TSU astronomer contributes 11 years of data

(November 6, 2007, Nashville, TN)--The National Aeronautics and Space Administration (NASA) announced today that astronomers have discovered a fifth planet circling the star 55 Cancri, leading scientists closer to the possibility of finding Earth-like planets in other solar systems. The new planet lies within the star's "habitable zone", meaning that it is at the right distance from the star for water to exist in liquid form.

55 Cancri is 41 light-years away in the constellation Cancer. Nearly the same mass and age as our Sun, it is easily visible with binoculars. TSU Astronomer Greg Henry contributed 11 years of data to the team's research. They were led by Debra Fischer, an astronomer at San Francisco State University and lead author of a paper soon to be published in the Astrophysical Journal (reprint). Other scientists on the team were from University of California-Berkeley, University of California-Santa Cruz, and Carnegie Institution of Washington.

"This is the first quintuple-planet system," Fischer said. "The system has a dominant gas giant planet in an orbit similar to our Jupiter. Like the planets in our own solar system, most of these planets reside in nearly circular orbits."

The newly discovered planet (shown here in a NASA/JPL artist's concept) weighs in at about 45 times the mass of Earth and may be similar to Saturn in its composition and appearance. The planet is fourth out from its "sun" and completes one orbit every 260 days. Because it is in the habitable zone, its temperatures would permit liquid water to pool on solid surfaces. It is slightly closer to its star (116.7 million kilometers or 72.5 million miles) than Earth is to the Sun, but it orbits a star that is slightly fainter than our Sun.

"Discovering these five planets took us 18 years of continuous observations at Lick Observatory, on Mt. Hamilton, east of San Jose, Calif., that were initiated before any extrasolar planets were known anywhere in the universe," said Geoff Marcy, astronomer at University of California-Berkeley and a contributor to the paper. "But finding five extrasolar planets orbiting a star is only one small step. Earth-like planets are the next destination."

The fifth planet's discovery was made using the Doppler technique, in which a planet's gravitational tug is detected by the wobble it produces in the parent star. The Doppler measurements were made possible with a spectrometer built by Steve Vogt of the University of California-Santa Cruz and with analysis techniques developed by Paul Butler of the Carnegie Institution of Washington. Fischer, Marcy, and their team of collaborators including Greg Henry of Tennessee State University, discovered their "mini-Saturn" after careful observation of 2,000 nearby stars with the Shane telescope at Lick Observatory and the W.M. Keck Observatory in Mauna Kea, Hawaii. More than 320 velocity measurements were required to disentangle signals from each of the planets.

TSU astronomer Greg Henry provided confirmation of the reality of all five planets from 11 years of precise brightness measurements of the star he made with TSU's robotic telescopes at Fairborn Observatory in southern Arizona (shown here). "These observations establish an extreme level of brightness stability in 55 Cnc," said Henry. "The fact that the star is so constant in its energy output means that other potential causes for the observed stellar wobble, such as pulsations in the star's outer atmosphere or dark starspots carried around the star by its rotation, can be ruled out," explained Henry. "This is good evidence that these planets are real."

The planet population around 55 Cancri is somewhat different from those orbiting the Sun. The inner-most planet is believed to be about the size of Neptune and whips around the star in just under three days at a distance of approximately 5.6 million kilometers (3.5 million miles). The second planet out from the star is a little smaller than Jupiter and completes one orbit every 14.7 days at a distance of approximately 17.9 million kilometers (11.2 million miles). The third planet out from the star, the Saturn-mass one, completes one orbit every 44 days at a distance of approximately 35.9 million kilometers (22.3 million miles). The fourth planet out is the newly discovered planet. The fifth and most distant known planet is four times the mass of Jupiter and completes one orbit every 14 years at a distance of approximately 867.7 million kilometers (539.1 million miles). It is still the only known Jupiter-like gas giant to reside as far away from its star as our own Jupiter.

"The gas-giant planets in our solar system all have large moons," said Fischer. "If there is a moon orbiting this new, massive planet, it might have pools of liquid water on a rocky surface. That would be a fascinating environment to explore from the perspective of biochemistry."

"Intriguingly, the planetary system has a huge, mysterious region between the fourth and fifth planets where no giant planet exists," said Marcy. "Perhaps that region contains rocky, Earth-like planets that our current technology cannot detect."

Funding for this study was provided by NASA and the National Science Foundation.