Planet Hunters on Safari
Of the 30 extrasolar planets around Sun-like stars detected previously, all have been the size of Jupiter or larger. The existence of these Saturn-sized candidates suggests that many stars harbor smaller planets in addition to the Jupiter-sized ones.
Above: This is an artist's concept of a giant planet recently discovered orbiting the sun-like star 79 Ceti, located 117 light-years away in the constellation Cetus the sea monster. The planet was not directly photographed but indirectly detected by its gravitational pull on the star. The planet is in an elliptical orbit about the star, which carries it closer to the star than Mercury is to our sun. Credit: Greg Bacon. [more information from the Hubble Space Telescope Science Institute]
Finding Saturn-sized planets reinforces the theory that planets form by a snowball effect of growth from small ones to large, in a star-encircling dust disk. The 20-year-old theory predicts there should be more smaller planets than large planets, and this is a trend the researchers are beginning to see in their data.
"Now we are confident we are seeing a distinctly different population of bodies that formed out of dust disks like the disks Hubble Space Telescope has imaged around stars," said Marcy.
These planets are very close to their stars and so have short orbits. They whirl around their parent stars with periods of 3.02 days and 75 days respectively. This allowed for their relatively rapid discovery.
|Mass of planet (Jupiter = mass of 1)|
|Number of Planets||10||7||5||2||3||1||2||1||0||1||0|
The astronomers detected the small wobble of a star caused by the gravitational tug of the unseen planets. For the past five years Marcy and Butler have used this technique successfully to catalog 21 extrasolar planets. Boosted by the light-gathering power of Keck, they have steadily increased the precision of their measurements so they can look for the gravitational effects of ever-smaller bodies. In this latest detection, the change in a star's velocity -- rhythmically moving toward and then away from Earth -- is only 36 feet per second, a little faster than a human sprints.
The Saturn-mass planets are presumably gas giants, made mostly of primordial hydrogen and helium, rather than the rocky material Earth is made of. They are so close to their parent stars they are extremely hot, and are not abodes for life as we know it. The planet orbiting 79 Ceti has an average temperature of 1530 degrees Fahrenheit (830 degrees Celsius). The planet orbiting HD46375 has an average temperature of 2070 degrees Fahrenheit (1130 degrees Celsius).
Right: Diagram shows the orbit of planet of HD 46375 as compared with the orbits of Mercury and Venus. The orbit of this planet is very close to its star - approximately 1/5th of the diameter of Mercury's orbit around our Sun.
Their research is part of a multi-year project to look for wobbles among 1,100 stars within 300 light-years of Earth. The project is supported by NASA and the National Science Foundation.Web Links
Planet Hunters on Trail of Worlds Smaller than Saturn - from the Hubble Space Telescope Science Institute.
More related links - about planet hunting.