The search for life on other worlds

Giovanna Tinetti is an expert on detecting signs of life across interstellar space. She has worked at JPL, Caltech and the Institut d’Astrophysique de Paris, and has just won an Aurora Fellowship to pursue her research on biosignatures at University College, London. We caught up with her as she made an exploratory visit to the city that will be her home for the next three years.

How do you describe yourself?

I’m an astrobiologist

Earth is the only planet that we know supports life, so what does an astrobiologist do?

A small fraction of the light that reaches us from stars that have planets is transmitted or reflected from their atmospheres. By studying the spectrum of that light we can learn a great deal about them. What I do is build detailed models of planetary atmospheres, and use them to identify the signatures of life and how we can detect them.

To do that for extrasolar planets we need first to understand in detail the atmospheres of planets in our solar system – which in a sense is our laboratory. Basically we are searching for disequilibria in the chemical state of the atmospheres of other planets.

Can you give me an example?

Many of our ideas go back to the work of James Lovelock, who was really the first astrobiologist. He pointed out that if an atmosphere is completely in equilibrium, it’s likely that the planet does not harbour life. So aliens looking at the spectrum of Earth would notice the large quantities of oxygen and ozone there. Oxygen can be produced in the atmosphere of an abiotic planet, but not in the quantities that we see on Earth – and it’s very reactive, so you need a constant source to keep the level at 21%.

Oxygen is not the only biomarker. Another disequilibrium we are very interested in is called the red edge. A leaf is a complex structure of cells and empty spaces, in which sunlight is scattered back and forth. Photons that are good for photosynthesis are absorbed very efficiently, while the rest are scattered back as a waste product. This gives a very distinctive signature – a high reflectance in the far red of the optical spectrum.

So if you look at the spectrum of Earth from space you get this incredible increase of signal in the red part of the spectrum, which tells you that vegetation is there.

So is that something we can detect on exoplanets?

That’s one of the questions I’ve been working on. You start from a 3-D model of an Earth-like planet and you play with the parameters. You look from different directions and as the planet rotates; you see how the signature changes in varying conditions of the atmosphere. It turns out that you need the co-operation of clouds to detect a signature. But on Earth there are enough spaces, even with the clouds, to detect the red edge pretty well. So at least for planets like ours this is a signature you can see from space.

What if the planets are quite different from Earth?

We looked at that while I was at Caltech, using a model developed by Wolstencroft and Raven at the Royal Observatory, Edinburgh. Raven came to present this fantastic paper at one of the first ever astrobiology conferences at NASA Ames. They had looked at how life might select a chemistry that could make best use of light from a star that was unlike our sun. In particular they studied photosynthesis on planets orbiting M-stars – which are cooler than our sun, so the whole spectrum is shifted to longer wavelengths.

They provided a scheme for photosynthesis that uses three photons instead of two, as vegetation does on Earth. They showed that you can still have photosynthesis with a cooler star. So that started me thinking about what would happen to the red edge. I showed that it would be shifted, so you would need to look for a slightly different signature.

The next question of course was whether planets of such a star would have the same atmospheric characteristics as Earth. Now a scientist called Joshi had already provided a 3-D model for a terrestrial planet in the habitable zone of an M-star. He’d shown that you probably need more greenhouse gases to warm up the area not illuminated.

This is because for such a cool star the planet has to be very close. So it could be tidally locked, with one face always illuminated and the other always dark. That meant you needed a circulation of the atmosphere and a particular composition. We put that model together with my calculations on the shifted red edge, and discovered that the strength of the edge feature on an M-star terrestrial planet can exceed that on Earth, given the right conditions.

This is just one example of the sort of modelling you have to do to generalise the concept of biosignature from Earth to extrasolar planets.

How much of these biosignatures can we actually detect at interstellar distances?

Trying to study a terrestrial planet in the habitable zone is a real challenge. Some of the missions being planned now will enable us to do that, but unfortunately the information of an entire planet is likely to be collapsed into just one pixel.

