Exoplanet news part 3: There may be hundreds of *billions* of planets in our galaxy!

Bad Astronomy
The entire universe in blog form
Jan. 13 2012 7:01 AM

Exoplanet news part 3: There may be hundreds of *billions* of planets in our galaxy!

[I'm trying to catch up with all the news that's been released this week while I was off lecturing in Texas. This is Part 2 of a few articles just about exoplanets. Here's Part 1, and here's Part 2.]

A very interesting set of observations has resulted in a conclusion that is somehow, paradoxically, both expected and startling: there are hundreds of billions of planets in our galaxy alone!

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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It's expected because all the research being done for the past few years has been zeroing in on how many stars have planets, and it's looking more and more like they're very common. I'll get into that in a sec. But it's also startling, because HOLY COW THERE MAY BE HUNDREDS OF BILLIONS OF PLANETS IN OUR GALAXY ALONE!

Ahem. OK. So what's going on here?

The new result comes from what's called microlensing. The gravity of a star or planet can bend the light coming from an even more distant star, briefly magnifying it. The way the star light gets brighter over time can reveal the mass of the object doing the magnifying -- the "lens", as it were. If a star passes in front of another star, you get a rise and then fall in the brightness, but if a planet is orbiting that nearer star, you get a second, smaller bump as well.

This kind of event takes an extraordinarily precise alignment, so they're extremely rare. To compensate, you need to look at a lot of stars. So astronomers did: a survey using two telescopes covered several million stars every night, looking for the tell-tale bump(s). Over the course of six years, they found three -- yes, only three -- planets orbiting other stars acting like wee distant lenses. But that number is actually pretty good: when combined with previous surveys, and also taking into account how many lenses they didn't see (which is important, statistically), they can extrapolate with some confidence about the numbers and types of exoplanets out there.

Their most basic result, and the one causing the stir, is that they find that there are likely hundreds of billions of planets orbiting other stars in our galaxy alone. Given that there are a few hundred billion stars in the Milky Way, this means on average there are about one or two planets per star in our galaxy! Now, let me be clear: this is an average. I've seen reports saying every star in our galaxy has a planet, and that's not necessarily the case. You could have one star, say, with ten planets, and then nine with none and get the same results here.

The results get even more interesting when you break them down by planet type. They get that 17% (+/- 6/9%) of stars host Jupiter-mass planets, and 52% (+/- 22/29%) of stars host a planet with a mass near Neptune, and a whopping 62% (+/- 35/37%) have planets with a mass between 5 - 10 times that of Earth! These numbers have a somewhat high uncertainty to them (as you can see by the +/-) but even taking that into account, it shows that a lot of stars have planets, and it's likely that most do. In other words, stars with planets are more likely the rule rather than the exception!

That, my friends, is very cool indeed. Other astronomer agree, too.

A couple of things to note: their method was not sensitive to planets the mass of Earth. However, given other results that lower mass planets are also abundant, and their own results mirroring that, it would appear that Earth-massed planets are extremely common. It may be too early to make that statement firmly given these particular numbers, but other results -- like the three tiny planets found around a red dwarf -- do make that very likely.

Also, this finding is for planets orbiting other stars. An earlier study from last year talked about rogue planets, unbound from stars, wandering the galaxy. The number of those detected implied there were billions of those, comparable to the number of stars int he Milky Way as well. That makes sense; those planets would've formed around stars and then been ejected by the gravity of bigger planets, which would be left behind. So the two numbers should be close.

And finally, a thought: in 1990 -- when I was just starting to pursue my degree -- we didn't know of a single planet in the Universe orbiting another star. Not one. Then a couple of years later we found a few, then a few more, then more... and now look. Not only are we finding them, we're using multiple methods to do so, and the upshot is that it might be hard to find a place that doesn't have planets.

The Universe itself hasn't changed in that time by very much, but wow, has our perception of it changed a lot. But then, science evolves. That's what makes it strong: it seeks out more knowledge, more understanding, and better ways of knowing things. We sometimes have to abandon old thoughts, outdated notions, and comfortable misconceptions. But look what we get from it! An amazing and profound change in the way we perceive everything around us, including a galaxy overflowing with planets.

That sounds like a pretty good deal to me.

Image credit: ESO/M. Kornmesser



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