JACKSON LAKE, WYOMING—In the early days of our solar system, rocky bodies collided, asteroids and comets whizzed past one another, and giant planets slowly drifted toward or away from the sun. But if this sounds like an extreme display of cosmic chaos, think again. Compared with planetary systems elsewhere in the Milky Way galaxy, our system was relatively calm. And that's one reason astronomers may have difficulty recognizing Earth-like planets in the universe.
That was the gist of a panel discussion held here last night at the Extreme Solar Systems II conference, where astronomers have been presenting their latest results on the study of planets beyond our own solar system. Astronomer Alessandro Morbidelli of the Nice Observatory in France explained how computer simulations reveal the intrinsically chaotic nature of planetary systems that contain massive, Jupiter-like planets. "Habitable Earth-like planets are very vulnerable if there are giants in the system," he said. The giant planets tend to migrate through the system, flinging small planets into their parent stars or into interstellar space in the process. "Solar systems like ours may be extremely rare cases."
Slowly but surely, astronomers are coming to grips with the chaotic evolution of solar systems. Giant planets frequently end up close to their star because of friction with the remains of the planet-spawning disk of gas and dust surrounding the star, or as a result of gravitational perturbations from another massive planet. According to calculations presented at the meeting today by theoretical astronomer Smadar Naoz of Northwestern University in Evanston, Illinois, such "jumping Jupiters" may end up in strongly skewed or even "retrograde" (backward) orbits, wreaking gravitational havoc in the inner parts of their systems.
As a result of tidal effects, strong irradiation from their host stars, and energy dumped in their interior by electrical currents that result from their high temperatures, the giant planets subsequently swell and start to lose their outer layers into space. Many bloated and even evaporating planets have been found, and although no one understands the precise details of this planetary inflation, "some small, low-mass planets in tight orbits may actually be the remains of evaporated hot Jupiters," Konstantin Batygin of the California Institute of Technology in Pasadena said at the conference today.
And that's where the problem lies for planet hunters who want to catch a true analog of Earth. Telescopes on the ground and NASA's prolific Kepler space telescope have detected many "super-Earths"—worlds just a few times larger or more massive than our own—orbiting close to their parent stars. Some of these have just the right temperature for liquid water to exist on their surface, which would make them potential habitats for life. But if they're gaseous instead of rocky, they don't have a surface at all, and they may be very different from our home planet, Morbidelli says.
More data could provide the answer to the true nature of small exoplanets. For example, if astronomers know both a planet's size and its mass, they can calculate the density, which provides information on the composition. During last night's roundtable, Kepler principal investigator William Borucki of NASA's Ames Research Center in Moffett Field, California, stressed that "we need to know if they're rocky, and we need to do our job so well that we might convince NASA to build a future flagship mission to study the atmospheres of these planets," in search of chemical evidence for biological activity. However, Borucki admits that the measurements "will be difficult and need a lot of work."
So is our solar system really an exceptional place, or are habitable, Earth-like planets very numerous? Kepler co-investigator Natalie Batalha, also from Ames, is confident that astronomers will eventually be able to find out. "We know what to do to answer that question," she says. "We're limited not by science or technology, but by economics."