The search for the perfect planet

地球已经很长一段路要走,喜欢它的大气。How would we find a suitable new planet? Are there any planets with atmospheres like ours? Izzie Clarke spoke with Nikku Madhusudhan from Cambridge University’s Institute of Astronomy, about what makes Earth so special, and with Amaury Triaud, of Birmingham University, about the hunt for new planets...

Nikku - The earth has a unique combination of chemicals on its surface and in its atmosphere which makes it very conducive for life. So here's a planet which is at the right temperature with an atmosphere which can sustain liquid water on its surface. And we know that life can thrive in such an environment.

Izzie - Now you look into atmospheres of exoplanets, planets outside of our solar system. So when you're looking for these, what exactly are you looking for?

Nikku - Ultimately, The Holy Grail of the field is to find a planet that’s exactly like the earth in all manner of things. It's one thing of finding a planet that’s at just the right distance from the stars so you could have the right temperature to have liquid water on its surface. But it's quite another major effort to be able to find what is the composition of its atmosphere. So when we look at Earth-like planets around other stars, probably we will start with looking for the same signatures that we have in our own Earth; signatures of oxygen, ozone and other chemicals like methane and so on.

Izzie - Have you had any luck with this? How common or uncommon are these biomarkers?

Nikku - So far we do not have any atmospheric signature of an Earth-sized planet. So we're still looking. But what we do have are lots of giant planets, like Jupiter-sized planets, for which we have found several chemicals in their atmosphere. So that's a place we are at right now with our current facilities. And that's actually looking quite promising. We have discovered molecules such as water vapor. We have discovered carbon monoxide in several planets. We have discovered exotic chemicals, like titanium oxide for example. These giant planets open a wide range of scientific questions on their own even before we get to a smaller, Earth-sized, cooler planet. So when we go to smaller planets, that’s the challenge; we don’t know what their atmospheric composition will be.

Izzie - So how can you go about looking for these smaller planets?

Nikku - So if you want to detect the atmospheric composition, or the atmosphere of this Earth-like planet around a sun-like star, that's extremely hard.

Izzie - So that's looking at a situation like us. A nice, tiny, little planet like ours next to a sun that is pretty big in comparison.

Nikku - Exactly, and that is really hard to measure. So why not look at stars that are much smaller than the sun? There are these classes of stars that are about a tenth of the solar size. And that becomes much more feasible. An advantage of this is that these small stars are also cooler. If the star is cooler, we can be closer to the star and still maintain Earth-like temperatures. So now we are talking about a scenario where you could have habitable planets, which are similar temperatures to Earth, but are much closer to the star. So you can observe them much more frequently. So that's where we see some hopes, and maybe within the next four or five years we actually might be able to find atmospheric signatures of Earth-like planets around small stars.

Izzie - And what is the process that you use to say “oh look at that star over there, there is a bit of hydrogen there. There is some nitrogen over there”.

Nikku - the most common type of observation we make is called transit spectroscopy. You are looking at a star constantly. So if nothing passes in front of this star, you see that the light from the start is constant. When a planet comes in front of this star, you see this light dipping by a little bit as long as the planet is passing in front of the star. What you’ll see is that the atmosphere absorbs in some wavelengths and not in others. Then we use detailed numerical methods to extract the chemical information that's in that spectrum.

Izzie - Nikku Madhudsudhan. Now one candidate system has been found: TRAPPIST 1, at a mere 40 light years away. That's roughly two hundred and thirty five trillion miles. It's a system with a small star, like Nikku looks for, with seven Earth-like planets orbiting around it each with masses and radii similar to those of our own.

Amaury - They have a density which is not that different from the planets of this solar system.

Izzie - That's Amaury Triaud, from Birmingham University. He's discovered over 100 planets outside our solar system.

Amaury - For instance, TRAPPIST 1E, one of the planets of that system has a density which is very very similar to that of the Earth. As far as surface conditions or whether they have an atmosphere we still do not know. With telescopes like the extremely large telescope in Chile, or the James Webb Space Telescope that NASA and the ESA are about to launch in two years time, we will have a
chance to find out whether the planets have an atmosphere, what the climate is, what's the chemistry of the atmosphere, how much greenhouse gas they have, and therefore deduce the conditions on the surface: how hot it is and whether liquid water could exist. However, this at the moment still remains unknown and we are very much looking forward to the day that we can say something about this.

Izzie - Why are we always focusing on on this side of water?

Amaury - Liquid water provides a really good substrate for molecules to move about; for life to use. You need something neutral in which your molecules are going to combine with others in order to produce something more interesting. And water is one of the most abundant molecules in the universe and is one that is actually really practical to make chemistry. It would be strange if a majority of biology in the universe didn't use it.

Izzie - Now you said you're comparing the density. Why is that important?

Amaury - Density is important because it's telling you something about the composition of the planet. Here, when we look at the planets we see that the density is similar to many of the planets of the solar system, so it indicates a lot of rocks. But, interestingly, some of the planets of TRAPPIST 1, like TRAPPIST 1B for instance, which is the closest to the star, has a density which is lower than what you expect for rocks and an iron core like the earth has. And it implies that there is something lighter on the planet. Either a very thick atmosphere, or maybe a big layer of water or ice. We don't really know but it's really intriguing.

Izzie - That would be great news if it might be some sort of ice or water, surely.

Amaury - Well although you want water you don't want too much water either. Here our numbers would be consistent with the planet having two hundred and fifty times more water than the Earth has. So it means no land. It means incredible pressures at the bottom of the ocean. And so it may not be that great. I think you want some water but not so much water.

Izzie - Now we talk about our system here on Earth and the system that we're in being quite unique. But we've got this TRAPPIST 1 which looks quite similar. Are these quite uncommon systems? Are there other candidates like this?

Amaury - It's a very good question. TRAPPIST 1 is similar to the solar system in the properties of the planets but is very different in other properties. For instance, the planets are really close to the star. The star is small, it's cold, and so in order to have the right temperature, the planets are really hugging the star. The entire system fits within 6 percent of the distance between the earth and the sun. So it's a tiny tiny planetary system compared to the solar system. In terms of uniqueness, We feel that solar systems around stars like the sun exist about 10 percent of stars like the sun. We don't really know yet. For systems like TRAPPIST 1, we also are quite uncertain how frequentative they’re found. Our early numbers are rough, but they indicate 30 maybe up to 50 percent of stars that are this small (10 percent of the size of the Sun) could have planets that are terrestrial. And a large fraction of those would have planets that are temperate as well as being terrestrial, meaning that they're very interesting for us to study.

Izzie - And when we say temperate we mean having those conditions where water could potentially exist.

Amaury——通常人们过去使用了我们rd “habitable” to say the planet is within the habitable zone, but I think this word is fraught with misconceptions and preconceived ideas. We see habitable as “OK if I land there it'll be okay for me”

Izzie - Because that's what I'm thinking, I'm like “Oh yeah it's habitable, great, let;s go!”.

Amaury - Let's let's go let's have a drink on the beach! So the word “temperate” I think has less of that baggage, and that's why I think we prefer using that now, at least within our team. So temperate means that the planet has the potential to be habitable.

Izzie- Okay. Is there any way that we might be able to go there?

Amaury - I think going there is so far the stuff of science fiction, it's incredibly far away. I don't think we quite realize how far distances are, how long or how big distances are in space. It's 10 times further than the nearest star is to us. The current speed of a spacecraft, the fastest of them, one of the Voyager probes, is going to take about 50 thousand years to reach that distance. It's a quarter of the lifetime of our species on Earth. It's really long.