would like to see a definitive survey of every nearby star but do not know how that might be done. Even the HARPS technology does not work well for bodies with orbital planes tilted away from earth.

  • jkittlejr December 6, 2011, 13:34

    I would also like to point out that it is highly likely this world would have a lot more retained helium and hydrogen. Much of earth’s oxygen is really a “Fossil ” resource, where Hydrogen from water /ammonia/methane ( interchangeable in geologic time) is photolyzed in the upper atmosphere and hydrogen atoms excape. This would not happen as readily in a heavier world where the sun is a bit cooler. Thus -do not get too excited about an oxygen atmosphere just yet.. however life does not require oxygen gas. This world will likely have a very thick atmosphere of water and methane as well as ammonia. – the molecular biologist

  • ecc December 6, 2011, 14:06

    I don’t mean to sound cynical — this is exciting news, and the science coming out of Kepler’s data is amazing — but I worry that these types of findings are oversold to the press and public By calling a planet of unknown material and chemical composition “a super earth” (or “second earth”), strong tacit claims about climate, biosphere etc are being implied, for which we have no real data. A planet being in a temperature zone around a sun where water, if present, could exist in a liquid state is necessary but hardly sufficient criteria for being “earth-like”.

  • Erik Anderson December 6, 2011, 14:15

    A planet with 36x the Earth’s mass and 2.4x the Earth’s radius would have a surface gravity of 15 g’s. Supposing that its density were similar to Earth, it would still be 14x as massive with 6 g’s. I cannot help but suppose that a planet such is this would have an atmosphere of insufferable density. I doubt that future generations of Galactic explorers would find this world to be “habitable.”

  • andy December 6, 2011, 14:23

    I don’t think you get to say you’ve found a habitable planet when the world in question has a radius 2.4 times that of Earth. Plugging the numbers into the mass-radius relationships for solid exoplanets, a pure silicate planet would be 21.5 Earth masses, an Earth-composition planet would be a whopping 32.4 Earth masses. These seem fairly implausible: your planet ought to have started picking up substantial quantities of ices and gas well before it got that big.

    Most likely in my opinion is that we’re looking at a mini-Neptune, in which case there may not even be a well-defined surface to the planet. In the most optimistic scenario we might have an ocean world with a very high ice fraction (the 75% ice “ocean planet” model in the paper gives a mass of 9.21 Earth masses) – but then you have the issue that the silicate core where most of the minerals and heavy elements are being separated from the ocean by a mantle of thousands of kilometres of high-pressure ices. Life needs more than just water. Certainly in neither scenario does the habitable zone model being used apply, as it is based on a silicate-carbonate cycle that could not exist on a planet where the atmosphere and silicate core are so decoupled.

  • JohnHunt December 6, 2011, 15:15

    Any report about what the surface gravity might be? I would think that, without being able to measure a wobble of it’s host star, I’m guessing they don’t know the density so no mass so no gravity estimate. If the same density as Earth what would 2.4 times the size calculate for gravity?

    From an interstellar mission standpoint, 600 ly is far too far for consideration. My guess is that we’ll end up with the nearest liquid water planet having no evidence of life in it’s atmosphere and being further than we’d like for the first true interstellar mission. So, we’ll have to choose a system with non-Earth-like planets. However, since we are just now getting down to Earth-sized detections, perhaps smaller rocky planets will be common.

  • bigdan201 December 6, 2011, 15:52

    So far away. But still a very intriguing find. This planet will be a great object of study not only in its own right, but it will also give us experience and information on earth-like exoplanets in general.

  • FrankH December 6, 2011, 15:59

    The surface gravity is going to be a killer – 2.5g Earth, if it’s the same density, and around 1.5g if it’s around the density of the Moon (or a dense rocky core and a deep, deep ocean).

    If the density is 3 g/cm^3 or greater, it’s escape velocity will be higher than the average escape velocity of hydrogen at that temperature (300K) so I’m betting it’s a warm, wet Neptune-y place.

