Potentially habitable exoplanet: The fine print

Kepler-22b is a newly confirmed exoplanet, orbiting a Sun-like star 600 light years away from Earth. The exoplanet sits in the "habitable zone"—a range of orbits around a star that are, based on what we know about life on Earth, most likely to provide the right conditions for life to happen.

That is pretty damn cool. But it does not mean there must be life on Kepler-22b. As Phil Plait explains on the Bad Astronomy blog, there's a lot we don't know about this exoplanet yet, and "within the habitable zone" is not a guarantee of habitability. Case in point: Our solar system. Earth is within the Sun's habitable zone. But so are Mars and Venus, and you may have noticed that they are not especially teeming with life.

Kepler detects planets when they transit their star, passing directly in front of the star, blocking its light a little bit. The bigger the planet, the more light it blocks. The astronomers going over the data determined that Kepler-22b is about 2.4 times bigger than the Earth. The problem is, that and its distance from its star are all we know. We don’t know if it’s a rocky world, a gaseous one, or what. It may not even have an atmosphere!

Another good post to read on this subject is Matthew Francis' explanation of "habitability" on the Galileo's Pendulum blog. Even the statement, "Kepler 22-b is within the habitable zone," comes along with a lot of assumptions that may or may not turn out to be true.

The following factors are needed to calculate whether a planet is in the habitable zone: The temperature of the host star: the hotter the star, the more it emits light of all wavelengths ... The size of the host star: a large star emits more light from its surface simply because there is more surface area ... The albedo of the planet: how much light gets reflected back into space ... Hand in hand with albedo comes the composition of the planet’s atmosphere—if it has one.

When we say Kepler-22b is in the habitable zone, we're assuming that it has the same atmospheric composition and albedo as Earth. We don't know that. And it's a big leap, bearing in mind (again) that there's not even another planet in our own solar system that shares those characteristics.

I swear, I'm not a fun-hater. Kepler-22b is awesome. Just keep it in context and know that there's still a lot we don't know about this thing.

Size comparison of Kepler-22b via Galileo's Pendulum.


  1. I’m not going to be thrilled about all these exoplanet discoveries until we have the technology to actual take a real picture of the planet’s surface instead of all these photoshop jobs.

  2. My favorite thing from the books The Quiet War and Gardens of the Sun by Paul McAuley was the offhand references to the study of expolanets – the building of enormous arrays of telescopes to gain better resolution images of the planets, so that people can argue about what they’re looking at.  Are those green patches forests, or just enormous mats of algae…!? und so weiter.

    I really hope wee get to that point in my lifetime, not because i want to participate in such arguments, but because pictures of other planets are so very cool

    1. I’ve been wondering if very long baseline interferometry (VLBI), using space-based arrays, might get us more actionable information about exoplanets. Not perhaps at the Google Maps level of detail, but enough to say with some confidence that a fuzzy blip really is a planet and approximately how big it actually is.

  3. As Phil Plait explains on the Bad Astronomy blog, there’s a lot we don’t know about this exoplanet yet…

    For starters, what kind of alien seductresses live there? Do they have beehive hairdos and fishnet tights, or are they the more modern variety that have slightly exotic forehead appliqués? What social issues are they dealing with that seem strange at first but are actually suspiciously similar to contemporary events on Earth? I bet they have some really crazy recipes for cooking chicken.

  4. You know, I appreciate all the accuracy, but Maggie, you’re becoming like the Science Killjoy over here. All your articles are about rolling back the excitement of some new discovery. Why do you have to be so damn factual? :)

  5. Astronomers have been only finding confirmation of extrasolar planets since 1992. Since then, 708 have been found and the means of detection are getting better.  (Extrasolar planet Wikipedia page). That’s pretty exciting.

    Do the writers of the articles ask the astronomers a lot of what-if questions about the possibility of life on these worlds? It seems like every time a new exoplanet is discovered, there’s a lot of speculation about potential life. It makes me think that even though there are seven billion of us here, we’re still lonely.

    1. It seems like every time a new exoplanet is discovered, there’s a lot of speculation about potential life. It makes me think that even though there are seven billion of us here, we’re still lonely.

      I found this part of your comment extremely poignant.  I think it’s because our world keeps expanding, mentally I mean.  Physically there’s less room but mentally there’s always more.   We now know there are different continents, deep oceans, whole other planets and yet we still haven’t found anything like us to fill that new space.   It’s like living by yourself in a giant house, eventually you wish there was someone else to share it with.

