Searching for a Goldilocks planet
Alien life is almost certainly real; they probably just haven’t visited Earth yet. That’s according to Sara Seager, a woman described as a pioneer in the search for exoplanets — planets that orbit a sun other than our own. She’s also leading the quest to
Q You have said we’re going to be able to both find and prove the existence of another habitable planet, even one with life on it, within the next 40 years.
A We try not to overpromise, so I would start by saying that with the next generation of telescopes, we’ll have the capability to find habitable planets. But there’s a bit of a stretch between the portrayal in science fiction, movies like Avatar and Interstellar, where we can go and actually visit the planets and what scientists are doing.
We’re looking for a habitable planet … a Goldilocks planet: not too hot, not too cold, not too big, not too small, just right for life. So far, most things we can measure about planets consistently are their mass and their size, sometimes both, and how much energy they’re receiving from their star. Really, it’s just those three things we have to go by. Sometimes we can measure atmospheric properties.
We have this thing called the habitable zone, it’s the distance from a star where the planets will be heated just enough, assuming they have a thin atmosphere. A number of planets have been found in their star’s habitable zone, most of them are probably too big or too hot, but we don’t really know. ... Now we’re just waiting till we have the next generation of telescopes (such as the James Webb Space Telescope and the Thirty Meter Telescope, both of which have a Canadian connection) so we can more easily access these smaller planets.
Q How will you know you’ve not only found a habitable planet but signs of life?
A What we call habitable now is quite simple: All life on Earth needs water, so we call a planet habitable if it has liquid water. Now, on a planet we can’t get a beautiful image like the Apollo images of Earth, we can’t see an ocean. But what we will be able to see (from telescope images) is vapour in the atmosphere. Water vapour is actually a very strong absorber (of energy from a star). ... So, if we see water vapour on a small rocky planet, that should be indicative of a reservoir. ... We’re confident we’ll find that: rocky planets with liquid water.
Q Once you find a small rocky planet with liquid water, how do we detect life from this far away?
A What we’re looking for are gases in the atmosphere that don’t belong. The reason we fixate on this is we have that on our own planet Earth. We have oxygen, but oxygen is a highly reactive gas. It shouldn’t be in our atmosphere. If we didn’t have life on our planet … we would have virtually no oxygen.
Q It wouldn’t necessarily need to be oxygen though, right?
A Right, then the question is the list of gases, which raises a bit of a problem. Because what we end up finding is, pretty much any other gas, including oxygen at some level, though it’s produced by life, it usually has another source.
We’re actually in a bit of a limbo right now, because there are gases produced in large quantities but they are created by geophysics and atmospheric chemical reactions as well. There are gases produced in small quantities specific to life on Earth (but there isn’t enough to detect them from this far away). So we’re looking for gases that are present in quantities that can’t really be explained any other way.
Q They say theoretically one in six stars has something in that Goldilocks zone, what does it take to prove we think this is habitable? Are there any planets we think hold life already?
A Not a single one right now. And that one in five, one in six number is still a bit speculative, actually. It’s sort of an extrapolation after a very long scientific inquiry. We know that exoplanets are extremely common … evidence is showing that every star has planets, and rocky planets are common also.
Q Is it possible life could be on a moon?
A It could, but planets are so hard to see themselves and moons are even harder. It’s possible we could see the moon of a giant planet. But we’re not ready yet.
Q People talk about these exoplanets circling “nearby” stars, but what does that actually mean?
A We think our Milky Way looks like a spiral galaxy (with) hundreds of billions of stars, and in our universe we think there are hundreds of billions of galaxies. But the only region we can search for habitable planets are the very nearest stars to us in our galaxy.
Q So how far away is that?
A Our nearest star is about four light years away. The planets we have found so far range anywhere from 10 to 20 light years away to thousands of light years away.
Q Do we need to send satellites to be closer to these stars to definitively prove the existence of life on other planets?
A We’d like to ... if we could. We do not know how to send interstellar spacecraft to other stars. ... Our fastest Voyager 1 spacecraft has left our solar system; it’s billions of kilometres away now. If it were on its way to the nearest star, it would still take about 50,000 years to get there.
If we could figure out how to go even faster — that one’s going 20 kilometres a second — then maybe we send a probe sooner. Some people think they could find a way to travel a tenth of the speed of light, and that way it would still take 40 years to get there.
Q What about the suggestion there was, or maybe still is, life on Mars?
A It’s very similar actually (to planets in the Goldilocks zone). There’s evidence of very tiny variable methane that appears to come and go, and it’s in much smaller quantities than we could observe on an exoplanet. But it’s the same thing: could this be produced by geophysics or is there actually some subsurface bacteria? This conversation has been edited and condensed for clarity and length.