?In the coming years we might be able to detect smaller planets and maybe approach what could be a telluric planet in the habitable zone?

Can you briefly describe the system these three extra-solar planets form?

We have discovered three very low-mass planets around the nearby star HD 69830. This star is much like our Sun, a little bit less massive and a little bit less luminous than our Sun. The planets orbit in 9 days, 30 days and 200 days around their star. Both inner planets are quite close to the star and therefore quite hot. Their mass is 10 times the mass of the earth, for the first one, and 12 time for the second. The third planet is further away and begins to looks more like our 365 days orbital period of the Earth. The mass of this planet is about 18 times the earth mass. We have made theoretical calculations to know what these planets are made of, and we think that both inner planets should be telluric: they are probably made of rocks essentially so they should look like the Earth. The third planet is probably not only rocky; it probably has a rocky core but it also has ice and a lot of gas, like Uranus, Neptune, Jupiter and Saturn in our solar system. The interesting point with the third planet is that it is situated in the so called habitable zone, which is the zone where the temperature is not too low and not too high so that water could be liquid on the surface of a telluric planet. But as I said before, unfortunately this planet is not telluric because it is too massive.

Does it mean that we can take in perspective to find life outside the solar system?

Yes and I think this is one of the most interesting points of this discovery. We have been discovering planets over the past ten years and each time the question was raised on the possibility of life to develop around those planets. Of course it was difficult to think of life because we had only very massive planets like Jupiter and it’s unlikely that life can develop on such planets. With this discovery we show that there are probably a lot of low-mass planets. A few of them have already been discovered over the past two years and now with three more Neptune-mass planets we think it’s the signal that there must be many and many of them, probably billions of low-mass planets in our galaxy. Of course we can imagine that life could also evolve and develop on those low-mass planets provided they are made of rock or ice and not gas and provided they are situated at the right distance of their star. The discovery we have made shows that we are now able to detect low-mass planets even within the habitable zone so in the coming years we might be able to detect smaller planets and maybe approach what could be a telluric planet in the habitable zone. That would be very exciting.

In your work you question the formation history of this system and you present a preliminary calculation to show that the inner planet probably formed inside the ice line. Can you introduce us the notion of ice line?

When the planets form they form in a what is called a proto-planetary disc which is a disc of material, ice and gas, that is orbiting around the star. We are just beginning to understand how exactly they form and this is a complicated mechanism which needs a lot of knowledge in physics to understand. One of the important points is this ice line which is the limit behind which you will find ice in the form of small crystals and within which ice will sublimate, so that you’ll have water vapour and not ice. If a planet forms outside the ice line, its composition will contain a lot of ice. If a planet starts to form within the ice line it won’t have water at all.

Your discovery was made possible by the use of refined techniques which permit to reveal lower mass planets. What are these techniques?

We used the improvement made on the radial velocity technique. We improved the accuracy of the techniques, so we have an instruments that is able to measure this radial velocity with a very high accuracy. I would like to mention that another telescope has also studied these same stars: the Spitzer Space telescope. It observed the star about one year ago and what would be an asteroid belt around this star but it did it with a complete different technique. A quite indirect detection: it detected the infrared lights and showed that there is too much infrared light coming from this star which is actually caused, this is an interpretation, by the collision of asteroids.

What will the use of this improved technique change in the research for extra solar planets?

We believe now that we will discover many of these Neptune-mass planets in the coming years and it will completely improve our knowledge of the properties of extra-solar planets. Up to now we know 180 extra-solar planets, most of them are very massive. We can do some statistics to try to understand what are the main characteristics and how in general this planetary system looks like. We are now able to say few things about massive planets, how massive they are, at which distance they orbit and how they were made, but we believe that with the discoveries of these low-mass planets coming in the next years we will be able to do also statistical studies on low-mass planets, to see if they are made of rocks, if they are made of rocks and ice or gas. The studies will become exiting in one year or two because of, for example, the COROT satellite launched by France, which will be able to detect transits of extra-solar planets in front of their star. This satellite will be able to detect transits from low-mass extra-solar planets. If at the same time we can detect these planets with our techniques then we will know the composition of these planets, we will be able to measure the radius, the mass and the density of these planets, so we will have their composition and we will be able to know from what they are made.

What are the next steps of your research?

For us there would be two main goals: extend the sample of extra-solar planets, especially low-mass, so that we can make some statistics and understand their properties in a general way and also to push the technique to the limit to detect very low-mass planets in particular we hope to detect telluric planets very soon.

Dr. Lovis thank you.

[1] Nature 441, 305-309 (18 May 2006)

Christophe Lovis works at the Geneva Observatory

Interview by Francesca Gilibert