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About Those Reported 8.8 Billion Earth-Like Planets

800px-Kepler-69c-_Super-Venus.jpg

David Klinghoffer commented on the story about the 8.8 billion Earth-like planets ("Our Vexing Planet: Still Privileged After All These Years"):

They calculated in the sense of "extrapolating" to "come up" with a figure. In other words, they estimated. The figure of "8.8 billion stars with Earth-size planets in the habitable temperature zone" comes down a bit too when you talk about actual planets that have been observed instead of being merely conjectured and "calculated."

Scientists at a Kepler science conference Monday said they have found 833 new candidate planets with the space telescope, bringing the total of planets they've spotted to 3,538, but most aren't candidates for life.

Kepler has identified only 10 planets that are about Earth's size circling sun-like stars and are in the habitable zone, including one called Kepler 69-c.

Ah hah. So from the initial, trumpets-blaring figure of 8.8 billion we're down more realistically to 10. Not 10 billion, just 10. Meanwhile the silence from space continues absolutely unabated.

Readers may wonder why 10 out of 10,000 planets observed have been "Earth-like" in the "habitable zone," whereas 8.8 billion are extrapolated/calculated to exist. The answer is that they also extrapolate/calculate 8.8 trillion planets in the Milky Way. Then if we take a random sample of 8.8 trillion, of course we see only 10/10000 usable planets. But is this plausible?

The number 8.8 trillion is a bit of a stretch too. There are some 100 billion visible stars in our galaxy, plus another 700 billion or so invisible red dwarfs, brown dwarfs and failed stars -- if every bit of the dark matter is in the form of dim stars. These might have habitable zones, but they would be so close to the star itself as to cause "tidal locking" so that like the moon, only one side faces the star. Some have wondered if such planets without weather would be habitable. But we'll include them, even if their habitable zone is about the size of South Pole Station.

But of those 0.8 trillion stars, most of them are binaries, where two stars orbit each other. This makes it hard for a planetary system to avoid getting ejected in the complex "3-body" dance. At best we have perhaps 500 billion "singleton" stars. Since most of the solar systems we have observed sufficiently have multiple planets, we'll be generous and say 5 planets per star. That gets us up to 2.5 trillion planets. 

Whew, I'm running out of generosity, and we still have a factor of 3 to go! But you get the idea. This is a looong stretch.

I did think of a way to be more generous, though.

If only 10 of the first 10,000 planets found are Earth-like, it could be that there is "observer bias." That means it is harder to see an Earth-like planet (because it is smaller) than a Jupiter-like planet (which is bigger). So if we say that the bias is a factor of 3, then we can lower the total number of planets from 8.8 trillion to something like 3 trillion. If we put in a bias of 10, then we can lower it to 1 trillion, etc. But as you can see, we need a bias of something like 100 in order to remove the red dwarf and binary star generosities.

In other words, we have to accuse astronomers of extreme prejudice to make ourselves look good.

Image: Kepler-69c; NASA Ames/JPL-Caltech.