About the Kepler mission’s announcement last week of tons of extrasolar planets:

1. A local Richmond TV station had me on the news to talk about the announcement. (The fact that they asked me primarily goes to show that astrophysicists are not exactly thick on the ground in Richmond.) I can’t stand to look at myself on video, but if you want to see it, go ahead.

2. Via Sean Carroll, some cool visualizations of the Kepler data, showing number of planets by size, distance from star, temperature.

3. Sean says

A back-of-the-envelope calculation implies that there might be a million or so "Earth-like" planets in our Milky Way galaxy.

I’d go much higher than that. Kepler looked at about 150,000 stars and found five Earth-like planets (meaning roughly Earth-sized and in the habitable zone where liquid water could exist). If you imagine that they had 100% efficiency — that is, that they found all the Earth-like planets there are in the sample — then one in 30,000 stars would have an Earth-like planet. Multiply by 100 billion stars in the galaxy, and you get about 3 million Earths.

But here’s the thing: Kepler’s efficiency can’t be more than about 1% or so. The mission works by looking for eclipses, which occur when the planet passes directly in front of the star as seen from Earth. That means that it only has a chance of detecting a planet if the geometry is fortuitously aligned. The probability of such an alignment occurring depends on the size of the star and of the planet’s orbit (in fact, it’s just the ratio of the two). For the actual Earth and Sun, the probability works out to 1%. Many of Kepler’s planets are closer in and have higher probabilities, but at best the geometrical alignment can only occur a few percent of the time on average.

Even with the right geometry, they don’t have a 100% chance of finding a planet, of course. Once you fold in all sources of inefficiency, I’d be very surprised if they have a better than 1% chance of finding any given Earth-like planet. I wouldn’t be surprised if it’s more like 0.1%

Just to be clear, that’s not a criticism of Kepler. It’s just an acknowledgment that this is a hard task they’ve set themselves!

So my back-of-the-envelope estimate is hundreds of millions, if not billions, of Earth-like planets in the Galaxy.