The pace of planet discoveries beyond our own solar system is accelerating -- but some of those discoveries have raised new puzzles as well. Take the case of planetary pulverization, or the latest example of extrasolar extravagance.
First, let's consider the pulverized planets: NASA's Spitzer Space Telescope has detected a surprisingly large amount of dust around three pairs of mature, close-orbiting stars. Astronomers say these are not the dusty cradles of planets yet to come. Instead, they appear to be the dusty graveyards of planets that have been ground down to bits. Their findings appear in the Aug. 19 issue of the Astrophysical Journal Letters.
"This is real-life science fiction," Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics, the principal investigator behind the research, said in a news release. "Our data tell us that planets in these systems might not be so lucky -- collisions could be common. It's theoretically possible that habitable planets could exist around these types of stars, so if there happened to be any life there, it could be doomed."
The types of systems in question are called RS Canum Venaticorums, or RS CVns for short, after one example of the category in the northern constellation Canes Venatici (the Hunting Dogs). Such sets of binary stars are typically separated by only about 2 million miles (3.2 million kilometers), or one-fiftieth of the distance between Earth and the sun. The stars spin around each other in just a matter of days -- and all that jostling creates a wild gravitational ride for any planetary bodies in nearby orbits.
The three star systems studied by Spitzer are old enough that most of the dust around them should have dissipated, as it has in our own solar system. Nevertheless, the telescope has detected the infrared glow of hot, dusty disks -- as hot as molten lava on Earth. Drake and his colleagues suggest that the dust was kicked up by planets smashing into each other. Additional detections of RS CVns have convinced the astronomers that what they're seeing is not a fluke.
"These kinds of systems paint a picture of the late stages in the lives of planetary systems," said Marc Kuchner, a co-author from NASA's Goddard Space Flight Center. "And it's a future that's messy and violent."
Plenitude of planets
And now for the extrasolar extravagance: The discovery of as many as seven planets, announced today by European astronomers, could represent a record for the number of worlds detected in orbit around one alien star. But the orbits for two of those seven still have to be confirmed. The biggest challenge is to nail down the detection of a close-orbiting, rocky world that may represent the smallest known extrasolar planet (potentially only 1.4 times as massive as Earth).
"This is the planet that has the highest uncertainty, if you like, and in order to conclude on it, we have to take several measurements on the same night," Christophe Lovis of Geneva University, the study's lead author, told me today.
One of the weirdest things about the HD 10180 planetary system is that the supposed super-Earth and five Neptune-scale planets are all orbiting within what would be Mars' orbit in our own solar system. How could so many big planets exist in that space without getting in each other's way, as the pulverized planets did?
"They cannot form where they are now," Lovis said. "You have to imagine a situation where they all migrate together, harmonically."
He suggested that a large number of protoplanets built up around the parent star, and "through ejections and accretions on each other, the number of bodies was reduced until a stable state existed."
Is this system really stable, though, or are HD 10180's planets still in flux? "We made an extensive dynamical study in the paper," Lovis said. "It is stable, provided we really integrate all the ingredients into the model."
That includes the gravitational attraction exerted by the star, the mutual gravitational influences of all the planets on each other, and tidal forces. Lovis said the calculations even include the effects of general relativity, "which are usually small, but in this case we have to include it in the model."
Another weird angle is that if you chart the planets' distances on a logarithmic scale, they fall almost precisely along a straight line, much as the inner planets of our own solar system do. Back in the 18th century, this logarithmic arrangement was considered a law of physics, known as the Titius-Bode Law. The formula fit neatly with the discoveries of Uranus and the dwarf planet Ceres ... but it failed to predict the orbit of Neptune, which was discovered in 1846.
Lovis said the idea behind the Titius-Bode Law might merit a second look, based on the arrangement of HD 10180 and several other extrasolar multi-planet systems. "I'm not claiming that this is something fundamental," he told me. "If you assume some kind of criterion that will assure stability, then you end up very often with some kind of Titius-Bode Law. ... I think it's an interesting thing to investigate."
More interesting things could come out on Thursday, when researchers on NASA's Kepler mission team are to announce the discovery of an "intriguing planetary system." Further details are embargoed until then, but be assured that we'll have the latest smashing news right here.
In addition to Drake and Kuchner, the researchers behind the Spitzer study include first author Marco Matranga, V.L. Kashyap and Massimo Marengo. In addition to Lovis, the researchers behind the HD 10180 study include D. Segransan, M. Mayor, S. Udry, W. Benz, J.-L. Bertaux, F. Bouchy, A.C.M. Correia, J. Laskar, G. Lo Curto, C. Mordasini, F. Pepe, D. Queloz and N.C. Santos.