    You can play with various parameters with this handy calculator:


    2.4x the radius of the Earth is around 15290km

  • Gunnar Larsson December 6, 2011, 16:05

    I get the feeling that we know pretty much have confirmed that there are planets that with regards to size and incoming solar radiation are earth-like. Obviously there will be some further progress, I guess the first detection of a (“normal”) planet smaller than Earth as well as the first exomoon will be milestones that will be reached reasonably soon. However, without knowing any more variables it is hard to really say how earth-like these planets really are. Anyone know roughly how large a (space?) telescope would need to be in order to do decent spectroscopy (detect the most abundant elements, temperature etc.) of super-Earths within the “habitable” zone?

  • Andrey Cavallazzi Freitas December 6, 2011, 17:38

    Does anyone know about the age of Kepler-22b’s host star? I couldn’t find it anywhere.

  • Daniel Suggs December 6, 2011, 18:22

    If this planet has several moons, is their mass included in the estimated mass? Would the existence of moons mean the planet could be smaller than the estimate? Would the moons (or rings for that matter) affect the transit shadow, making it appear larger? If the planet is a ‘small Neptune’, could it have an Earth size moon? Thanks for keeping us informed!

  • Abelard Lindsey December 6, 2011, 18:30

    At 2.4 times the size of the Earth, its a big planet. I will bet you donuts to dollars that, if the density can be measured, it will turn out to be a warm Neptune or at least some kind of water world. I doubt this planet is as dense as the Earth.

  • Abelard Lindsey December 6, 2011, 18:37

    I have a question for all of you in this blog.

    Are Eukaryotes necessary to generate an Oxygen atmosphere of sufficient concentration to be breathable for humans? Or can prokaryotic photo-synthetic bacteria (cyanobacteria?) produce such an Oxygen atmosphere?

    Any thoughts?

  • Phil December 6, 2011, 19:07

    I agree that this is very interesting but Kepler 22b as a potential habitable planet is overhyped. What Kepler is doing is unique, and some of the remaining KOIs look very intruiging, but from the point of view of really studying Hz planets I’m more interested in what HARPs announces. Their planets are close by and their RV sensitivity seems to be improving, at least with quiet stars. I’m hoping that within 5 years they might start to reveal some information about planets in the 0.5-1.5 (sin i) earth mass regime – possible true Earth analogues. For stars witin 10pc, thats going to mean M and K class star/planet combinations and tidal locking to boot. I’m waiting for Delfosse’s upcoming paper on Gliese 667Cc for starters. Then an update on A Cen B please!


  • Rob Henry December 6, 2011, 19:27

    Why is there so much talk here of this planet having almost Neptune’s mass. If it had accumulated so much mass, how has it managed to cool off so much more than Neptune from its heat of formation, through a much smaller surface area, and despite it being much closer to its star, as to allow its interior to condense when Neptune’s has not?

    I know little about the theoretical structure of planets this size but wonder from the comments if they are often expected to contain only a tiny fraction of their weight as volatiles. If this was not the case wouldn’t the expectation become a very low density, possibly even lower than that of Neptune as measured from the cloud tops (as we have to do here).

  • yeti101 December 6, 2011, 19:41

    jkittlejr, kepler will tell us how many we should expect to find within 30 light years. As for surveying all stars within 30 light years? very difficult. The scope of proposed missions was to be able to search 100 nearby stars.

  • FrankH December 6, 2011, 19:47

    There’s a limit to the size of an icy moon around a gas giant (slightly smaller than the size of mars) and an icy moon in the HZ of a star will quickly become an ex-moon.
    The only way you’re going to get an Earth sized moon around a gas giant is through the capture of an existing terrestrial planet. Good luck with that.

  • Michael Simmons December 6, 2011, 20:21

    This would be a lot more interesting if it was near the outer edge of the stars habitable zone rather the inner edge. Assuming a larger planet means a larger atmosphere and hence a larger green house effect.
    i.e. my guess is that if its a rocky planet then its probably like venus.
    Is someone using the doppler/wobble method on this system to find out its mass?

    Hopefully the Kepler Mission will be extended and planets 1-4 x earth mass further out in the habitable zone can be found.