      1. I’m not lonely; I just keep hoping we’ll meet a super-intelligent alien race who can help humans stop being such complete and utter fuckheads most of the time.

  6. I don’t think it’s too early to declare our fealty.  I for one welcome our new Keplerian overlords.  I just hope their not fundamentalist Republicans and/or consume human flesh. But if they are, that’s ok, too (see above), I’m sure they’ll have a good justification for it.

  7. “When we say Kepler-22b is in the habitable zone, we’re assuming that it has the same atmospheric composition and albedo as Earth.”
    That’s not the way I was taught.  
    The habitable zone is a distance range from the star in which water can phase change.  Is there a new definition?

    1. The habitable zone is specifically the distance from a star where an earth-like planet can maintain liquid water, effectively guaranteeing a certain temperature range as well.

    2. See this wikipedia article on the Black body. Notice that both the albedo fraction and the greenhouse effect are planet specific. Even with the correct albedo fraction, the Earth’s temperature is too low to support liquid water (254 Kelvin), were it not for a greenhouse effect.

  8. Here on BoingBoing, on Dec. 6th, 2011, at around 1:50pm EST, I officially make my prediction: this Kepler-22b will be an ocean planet, with a rocky core about the size of Mars.  So, basically, a big ball of water, with a smaller, rocky core. 

    Why?  Because, with the provided data, that’s what would lead to something that’s closest to Earth’s mass, leading to it being somewhat inhabitable, if we ever get there.  A rocky planet of that size would have too strong a gravity for us to land on it, and if it was a gas giant (made up of anything other than the composition I described above) then it wouldn’t be as interesting to us.

    I’m not making any predictions on whether it will contain life, but I sure hope it does.  That would just make the universe a much more interesting place to be in.

  9. Of course there is always the possibity of life that doesn’t follow our own earthly model. I have never heard of water being the sole determinant of something that is alive.Why would water be necessary for an organism that breathes methane and eats nitrogen?

    1. Water is really a most unusual fluid. Because of the large difference between oxygen and hydrogen in electronegativity, and the geometry of oxygen, water can form up to four hydrogen bonds. No other remotely common compound does this so water is an amazing solvent and fosters structures through hydrophobic and hydrophilic interactions. As a biologist I really really doubt that there will be any life forms using any solvent other than water, for these physical reasons.

      1. Good explanation. It was on point and directly answers my comment. The rest of my response may sound dismissive. Oh well. I grok that carbon based life with an h2o solvent and amino acids and DNA and RNA seems to be the ideal way for life to form, but that doesn’t automatically eliminate the possibility of silicon based or other types of life. While I’m loathe to resort to anything that resembles an ad hominem argument, I think it would be difficult for anyone who has spent a lifetime studying life on earth to be receptive to non-carbon based life.

  10. Potentially****

    At such time as a future life form resembling what we colloquially know as “humans” can travel the 600M light years.  Pioneer 10, which launched March 3, 1972, is ~14.38 light hours from earth.

    1. By my calculations, at that rate of speed using 1972 technology, which sounds about right given our current state of space exploration, it would take about 13,903,310.15 years to get there.

      If their ain’t life there now, perhaps it will develop by the time we get there? :)

  11. According to one of my favourite authors (guess his name) a planet like this is where the Invaders will originate. In 2050 the Invaders will remove every sign of civilization – from the smallest woodshed to the largest dam. Ten billion humans will starve to death in the following months. The only humans  to survive are the ones who manage to make it to a spaceport before it’s destroyed, and the humans on lunar colonies, Mars, in orbit and so forth.  

    The invaders will do this because they consider whales and dolphins to be superior life forms and humans to be lesser forms of life. The only humans to survive on Earth will be native, tribal peoples who will grow to  worship Cetaceans.

    Just remembered – the Invaders were from a gas giant and only recognized their form of life and that like whales et al. as truly intelligent. We’ve found plenty of gas giant exoplanets though…The author’s name is John Varley – well worth reading.

  12. With Kepler-22b’s larger size and, therefore, higher gravity, I expect that, if we ever make contact with them, they will come to Earth to use their superior strength to become superheroes. Or supervillains. Possibly, linebackers. 

    1. Interesting.

      In 100 years I think we Humans will have suffered through one or more Great Declines; due to war, plague or general tomfoolery.

      And the human race will resemble a kicked termites nest, hardly interested in the impossible to ascertain unlikelihood of intergalactic sentience.

      Give or take a few centuries; but…I just feel it.

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