SINCE the discovery of the first exoplanets, planets orbiting stars outside of our solar system, astronomers have been looking for planetary systems similar to our own. Most exoplanets are far larger than Earth, and many orbit in wild, eccentric orbits quite unlike the relative order found in the solar system. Now, according to a report from MIT, a team led by a physics graduate student at MIT has found an extrasolar planetary system which has three planets orbiting their parent star in a similar fashion to the solar system. The team, led by Roberto Sanchis-Ojeda, used data from NASA's orbiting Kepler space telescope to find their evidence. Kepler finds evidence for planets orbiting distant stars by measuring dips in light from the stars, which happen when a planet passes in front of the star. By measuring the timing and frequency of these dips, astronomers can get an idea of the distance and orientation of an exoplanet's orbit. The star studied by Sanchis-Ojeda's team, Kepler-30, is known to have three planets, all orbiting in regular, fairly circular orbits around their star. However, Kepler-30 also has another feature: large spots, like the sunspots that occur on our own Sun, that frequently cross the face of the star. The team used these spots to deduce the plane of Kepler-30's rotation. When a planet orbited in front of Kepler-30, the star's light dimmed. When the planet passed in front of a spot, the light dimmed even further. By measuring the timing of the extra dimming, the team was able to plot the rotation of the star itself—leaving them knowing both the plane of Kepler-30's spin, and the plane in which its planets orbited. “In our solar system, the trajectory of the planets is parallel to the rotation of the sun, which shows they probably formed from a spinning disc,” says Sanchis-Ojeda, “In this system, we show that the same thing happens.” His co-author, Josh Winn, explains the significance, “It's telling me that the solar system isn't some fluke. The fact that the sun's rotation is lined up with the planets' orbits, that's probably not some freak coincidence.” Astronomer's hypothesize that planets form from the same rotating disk of material that their parent star forms from. In this case, planets should orbit in roughly the same plane as their star rotates. The rotation of the stars is unknown, however, and in many systems, large planets, such as “hot Jupiters” are known to orbit erratically. This new study shows that, in a case where orbits and rotation can be measured, they match, showing that the understanding of planetary formation is basically correct. The team expects that an erratic orbit is a result of orbital interactions after the planet's formations, not a result of their formation itself. Winn says, “We've been hungry for one like this, where it's not exactly like the solar system, but at least it's more normal, where the planets and the star are aligned with each other. It's the first case where we can say that, besides the solar system.” — Agencies
