May 8, 2012
Earth-like Planets May Be Scarce In Systems With Hot Jupiters
Solar systems that have “hot Jupiter-type” planets closely orbiting host stars likely do not have Earth-like planets neighboring them, according to a new study by researchers from University of Florida (UF), Fermilab, and Carnegie Institution.
“Hot Jupiters” are giant planets that are roughly the size of Jupiter but orbit more closely to their parent stars, making them much hotter than Earth or Jupiter, according to Eric Ford, researcher at UF. These planets have very short orbital periods of 10 days or less, he added.
In searching for Earth-like planets, researchers say it is helpful to look for clues and patterns that help narrow down the types of systems where potentially habitable planets are likely to be discovered.
Ford noted that Earth-like planets would likely not be found in systems that have a “hot Jupiter,” largely due to chaotic paths these giants take to end up close to their parent stars.
Publishing the findings in this week´s Proceedings of the National Academy of Sciences (PNAS), Ford and his colleagues, which include Carnegie´s Alan Boss and Fermilab Center´s Jason Steffen, narrowed down their search for Earth-like near Jupiter-like planets. Their findings indicate that the chaotic movements of hot Jupiters most likely would have disrupted the formation of smaller Earth-like planets.
The team used data from NASA´s Kepler mission to look at so-called hot Jupiters with orbital periods of around three days. If a Jupiter-like planet is discovered by a slight dimming of brightness in the star it orbits as it passes between the star and Earth, it is then possible to determine whether the hot-Jupiter has any companion planets.
“If you have a [previously] measured planet, which in this case is a hot Jupiter, you can look for deviations that result from gravitational interactions with other planets in the same system,” Steffen told Darren Osborne of abc.net.au.
Using Kepler, the team identified 63 hot Jupiter systems, of which none gave up any evidence for nearby companion planets. They also looked for stars with “warm Jupiters” -- large planets that orbit host stars every few weeks -- and “hot Neptunes” -- close orbiting mid-size planets. They detected the presence of Earth-like planets around 10 percent of the warm Jupiter systems, and 30 percent of the hot Neptune systems.
The team assumes that either there are no companion planets in hot Jupiter systems; or these companions are so small, they cannot be detected by the means used in the study. They also add that it is possible the companion planets, if they exist, may be in different orbital paths, also making them undetectable using the study methods.
Steffen said the findings confirm Ford and Boss´s theory for hot Jupiter formation, first put forward in 1995.
That theory suggests hot Jupiter planets are probably formed farther away from their host star, and the gravitational interactions with another body cause their orbits to become highly elongated. During each orbit, the hot Jupiter passes very close to the host star and then travels far away. The star raises tides on the planet, repeatedly stretching it and causing its orbit to become smaller and more circular. This process would likely to destroy other low-mass planets that originally formed between the star and the giant planet.
“The lack of nearby planets supports the theory that a close encounter with another body in the system caused the elongation of the orbit,” said Ford. “When a giant planet repeatedly passes through the inner regions of a planetary system on an elongated orbit, it would wreak great havoc on any planets that had formed there. The other planets would either fall into the star, collide with the hot Jupiter or be kicked out of the system via a gravitational slingshot.”
Steffen said the lack of smaller rocky planets alongside hot Jupiters confirms the migration theory, and it also narrows down the number of solar systems astronomers need to focus on when looking for Earth-like planets.
Ford said that, based on their findings, they now know that “less than 1 percent of stars harbor hot Jupiters.”
“Since we believe that hot Jupiters formed farther out, and then migrated inward toward their stars, the inward migration disrupted the formation of Earth-like planets,” added Boss. “If our sun had a hot Jupiter, we would not be here.”
However, some computer simulations of planet formation have indicated that terrestrial-sized planets could perhaps survive the inwards migration of giant planets in about 30 percent of cases. Other simulations predict that to make a hot Jupiter a system must sacrifice many of its worlds.
The study was supported in part by NASA via the Kepler Participating Scientist program and Hubble Fellowship grants.