Now you’re probably thinking it’s impossible to learn anything about a planet from one pixel. But not so. I’ve been working a lot on what you can still understand when you start from a 3-D, very complex model of a planet and collapse it.

I’ve done this in particular for the Earth and Mars – remember what I said about the solar system being our laboratory? I’ve shown that you can still pick up biosignatures from Earth, the difference in seasons on Mars, the variation of the polar caps, the composition of the atmosphere – a whole lot of interesting information.

But can you make much progress now before terrestrial exoplanets in the habitable zone are actually detected?

We need to model for future missions, and at the same time to understand what is going on with present measurements. The extrasolar planets we can already probe are hot Jupiters. So I’ve published a few papers on those, and have projects with Spitzer and Hubble to probe the atmospheres of those planets. It’s great to understand how all this modelling can relate to the few observations that we have.

It also tells you about the difficulties of missions that will provide extra information, and about your capability as a modeller to interpret these. It is important to start to use these observations, although it's clear that with hot Jupiters you can't talk about biosignatures.

Still an atmosphere is always an atmosphere, and it's pretty amazing how much you can say about these planets from very few observations. It makes you optimistic about future missions, which are very difficult.

What do you expect to learn from these future missions?

Both Darwin and the Terrestrial Planet Finder interferometer will look for signatures using free-flying telescopes to study objects in the infra-red. In addition TPF will use a very big telescope with a coronagraph – which masks light from the star – to detect planets in the visible range.

The timescales for these missions have changed in the past year, because it’s impossible to separate the science from the funding – unfortunately. These will be the first-generation missions dedicated entirely to the search for life on terrestrial planets in the habitable zone. But there are other missions that will provide valuable information before them.

The European Space Agency mission Corot, which successfully launched in December, is now searching for extrasolar planets down to about two Earth radii, using the transit method. So we are expecting exciting new data soon. We will certainly get statistics of new planets that are smaller in size. And we should also get information about the composition of these planets.

That’s because Corot will give us their radius, then astronomers will follow up with radial velocity measurements, which give will us their mass. So right away you’ve got the density of the planet, and can say if it’s a giant or a terrestrial – and also maybe what kind of terrestrial, whether it’s a silica planet like Earth or Mars, or an icy body like Titan.

We would hope to learn more about these planets using the Hubble and Spitzer telescopes. Then next year there ‘s another mission called Kepler which will take us down to Earth-size planets, and let us look further out from the star towards the habitable zone.

Within a few years we will have the statistics of extrasolar planets. That’s why we need to do a lot of work now on modelling planets and their atmospheres, so that we understand how they are formed, what characteristics they have, and of course whether they are habitable.

I guess as an astrobiologist you must believe we are going to find inhabited planets?

I actually began as a particle physicist, and had already started my PhD thesis when I read Lovelock’s book – which I thought was just amazing. In that same year the first extrasolar planet was discovered. It was really the dawn of astrobiology. I realised that was what I wanted to study, so I finished my thesis and sent my CV to NASA centres where they were interested in extrasolar planets. They said come and work with us.

I have no doubt now that if life can set up on a particular planet it will. I think it's much less probable that it will evolve from an elemental to a more complex sort of life. On Earth that became possible because at one point life succeeded in using the light from the sun as a source of energy, and this is just incredible.

Micro-organisms can get their energy from fairly simple chemistry. But complex lifeforms burn much more energy, and have to get it at a far higher rate than simple organisms. The key to that is photosynthesis combined with the use of oxygen.

So life is almost certainly very common in the universe. Complex life probably is not.

So you think we live in a universe full of micro-organisms…

Exactly

…and hardly any humans.

Hardly any complex lifeforms. But I would be very surprised if we were the only ones. It is a huge universe.

Further reading:

Giovanna Tinetti's homepage

Archive for exoplanetary science at Centauri Dreams

Searching for terrestrial planets with Corot

Searching for Earth-size planets with Kepler

The Darwin mission

Terrestrial Planet Finder

Aurora Fellowship