Kepler stuff

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.

Ted Bunn

I am chair of the physics department at the University of Richmond. In addition to teaching a variety of undergraduate physics courses, I work on a variety of research projects in cosmology, the study of the origin, structure, and evolution of the Universe. University of Richmond undergraduates are involved in all aspects of this research. If you want to know more about my research, ask me!

One thought on “Kepler stuff”

1. Does this mean that Gott (with his “from my visit to the Berlin Wall, I can predict how long it will last” argument) was right?

For more than half of the history of the Earth, there was nothing more complex than algae. Thus, “higher” life and hence intelligence might not have happened at all. Then again, they probably could have happened earlier. Unless the development of intelligence itself is rare, now that the number of planets looks to be large, the Drake Equation tells us that a large number of intelligent beings must have emerged. (OK, maybe all don’t develop a technology capable of interstellar communication or travel.) However, if we believe the Copernican Principle in the sense that we are nothing special, then neither intelligence, nor civilisation nor development of interstellar communication and travel (though we’re not there yet) is anything special. So, as Fermi asked, where are they?

One answer is that the lifetime of a technical civilsation is short. This is also the result of Gott’s argument applied to us.