  • Mike December 6, 2011, 20:24

    To jkittlejr, I think the cancelled SIM space telescope would have had the capability to detect earth-mass planets orbiting all stars within 30 light years regardless of their alignment to our line of sight thanks to astrometry characteristics. And possibly considerably much further for stars brighter then M-dwarfs.
    The findings discovered by SIM could then be followed up and examined by a space telescope designed for visible and infra-red spectroscopy. Something like DaVINCI or some similar design. Perhaps if NASAs’ long-term funding plans had a little more sanity this would have happened.
    Perhaps something like this will anyway. However I, and presumably most interested people want to see if bio-signatures are detected from exoplanets
    while they are still around to enjoy and appreciate such new knowledge .
    To paraphrase Geoff Marcy,we’ve lost ten years. I’m hoping Keplers’ findings will encourage support for funding further explorations of the kind I’ve described.
    More good things coming from Kepler, stay tuned.

  • Tony P December 6, 2011, 21:04

    Abelard, I think Cyanobacteria is responsible for the large quantities of oxygen in our atmosphere, so, prokaryotic life is all that is needed, but it would likely take at least a billion years for appreciable oxygen.

  • Tom Baty December 6, 2011, 21:27

    Abelard: I don’t have an answer to your question, but it is a good one. I have a copy of “Rare Earth” here on hand; I did a quick check to see if there was any reference to this in the text but came up empty . Perhaps someone else can answer it better then I can.

  • Dave December 7, 2011, 0:21

    Has anyone seen any data on how old Kepler 22 might be? Obviously that would have a big impact on the possibilities for life.

  • Enzo December 7, 2011, 1:08

    I was going to write pretty much what Andy did, so no point in doing that.

    However, in this article, there’s a mention of a yet to be announced “double planet” discovered by the Kepler team :

    “The team has also used HET to confirm the planet Kepler-17b and four additional Kepler planets, including a double-planet system, that will be published soon.”

    Anyone heard this somewhere else ?

  • djlactin December 7, 2011, 2:22

    Mr Lindsey:
    In fact, cyanobacteria are responsible for the oxygenation of the Earth’s atmosphere.

  • Erik Anderson December 7, 2011, 3:48

    Whoops, I forgot to square the radius to get the surface gravity. So it’s 6 g’s with 36x the mass and indeed ~2.4 g’s at 14x (Earth density) as FrankH says.

    Some of the reporting I’ve seen makes worse mistakes — one version of the story I read states that Kepler 22b has “2.4x as massive as the Earth” (rather than 2.4x the radius). Ugh!

    Note to Abelard Lindsey, have a look at:

  • A. A. Jackson December 7, 2011, 8:18

    One should keep in mind that Kepler has a fixed field of view, about .25 per cent of the whole sky. I think we are looking at only roughly one year’s worth of observational data. It had been estimated in it’s three year operational mission it might roughly see 50 planets in a habitable zone. Kepler now has more candidates than that.
    The main idea will be to gather the statistics on the nearly 100,000 stars it observes and use that data to compute the statistics.
    There is still a debate about ‘habitable’ zones around stars, since there are more M stars than any it is hard to say just how many ‘Goldilocks’ planets there are.
    I can remember when there was only one. If Kepler has found more than 50 in a year, no telling how many in the three year mission.
    (Kepler is not funded for the six year mission it could carry out, one hopes this will change.)
    In their infinite wisdom the Washington powers-that-be canceled The Terrestrial Planet Finder mission this year.

  • Abelard Lindsey December 7, 2011, 12:27

    Thanks for the responses. I did some digging around myself and found the following:


    In short, cyanobacteria (pro-karyotes) generated only about 10% of the Oxygen in our modern atmosphere. The rest require Eukaryote algae. At least, this is how I read it.

  • FrankH December 7, 2011, 13:02

    @Erik – a very easy way to get a good approximation of a planet’s surface gravity is to multiply the ratio of the radii (Rplanet/Rearth) by the ratio of the densities (dPlanet/dEarth).

  • andy December 7, 2011, 13:24

    Regarding the “double planet system”, that could also be interpreted as a two-planet system… English is a wonderful language for ambiguities…

  • A. A. Jackson December 8, 2011, 8:43

    I don’t really have the data at hand, but wondered , just how many super Jupiter sized planets have been found in habitable zones?
    Article here in September:
    I can remember speculation that there maybe terrestrial sized satellites about gas giants in prose SF before Avatar.
    I am not sure these planets could be detected by transit , but maybe Doppler spectroscopy? Tho methinks that might be hard.
    It have only seen one thumbnail estimate of habitable ‘exomoons’ that is 25 million in the milky way. Only a small modification to the Drake equation.
    Yet still I have not seen a definitive quantitative analysis of the statistics of possible exo-moon-planets about gas giants in habitable zones about other stars.
    There are some papers that give constrains , orbital stability, tidal heating, other factors, seems a field rich with questions.

