Posted Wednesday, Feb. 6, 2013, at 11:44 AM
Image courtesy of ESO
Over the past few years we’ve learned a lot about planets that orbit other stars. Called exoplanets, so many have been found in fact—over 800—that we’re starting to get a good sense of them statistically; the sizes they are, the kinds of stars they orbit, and the relationship between the two.
The big goal is to find a planet the same size as Earth orbiting a star at the right distance such that water can exist on the planet—that way, the conditions are right for life to arise. (It’s possible conditions can vary and life can still turn up, but that’s the way it happened for Earth, and we know that worked, so it's the logical place to look.) We’re pretty sure such planets are out there, but how many are there, and how far away are they?
New results just released today may have an answer to that. A team of astronomers examined thousands of stars looking for planets, and what they found indicates the closest Earth-like planet might be very close by indeed. Statistically speaking, we’d expect the nearest one to be a mere 13 light-years away!
There are some caveats here, of course. But the takeaway idea is that planets like ours are almost certainly very common in the galaxy; so common that the distance between them on a galactic scale is small. That’s amazing.
Image credit: Adapted from a diagram by Greg Loughlin
The study used data from NASA’s Kepler mission. This space-based observatory stares at one spot in the sky, observing about 150,000 stars. If a planet orbits a star, and the orbit happens to take the planet directly in front of the star from our view, we see the starlight dim as the planet blocks the star. This is called a transit. We don’t see the planet directly, but we can infer a lot about it, including its size, estimated temperature, and in some cases its mass. (I have a description and details in an earlier post about looking at exoplanets.)
This new result comes from looking at red dwarfs, dinky little stars less massive and cooler than the sun. They are so dim that not a single one is visible to the naked eye, but they are by far the most common type of star in the galaxy—there may be 75 billion of them in the Milky Way alone. They make great targets for Kepler: Because a red dwarf is smaller, even a planet as small as Earth blocks a higher percentage of the starlight, making them easier to detect. Also, for a planet to be warm like Earth, it has to orbit the star closer in; that makes its period (its year) shorter, again making it easier to find: You see more transits from it in a given time. And there are simply so many of these stars it makes observing lots of them easy, and the statistics you get are better.
Looking at thousands of red dwarf stars in the Kepler field, the lead investigator (Courtney Dressing from the Center for Astrophysics) found several dozen stars with candidate planets (probable companions that have not yet been confirmed). Out of those, she found three that were about the size of Earth, as well as being in their stars’ habitable zones, the right distance from their stars to have liquid water. Accounting for planets with orbits that don’t let them transit from our view, what she found is that 6 percent of red dwarf stars have Earth-sized planets at the right distance from the star to be potentially habitable.
That’s 1 out of 16. Out of 75 billion stars. That’s a lot of Earths. In fact, using that number and applying some statistics, Dressing and her team calculate that on average, in this part of the galaxy, Earth-like planets are only 13 light-years apart. That’s a long way to walk, but in galactic terms, that’s incredibly close. Only about three dozen stars are known that are within 13 light-years of Earth. Could one of them bear a planet like home?
Image credit: Inga Nielsen
Now to be clear, this is a statistically determined number; we haven’t actually found any Earths that close. (The three Earth-sized planets found by Dressing’s team are 300‑600 light-years away.) Still, it should be in the right ballpark—it’s consistent with numbers from previous research. I’ll also note we don’t really know the habitability of these planets. Many red dwarfs are extremely active, prone to huge eruptions on their surface like mega-solar flares, which makes the habitability of any orbiting planets questionable (though some biologists wonder if such stress might add more pressure for evolution, accelerating the adaptation of life). Perhaps most red dwarf exoplanets won’t have atmospheres, or they might have such thick air that they produce a runaway greenhouse effect like on Venus, or or or ... We’re still new at this and gathering data. But these new results about the raw number of planets out there are very promising.
Another point that’s well worth noting: Red dwarf stars live a long time. When our sun is dead and gone in 6 billion years, a lot of these stars will still be merrily ticking away. Some of these red dwarfs are already old, so it’s not inconceivable we’ll find one with planets that are much older than ours, maybe even 8 billion to 10 billion years old. (Earth is a whippersnapper at 4.54 billion years old.)
That to me is fascinating. If life arises easily on a planet given the right conditions—and it did on Earth, eons ago, practically as soon as the Earth cooled enough for the surface to allow it—then this implies there has already been plenty of time for life to arise all over the galaxy. We only have one example to go by, so it’s hard to extrapolate from this. Still, the temptation is too hard to resist. What does it mean to have the possibility of older, habitable planets scattered around space?
If life arose on some small planet orbiting a red dwarf a few hundred million years before it did on Earth, what would life look like on that planet today? We have no idea if intelligence is inevitable once life gets its start, or even how long it takes to arise—it took more than 3 billion years here on Earth, for example. But given how old some red dwarfs are, we have a couple of billion years of slack here and there to play with.
It staggers the imagination to think that complex life on some warm world may have already been around for billions of years when Earth was still covered with algae. If true, it’s not silly at all to question if intelligent life besides our own exists in the galaxy and just how many civilizations may be out there. Could the galaxy actually be buzzing with life?
It’s all pure speculation at this point. Well, not 100 percent pure … We now know that there are lots of planets out there, billions upon billions of them. That’s an excellent first step in figuring this out. As we get better at finding these exoplanets, will we find the majority are lifeless and incapable of life, or will we find one after another that will remind us of home?
Either way, what an exciting thought! And it’s brought to you by human curiosity, by our desire to know, and by real science.