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KOI-172.02: An excellent candidate for a “Venus-twin”

posted Jan 12, 2013, 8:29 AM by Shawn Domagal-Goldman   [ updated Jan 14, 2013, 7:56 PM by Abel Mendez Torres ]

(Reposted from

It seems that we have our latest "Earth twin candidate." This time, the planet is in orbit around a star very similar to our Sun. It's been getting lots of press attention. A google news search turns up quite a bit.

Unfortunately, this planet is almost certain to be way too hot to support life, according to the metrics posted at the Kepler website.

Before I get into that, a quick primer on what the habitable zone is. It's the area around a star at which a planet could potentially maintain liquid water oceans at the surface, and therefore a global biosphere. For that to be the case, the planet cannot be too cold, in which case the oceans will freeze over... or too hot, in which case the oceans will be lost to evaporation. You can think of it like Goldilocks and the three bears: the porridge can be too hot, or too cold, or juuuust right.

What's more, on both boundaries of the habitable zone there are runaway feedback loops that amplify the things that would kill oceans, along with any life dependent on those oceans. On the "too hot" side of things, when you make the planet warmer more water will get into the atmosphere. Because water is a greenhouse gas, putting more water in the atmosphere will cause things to get warmer, which will cause more water to get into the atmosphere, which will cause things to get warmer.... and the wheels on the bus go round and round....

On Earth, this feedback loop is broken because as the Earth gets warmer, it radiates more energy back out to space. But if you put too many greenhouse gases into the atmosphere (way, way, way more than we're putting into Earth's atmosphere), you can make the greenhouse effect so efficient that this process get shut down: when you warm the planet, the surface radiates more energy, but essentially NONE of it escapes to space. You can also think of this in terms of "how close to the star are you?" If a planet is too close to its star, too much water will get into its atmosphere and the planet will not be able to radiate enough energy to cool back down. In other words, getting too close to the star will break the planet's "air conditioner." This is what happened to Venus, and it's why the planet has surface temperatures hotter than an oven on full blast - literally.

Unfortunately, KOI-172.02 is too close to it's star for the planet's air conditioner to work properly. It gets over 40% more energy from it's star than Earth gets from the Sun. Unless something else is going on (such as massive cloud cover), the air conditioner on KOI-172.02 should be busted. It's too hot, and probably WAY too hot.

That all said, there are three ways that KOI-172.02 could support life:

  1. the data posted on the Kepler database are inaccurate;
  2. our understanding of the boundaries of the habitable zone are wrong; or
  3. life could be "finding a way" to get by despite the super-hot, Venus-like conditions.

The first of these is just a matter of the uncertainties in the measurements. The posted values for the planet's orbit and the star's properties place the planet outside the star's habitable zone. But all measurements have some degree of uncertainty - a predictable chance that they could be wrong by a given amount. These particular measurements could be off by enough to place the planet in the habitable zone. (Most of this uncertainty comes from the measurements of the size of the star that KOI-172.02 is orbiting.)

The second possibility is tantalizing - what if we're wrong about the habitable zone boundaries? It's possible, for example, that the planet has a totally different kind of air conditioner. Maybe it has lots of clouds that reflect all that incoming energy back out to space or block it before it reaches the surface? (Titan, a moon in our solar system, has clouds that do this.) I think there's more potential here than most of my colleagues acknowledge. But the issue here is that we don't have any examples of planets that "work" this way, and any such planet would not "Earth-like" or have anything resembling our modern-day biosphere. So while such planets are possible, they aren't the best candidates for life.

The last possibility is probably the most unlikely. What if life can "get by" on incredibly hot planets similar to Venus? Some scientists have proposed that this happens on Venus itself, so why not on KOI-172.02? Again, I go back to "likelyhood." Most scientists do NOT think Venus has life, and even those that do think Venus has (or could have) life would also admit life is much more likely on an "Earth-like" planet than on a "Venus-like" one. So again, even the "Venus optimists" wouldn't call this planet "Earth-like" or suggest it's amongst the most likely to be habitable.

These expected similarities to Venus are the rationale for not expecting life on KOI-172.02... but they are also why KOI-172.02 is fascinating! It should be a "Venus-twin." That means we think we know what many of it's properties should be, and the planet is a place to test these hypotheses. It's a planet I want to study!  I want to study it because it *should* be like Venus, at least according to our current hypotheses. I want to study it because it provides a test for those hypotheses. And testing those hypotheses is how this field of science will progress.

Like all of Kepler's other discoveries, KOI-172.02 is a fascinating place with lots to tell us about planetary science, habitability, and life. But it's not fascinating because I think it could have life; in fact, the opposite is true. It is fascinating because I think it CANNOT have life.

Be patient, humans. Kepler will confirm a habitable planet soon. VERY soon. I can tell you that with confidence, based on the other available data. (Abel has done a great job of culling and sorting those data.) When Kepler does find a planet that will satisfy even skeptics such as myself, there will be a press conference. And that press conference will be on a confirmed planet. (This is another issue: KOI-172.02 is only a candidate for the moment.) But that press conference hasn't happened yet. You'll know when it does.

Additional Read: More on the prospects for life on KOI-172.02