18 May 2006
Under embargo till 17 May 19:00 CET

Trio of Neptunes and their Belt

HARPS Instrument Finds Unusual Planetary System

This image is taken from a point of view inside the asteroid belt, which is assumed here to lie between the two outermost planets.
Video 1 7.9MB mov	Video 2 17MB mov
ESO Video 18a/06 shows three computer animations. The first two ones are an artist impression of how the planetary system surrounding the nearby star HD 69830 may look like. The HARPS measurement indeed reveal the presence of three planets with masses between 10 and 18 Earth masses. The planets' mean distance are 0.08, 0.19, and 0.63 the mean distance between the Earth and the Sun. They orbit their parent star in 8.67, 31.6 and 197 days, respectively. From previous observations, it seems that there exists also an asteroid belt, whose location is unknown. It could either lie between the two outermost planets, or farther from its parent star than 0.8 the mean Earth-Sun distance. The second animation illustrates the system, seen from inside the asteroid belt, which is assumed here to lie between the two outermost planets. The third animation illustrates the method used to detect the planets. A planet in orbit around a star will manifest its presence by pulling the star in different directions, thereby changing by rather small amounts its measured velocity. When the planet is closer to us than the star, it pulls the star towards us, therbey decreasing its radial velocity. According to the Doppler effect, the lines in the spectrum will then be shifted towards the blue. When the planet is on the side, it makes the star slightly recedes from us. The lines are then shifted to the red. Using highly accurate spectrographs, like HARPS on the 3.6-m ESO telescope, astronomers therefore measure with the velocity of a star to detect the signature of one or more planets. The amplitude of the shifts provide astronomers with information on the mass of the planets. Video 2 Footage of ESO La Silla Observatory, located in the Atacama Desert in Chile. The Observatory is home to the 3.6-m telescope, equipped with the HARPS spectrograph, the most-precise planet hunter in the world.

"For the first time, we have discovered a planetary system composed of several Neptune-mass planets", said Christophe Lovis, from the Geneva Observatory and lead-author of the paper presenting the results.

During more than two years, the astronomers carefully studied HD 69830, a rather inconspicuous nearby star slightly less massive than the Sun. Located 41 light-years away towards the constellation of Puppis (the Stern), it is, with a visual magnitude of 5.95, just visible with the unaided eye. The astronomers' precise radial-velocity measurements allowed them to discover the presence of three tiny companions orbiting their parent star in 8.67, 31.6 and 197 days.

"Only ESO's HARPS instrument installed at the La Silla Observatory, Chile, made it possible to uncover these planets", said Michel Mayor, also from Geneva Observatory, and HARPS Principal Investigator. "Without any doubt, it is presently the world's most precise planet-hunting machine"

The detected velocity variations are between 2 and 3 metres per second, corresponding to about 9 km/h! That's the speed of a person walking briskly. Such tiny signals could not have been distinguished from 'simple noise' by most of today's available spectrographs.

The newly found planets have minimum masses between 10 and 18 times the mass of the Earth. Extensive theoretical simulations favour an essentially rocky composition for the inner planet, and a rocky/gas structure for the middle one. The outer planet has probably accreted some ice during its formation, and is likely to be made of a rocky/icy core surrounded by a quite massive envelope. Further calculations have also shown that the system is in a dynamically stable configuration.

The outer planet also appears to be located near the inner edge of the habitable zone, where liquid water can exist at the surface of rocky/icy bodies. Although this planet is probably not Earth-like due to its heavy mass, its discovery opens the way to exciting perspectives.

"This alone makes this system already exceptional", said Willy Benz, from Bern University, and co-author. "But the recent discovery by the Spitzer Space Telescope that the star most likely hosts an asteroid belt is adding the cherry to the cake."

With three roughly equal-mass planets, one being in the habitable zone, and an asteroid belt, this planetary system shares many properties with our own solar system.

"The planetary system around HD 69830 clearly represents a Rosetta stone in our understanding of how planets form", said Michel Mayor. "No doubt it will help us better understand thehuge diversity we have observed since the first extra-solar planet was found 11 years ago."

Measurements of the radial velocity of the star HD 69830 obtained by HARPS on the ESO 3.6m telescope at La Silla as a function of time, from November 2004 till February 2005 (a) and from mid-October 2005 till February 2006 (b). The two innermost planets are clearly revealed, while the presence of the third one becomes clear when removing the signal of the inner planets and binning the data points (one per observing run), as shown in (c). The lower parts of (a) and (b) show the residuals from the best fit indicated by the solid line.	The HARPS radial velocity measurements of HD 69830 are folded with the orbital periods of the three discovered planets: 8.67, 31.6 and 197 days, respectively. In each case, the contribution of the two other planets has been subtracted. The solid line shows the best fit to the measurements, corresponding to minimum masses of 10.2, 11.8 and 18.1 Earth masses. Note that the full span of the vertical axis is only 13 m/s! Error bars indicate the accuracy of the measurements. The integration time was 4 minutes on average for the first 18 measurements (shown as open dots), and was increased to 15 minutes for the remaining points (full dots). The latter measurements are therefore of much higher quality.	lustration of the possible formation process and present day structure of the planetary system around HD 68930. The three planets form from embryos originally located at larger distances (dashed ellipses) than the present ones (indicated by solid ellipses at 0.07, 0.18 and 0.63 the mean Earth-Sun distance). The embryos of the inner and middle planets start interior to the ice line, so that these two planets build up from rocky planetesimals and gas. The two planets consist of a central rocky core (in brown) and an envelope of gas (in gray). The embryo of the outermost planet starts beyond the ice line, and the planet accumulates a large amount of ice at the beginning of its formation. The planet finally consists of a central rocky core (brown), surrounded by a shell of water (ice or liquid - in blue), and a quite massive gas envelope (gray).