The harsh destiny of a planet?
Posted: May 10, 2001

Did the star HD 82943 swallow one of its planets? What may at a first glance look like the recipe for a dramatic science-fiction story is in fact the well-considered conclusion of a serious scientific study, to be published by a group of astronomers in Switzerland and Spain in the latest issue of the international research journal "Nature".

Artist's impression of the engulfment of a planet. Credit: Gabi Perez and the IAC
Using the very efficient UVES high-resolution spectrograph at the ESO VLT 8.2-m KUEYEN telescope, they have convincingly detected the presence of the rare isotope Lithium-6 (6Li) in this metal-rich, solar-type dwarf star that is also known to possess a planetary system.

Unlike the Lithium-7 (7Li) isotope of this light element, any primordial Lithium-6 would not survive the early evolutionary stages of a metal-rich solar-type star. The Lithium-6 now seen in HD 82943 must therefore have been added later, but from where? The astronomers believe that this observation strongly suggests that the star has at some moment engulfed one of its planets, whose Lithium-6 was then deposited in the star's atmosphere.

This surprising discovery represents important observational evidence that planets may fall into their host stars.

HD 82943 and its planetary system
The last few years have seen the discovery of more than 60 new planetary systems. One of the most prolific planet search programmes is being carried out by the Geneva Extra-Solar Planet Search Group, by means of the CORALIE spectrograph at the 1.2-m Leonard Euler Swiss Telescope at the ESO La Silla Observatory (Chile).

One of the stars included in this programme is the dwarf star HD 82943 in the constellation Hydra (The Water Snake). It is slightly hotter and larger than the Sun and was recently found to harbour a planetary system with (at least) two giant planets.

Like most extra-solar planets ("exoplanets") found to date, the orbits of the objects orbiting HD 82943 are quite unlike those expected from traditional theories of the formation and evolution of such systems. Contrary to the giant planets in the Solar System, those at HD 82943 have rather elongated orbits, and they are unsually close to the central star.

Astronomers believe that giant planets must form in comparatively cool environments, as this was the case in the solar system. The existence of systems in which the giant planets are much closer to the central star can only be explained by certain dynamical processes, e.g. significant orbital changes with time ("orbital migration") or the effects of strong gravitational interaction between several planets.

These processes can explain the short-period planetary systems found to date, in which planets are very close to the central star, and also the very elongated orbits found in some cases.

These theories also predict that it may be the fate of some planets to fall into their host star.

The significance of Lithium
Unlike most other elements lighter than Iron, the light nuclei of Lithium (both the Lithium-6 and Lithium-7 isotopes), Beryllium and Boron are not produced in significant amounts in the stellar spheres of fire.

In fact, Lithium-6 is extremely "fragile", being easily destroyed by proton collisions at a temperature of "only" 1.5 million degrees - by comparison, the fusion of Hydrogen to Helium takes place at about 10 million degrees. In the case of solar-like stars, any Lithium-6 atoms present in a newborn star will be ``burnt'' during the early evolutionary stages. Strong internal motions will thoroughly mix the outer (cooler) and inner (hotter) stellar layers, and Lithium-6 will completely disappear in just a few million years. We would therefore not expect to find any Lithium-6 in a developed solar-type star.

However, during the later evolutionary stages the outer layers of a solar-type star remain better "separated" from the hotter central parts. Thus, if some Lithium-6 is now picked up from the outside, it is therefore possible that it will be preserved in the upper, cooler regions for some time, possibly billions of years.

Unlike stars, planets never reach temperatures that are high enough to burn their initial content of Lithium-6. Consequently, planets will retain their Lithium-6. So, if a planet happens to fall into a solar-type star like HD 82943, we may then be able to detect this isotope in the stellar spectrum.

In the case of "metal-poor" stars - that are less rich in metals than the Sun - the mixing process in the early phase is less efficient and some original Lithium-6 may actually survive.

Detection of Lithium-6 in HD 82943 with UVES
The possible presence of Lithium-6 in a stellar atmosphere can be checked by means of a detailed analysis of the star's spectrum. For this, the astronomers search for a very small asymmetry in the "stronger" absorption line in the red spectral region that is caused by Lithium-7 atoms in the stellar atmosphere.

However, this type of investigation is critically dependent on the availability of very detailed and "clean" spectra (i.e., very high spectral resolution and excellent signal-to-noise ratio). This is a great observational challenge and to date, only about five stars are known to display the signatures of Lithium-6 in their spectra. In all cases, the measured isotopic abundance ratio is very small, with 6Li/7Li less than about 0.05. All of these stars are metal-poor and may have retained some of their initial Lithium-6, see above.

Until now, no convincing detection of Lithium-6 has ever been made in a metal-rich, solar-type star.

The UVES spectrograph at the 8.2-m VLT KUEYEN telescope is perfectly suited for this kind of study. Three high-resolution spectra of HD 82943 were obtained in June 2000 that show a significant asymmetry in the Lithium-7 absorption line, cf. PR Photo 17/01. After a careful analysis, this asymmetry is confirmed as the spectral signature of Lithium-6 atoms. The observed abundance ratio is 6Li/7Li = 0.12. This is unusually high when compared to the detections in metal-poor stars and is in fact more compatible with the value of 0.08, observed in solar-system meteorites!

HD 82943 has swallowed a planet
The astronomers believe they know the answer: "The simplest and most convincing way to explain this observation is that one or more planets, or at least planetary material, have fallen into the star, sometime after it passed through its early evolutionary stage", says Nuno Santos of the Geneva Observatory.

Garik Israelian of Instituto de Astrofisica de Canarias adds: "One may also try to determine the quantity of material needed to explain the observed isotopic ratio of 0.12. Based on the mass estimate of the star HD 82943 and the known Lithium-6 content of meteorites, it appears that the star has swallowed the equivalent of a giant planet with twice the mass of Jupiter". If the unlucky planet were of the terrestrial type, in which the relative Lithium-6 content is higher, it would have had a mass of about three times the mass of the Earth.

The observational search for Lithium-6 in other stars with planetary systems now continues. In due time, it will permit to better understand the formation and evolution of the newly discovered exoplanets. In particular, it will demonstrate whether the fall of planets into their host stars is a common process or not.

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