Planets around star HD69830
Astronomers at the Observatory of Geneva have discovered three Neptune-size planets orbiting star HD69830, which is much like our Sun. Although the planets are too small to be seen, astronomers predicted their existence from a complicated pattern of 'gravitational wobbling' the planets provoked around their star. Chris Smith talks to Christophe Lovis about the discovery and wonders whether these planets are of a type which may support life.
Chris Smith: HD69830 might not be the most excitingly named star in our cosmic neighbourhood, but it's certainly got researchers buzzing in Switzerland because Christophe Lovis and his colleagues at the Geneva Observatory have found in orbit three rocky planets about the size of Neptune. They're too small to see directly, in fact, it was the wobble they provoke in their home star that gave them away, but intriguingly at least one of them is in the habitable zone, like the Earth, and that means it might also be a home to liquid water.
Christophe Lovis: We have discovered a multi planet system with three very low mass planets, and it's the first such system with such low mass planets. Of course, we are now beginning to discover planets that are only a few Earth masses, so we are approaching planets of really small size and it's become very interesting.
Chris Smith: So you're into the kind of size which is almost Earth-like and therefore very similar in terms of their capacity to support life, perhaps.
Christophe Lovis: Yes, we are not sure now what is their exact composition, but of course if the mass is very low then these planets are very likely to be mostly rocky, and in that case they would really look like the Earth.
Chris Smith: Where are these planets?
Christophe Lovis: They are around a nearby star. This is a quite bright star that can be seen with the naked eye, and it's about 40 light-years away from the Earth. So these three planets are orbiting this star, which is quite like our sun, a little bit less massive, a little bit cooler, but still much like our sun.
Chris Smith: And how did you actually make these observations?
Christophe Lovis: We used dedicated instrumentation which was developed with the goal of being able to discover extra-solar planets by measuring very precisely the radial velocity of the stars. All stars are moving across space and each star has its own velocity, and so when observed from the Earth we can measure the component of this velocity along our line of sight. So some stars are just receding from us and some stars are approaching. And on top of that, when they are orbited by planets, this will introduce a little gravitational wobble on these stars and they will make oscillations around their centre of mass velocity and we are able to measure these very tiny radial velocity changes.
Chris Smith: So you saw this star literally wiggling a bit in space?
Christophe Lovis: Yes.
Chris Smith: But if it wiggles a bit, how do you know there's three planets, then, if you can't see them?
Christophe Lovis: The signal received from a planet is expected to be like a sinusoid curve and when we observed the star we noticed that the radial velocity showed complicated changes that could not be only explained by one sine curve, and after accumulating enough measurements and trying orbital solutions we found that the best solution was when we fitted three planets on these radial velocity curves.
Chris Smith: And where are these planets in relations to the star itself?
Christophe Lovis: The first one orbits with a period of nine days. It is very, very close to the star, much closer to the star than, for example, Mercury in our own system. The second planet orbits in 30 days, which is a little bit further away. And most interesting is the third planet, which orbits in about 200 days, which becomes quite similar to the Earth, and so it is about 60% of the sun/Earth distance.
Chris Smith: Is that within what we call the habitable zone for this particular star then?
Christophe Lovis: Yes, for this particular star it would be at the inner edge of the habitable zone, so that means just at the border where the temperature is low enough for water to become liquid, and that's one of the definitions of the habitable zone.
Chris Smith: I guess we'll just have to watch this space for more fertile findings from HD69830, and its clutch of rocky worlds. That was Christophe Lovis from the Geneva Observatory describing the work that he's published in this week's edition of Nature.
- Christophe Lovis
University of Geneva
- Chris Smith
- Presenter, Nature podcasts
- Robyn Williams
- Polly Rickard / David Fisher