"Solar Systems Common Across the Milky Way," NASA Probe Hints --Bodes Well for a Habitable Zone 'Super-Earth' Discovery
NASA's Kepler Mission astronomers are looking for environments that make complex chemistry viable --pathways to life in the Universe. Ideal planets will maintain surface temperatures in which large molecules can survive and can attain chemical concentrates that can be stable over time.
In his brilliant new study, The Life of Super-Earths, Harvard astronomer Dimitar Sasselov, observes that Mars never had moving tectonic plates and Venus appears marginally capable of moving its plates. Only Earth had active plates that promoted environmental complexity, stability, and chemical concentration.
A new study based on Kepler data says that such multiplanet discoveries similar to the Kepler Mission's announcement last week of 26 new planets in 11 star systems will become more common, because multiple-planet systems are much less likely than single candidates to turn out to be false positives. And the chances of finding a Super-Earth within the habitable zone increase exponentially.
"What we are finding is that, if you see more than one planet candidate in a system, then it's really likely that those are all real planets," said study co-author Elisabeth Adams, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts. "So, of the 170 systems that Kepler has found with multiple planet candidates"—representing a total of 408 possible planets—"probably all but one or two of the planets are real."
In the new study, to be published in an upcoming issue of the Astrophysical Journal, Adams and colleagues ran a statistical analysis and found that, if a star system has multiple planet candidates, the confirmation step can usually be skipped becuase their study shows it's very unlikely that a single star system will have multiple false positives.
On the other hand, "we know that planets form in multiple systems," Adams said. "Where there's one planet, it's much more likely that there will be other planets."
Thus, the more planet candidates a system has, the more likely that all of them are true detections.
"Having three or four or more false positives in the same system is just so much less likely than having them all be real planets," Adams said.
NASA’s Kepler Mission uses the transit method to search for planets. This method relies on the simple fact that when the Kepler spacecraft observes a star as a planet passes in front it, a tiny fraction of the starlight will be blocked—the star dims a minuscule amount. The word transit means “going” and in astronomy it means one heavenly object going in front of a larger heavenly object, like one of Jupiter’s moons going in front of (transiting) Jupiter, or a planet is going in front of (transiting) a star.
If we detect one transit, it could be a planet. But, the dip in light could be caused by other phenomena: the random changes of a variable star or starspots (sunspots on other stars). When there are repeated transits at regular times, we may have discovered a planet.
Other “false positives” must be ruled out, such as changes in brightness caused by a binary star that appears nearly in our line of sight with the target star. Earth-based observatories do follow-up work to study the target stars and eliminate phenomena that masquerade as transits.
Once a planet is discovered, we can determine the planet’s size from the drop in brightness—the “transit depth.” The orbital period of the planet is simply the time between successive transits. We use Johannes Kepler’s 3rd Law of Planetary Motion to calculate the average distance of the planet from its star from the orbital period. Knowing the planet’s distance from the star, we can estimate the planet’s surface temperature. Kepler seeks planets in the “habitable zone” of stars, the distance where liquid water can exist on the surface of the planet.
The likelihood that a multiplanet detection is real increases further if the candidates have period ratios of almost exactly 2:1, 3:2, or 4:3, said study leader Jack Lissauer, an astronomer at the NASA Ames Research Center in California. That's because such ratios could be caused by what's known as orbital resonance, when two bodies orbiting a third parent body have a periodic gravitational influence on each other, as is the case with Neptune and Pluto in our solar system.
"Planets that are in resonance tend to perturb each other in a predictable pattern," Lissauer said.
"That means there are going to be changes in their orbital periods, and that results in transit timing variations"—when a transiting object seems to slightly speed up and slow down during each periodic pass across the face of its star—a phenomenon Kepler can detect.
In each case of the multiplanet systems discovered by Kepler, the planets are tightly packed and are crammed close to their host star—which is one reason Kepler can detect the transits at all. In some, for instance, five or six planets are huddled in the same orbital space that's occupied by Mercury, Venus, and Earth in our solar system.