Three planets of Neptune mass have been discovered orbiting a Sun-like star known to have an asteroid belt. Exquisite measurements suggest that the search for habitable planets might be easier than assumed.
Our thirst for knowledge of planets orbiting stars similar to the Sun is tempered by the technological challenges of detecting them. We cannot see analogues of the Solar System directly; rather, the presence of extrasolar planets is inferred through effects that they induce on their parent star. The Doppler method, whereby astronomers search for subtle, periodic changes in the apparent speed of a star that result from its gravitational dance with an unseen planetary companion, has yielded all but a handful of the more than 180 known extrasolar planets1. Heavier planets produce larger stellar wobbles, so it is not surprising that most of the worlds discovered so far have more in common with the distant gas and ice giants of the Solar System (Jupiter, Saturn, Uranus and Neptune) than with the smaller, closer terrestrial planets from Mercury to Mars.
But as techniques have been refined, so planets of lower mass have been revealed in increasing numbers1. The current bestiary of extrasolar planets is therefore far from comprehensive. On page 305 of this issue, Lovis and colleagues2 report unprecedentedly precise observations of the nearby, Sun-like star HD 69830. The fruit of their efforts is not one, but three orbiting planets (Fig. 1). The discovery is exciting for two reasons. First, the authors' technological advance implies that further low-mass planets will be spotted orbiting other stars. Second, the architecture of this particular planetary system bears some intriguing similarities to that of our own Solar System.
The newly found planetary system is remarkable in that it possesses three planets located on nearly circular orbits within 1 astronomical unit of the star (1 AU is the Earth–Sun distance). The same is true of the Solar System. Where the HD 69830 system differs, however, is that the masses of the worlds range from 10 to 18 times that of Earth, and so are similar to that of Neptune. In the Solar System, the division between the low-mass terrestrial planets and massive gas giants was determined by the ‘ice-line’. This is the distance beyond which the temperature in the protoplanetary nebula — the reservoir of gas and dust from which the planets formed — dipped below the freezing point of various hydrogen compounds. Beyond this point, much greater amounts of solid material, and so planets of much greater mass, were created.
The formation history of the HD 69830 system is thus a puzzle deserving of detailed study. Lovis and colleagues present2 a preliminary calculation to show that the inner planets probably formed inside the ice line, and thus are likely to be predominantly rock, not gas. Their large masses require that HD 69830's protoplanetary nebula contained a larger quantity of solid material than did that of the Solar System. That inference is at odds with the observation that the star itself actually has a lower abundance of heavier elements, the stuff of planets.
HD 69830 is no stranger to the spotlight. Last year, researchers using the NASA Spitzer Space Telescope announced that it probably possesses an asteroid belt3 — the only star similar in mass and age to the Sun that is known to have one. This conclusion came from the observation that the star was brighter than expected at infrared wavelengths, suggesting the presence of grains of dust that were small and were located within 1 AU of the star. At that distance, small grains could not survive very long, as the star's light would cause them either to fall inwards towards the star, or be blown outwards. The researchers therefore inferred that the grains must be constantly replenished by material spun off in collisions of large bodies in an asteroid belt. Notably, the implied mass of HD 69830's asteroid belt is roughly 25 times that of our own, seemingly in line with the beefed-up values of the planetary masses.
Intriguingly, these researchers also posited3 the existence of an unseen planet that quietly shepherded the asteroid belt in its orbit. It would seem that Lovis et al. have found the (plural) shepherds. Working with the converse logic, they consider2 the gravitational influence of their three planets on an asteroid belt, and find that its position must be constrained to be either close to the star (0.3–0.5 AU) or far from it (beyond 0.8 AU). The earlier infrared observations favour the former location, which places the belt between the orbits of the centre and the outermost planet, but whether this location is truly stable and consistent with the infrared observations remains an open question. In the Solar System, the asteroid belt lies near 2.6 AU, between the orbits of Mars and Jupiter. The difference in its location in the HD 69830 system is surely a clue to differences in that star's planet-formation history.
One of the great quests of astronomy is to discover a small, rocky, Earth-like planet orbiting within the ‘habitable zone’ — the range of distances from a star for which the ambient temperature would permit a planet with liquid water and, perhaps, life as we know it. For the Sun, which is comparatively large and hot for our Galaxy, this orbital distance is great enough that the Doppler wobble induced by Earth would be a measly 9 cm s−1, nearly an order of magnitude below even the exquisite measurement precision established by Lovis and colleagues. But most neighbouring stars are significantly less massive and cooler than the Sun, so a search for habitable planets using the Doppler method is feasible for two reasons. First, the lower stellar mass means an Earth-mass planet will cause a larger Doppler wobble. Second, the reduced stellar brightness shifts the habitable zone inwards, reducing the orbital period of a habitable planet and further increasing the Doppler wobble. Should astronomers succeed in transferring the precision achieved by Lovis et al.2 to low-mass stars, they might just turn up a few planets akin to our own.
What the Doppler method will, unfortunately, never reveal is the composition of the planets it detects. An exciting synergy therefore surely exists between such high-precision Doppler measurements and measurements to be made by two planned satellite missions, the COROT mission4 led by the French space agency CNES, and NASA's Kepler mission5. Both spacecraft will seek to identify rocky planets by the complementary method of transits6, in which a planet is revealed as it crosses the face of its parent star by a minute dimming of that star's light. By observing both Doppler signals and transits, both the mass and physical size of a planet can be estimated. That in turn yields a density and (by inference) a composition. Previous limits on the Doppler method implied that any terrestrial planets detected by COROT and Kepler would not receive corroborative Doppler measurements. Excitingly, Lovis and colleagues2 have given us cause to rethink this pessimistic assumption.
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Space Science Reviews (2007)