  • Gunnar Larsson December 8, 2011, 10:59

    @A. A. Jackson: You can find some methods at http://en.wikipedia.org/wiki/Exomoon

  • ljk December 8, 2011, 11:29

    Some food for thought as we begin licking our chops over every announcement of an Earth-type planet in the galaxy:


  • Ronald December 8, 2011, 12:30

    More than the oversold and overpublisized ‘habitable’ planet kepler-22b, the really fascinating and encouraging news is the dramatically increasing trend in number and proportion of roughly earth-sized and super-earth planets.

    Another quote from the news release: “The number of Earth-size and super Earth-size candidates has increased by more than 200 and 140 percent since February, respectively”.

    Simple arithmetics: right now, the proportion per planet size category is about:
    – 9% earth-sized
    – 29% super-earth
    – 51% Neptune-class
    – 9% Jupiter-class
    – 2% super-gas giant

    I wonder what % of all stars observed has at least 1 planet. It should be over 50% by now?

    BTW, this also largely settles the relevance of large (habitable) exomoons around super-Jupiters: they must simply be very uncommon.

  • Tom Louden December 8, 2011, 17:01

    @A. A. Jackson – probably the easiest way to detect an exomoon would be through the technique of transit timing. The gravity of the moon tugs the planet slightly to and fro, changing the timing of the transit event by up to a few seconds, which should be detectible if the moon/planet mass ratio is high enough and the measurements sensitive.

  • Eniac December 9, 2011, 0:53

    Simple arithmetics: right now, the proportion per planet size category is about:
    – 9% earth-sized
    – 29% super-earth
    – 51% Neptune-class
    – 9% Jupiter-class
    – 2% super-gas giant

    Just to make sure, the fall-off a t the lower end is bound to be due to detection bias. Small planets are not as easily detected as large ones.

  • Ronald December 9, 2011, 8:18

    I have had a quick look back at our post on the initial Kepler results (first 136 days) in last February, see the thread Musings on Kepler’s Latest:


    At that time the proportions of the planet size categories was:
    Earth-sized: 6%
    Super-earth: 24%
    Neptune-class: 55%
    Jupiter-class: 14%
    Super-gas giant: 2%

    See present % in my previous comment: so, the proportion of the smaller two categories is growing and those of the Neptune-class and particularly the Jupiter-class are diminishing, maybe not surprising and a result of previous observational bias.

    Those results were up to about 0.5 AU from each star i.e. the innermost system. The Kepler team extrapolation was that (at least) “6% of the stars in our Milky way have Earth- and super-Earth size exoplanets, 17% of them have Neptune-size candidates and only 4% of them have Jupiter-size exoplanets”.

    I have two major questions now:
    – Up to which distance from the star (in AU) are the current data corresponding?
    – What is the present extrapolation for (minimum) occurrence of the various size categories in our MW?
    – What fraction of stars have an (near) earth-sized planet in their HZ, same for super-earth? If remember well, that was only about 0.5 – 1% last Feb.

  • andy December 9, 2011, 14:57

    By the way, the talks at the First Kepler Science Conference are available online here. Worth checking out.

  • Rob Henry December 9, 2011, 22:00

    The talk here has been of the use of densities as an almost unrestrained variable, so that we may pick an arbitrary composition of common materials and then find the implied density of an exoplanet and use this to calculate its surface gravity.

    This has started me thinking in the opposite direction, and I notice that we have two examples of the surface gravity of Earth size planets and two examples of cloud-top gravity on Neptune sized worlds. They form the following pattern.

    Mass/Earth’s “Surface” gravity
    0.8 8.9 m/s/s
    1 9.8 m/s/s
    14.5 8.7 m/s/s
    17 11.1 m/s/s

    Notice that they are all very similar. Also note that Saturn’s cloud-top gravity is also similar. It is just possible that that implies that as mass increases it is very hard to avoid picking up volatiles in such a quantities as to compensate for the increased mass.

    How naïve would I have to be to think that, in the light of the paucity of other data, a probable density might be in the 2-3 times that of water range, and that this would be a water world of very reasonable surface gravity.

    Perhaps though, the atmosphere is so thick that this ocean is frozen as exotic ices and life, if there is any, floats high in the atmosphere.

  • Danonino December 10, 2011, 16:50

    Why so much interest and research money spend on searching for planets hundreds of light years away? Seems pointless to me. Why dont we search in our neighborhood?

  • spaceman December 10, 2011, 17:53

    Given Kepler-22b’s position within its G5 star’s habitable zone, I am sure that work will be underway to get more data so as to constrain its composition. As truly exciting as this discovery is on an individual basis, remember that the Kepler mission is designed to give us data more statistical in nature and so media talk of the planet as being “a new Earth” is premature. It will be up to the next generation of telescopes and, for that matter, given the stark fiscal climate, the next generation of astronomers to gather enough detailed data to take “family portraits” of many systems of planets. Right now, the data we have is analogous to knowing the sizes of those in a family based on the shadows they cast with extremely limited information on the more exact nature of the family members.

    The most encouraging sign is that the rate of growth in the number of small candidates is outpacing the rate of growth in the number of larger candidates all of the way down to approximately Earth size. This is at least a strong indication that the initial data was biased in favor of finding larger planets and the observed drop-off below Neptune-size is likely a selection effect rather than a true indication of the actual distribution.

    A question I have has to do with the notion that the Kepler mission will be the humanity’s first science mission capable of pinning down the next term in the Drake equation, f(p), the fraction of stars hosting planets. I think at the time of the February 2011 data release the reported percentage of stars with at least one planet was about 34%. I am guessing it is higher now, perhaps near 50%. Has anyone heard about what that percentage is now after the addition of 1000+ new candidate planets?

  • Christopher Phoenix December 10, 2011, 18:26

    I remember reading Olaf Stapledon’s The Last and First Men, the part where the last men use advanced transhuman technologies to adapt themselves and numerous Earth species to the surface of Neptune. Before, they could only visit the surface of Neptune in armored pressure suits resembling deep-sea diving gear. I considered the idea rather outdated, as we know that gas giants like Neptune have no solid surface, eliminating them as potential landing sites (unless you care to build a floating city in the gas giants atmosphere).

    But now, with all the super-earths and mini-neptunes we are finding, might we discover a world with a solid surface, dense high-pressure atmosphere, and high surface gravity, like Olaf Stapledon’s Neptune? If there are living things at the bottom of the ocean, why not on a mix of Neptune and Earth, with gas, liquid, and plenty of rock? Could we visit such a world in an armored suit? Could we even adapt terrestrial forms of life for such an unusual planet?

    Finally, how do you suppose a walk on a planet with a 1.5 g surface gravity and a dense, high oxygen atmosphere would affect Lord Cockswain as he hunts for the rare Triple-Toed Shrobunk in the umbrella-tree forests of a distant super-Earth?


  • yeti101 December 14, 2011, 14:25

    Danonino, the purpose of the kepler mission is to deliver the number of earth sized planets in the HZ of sun-like stars . We can then extrapolate these numbers throughout the galaxy . This will tell us how many we should expect to find around stars near earth. We do not have the capability to do a search like this of close by stars.

    Kepler results will dictate what sort of mission we need to do next. How many stars nearby do we need to search until we find another “earth”? kepler will tell us.

  • amphiox December 14, 2011, 22:28

    Why so much interest and research money spend on searching for planets hundreds of light years away? Seems pointless to me. Why dont we search in our neighborhood?

    Keep in mind that we are, in fact, searching nearby stars too (like with HARPS). The famous Gliese 581 system is only 20 light years away.

    It’s just that the specific parameters for the Kepler mission cannot be achieved just by looking at nearby stars.

    Remember that Kepler is an attempt to obtain a large statistical sample of sun-like stars, and there aren’t many sun-like stars in our nearby neighbourhood. Most of our nearby stars are red dwarfs, like Gliese 581.