Boston University Astronomers Help Discover Extrasolar Planet

An international team of professional and amateur astronomers, using simple off-the-shelf equipment to trawl the skies for planets outside our solar system, has hauled in its first “catch.”

The astronomers discovered a Jupiter-sized planet orbiting a Sun-like star 600 light-years from Earth in the constellation Corona Borealis. The team, led by Peter McCullough of the Space Telescope Science Institute in Baltimore, includes researchers and students from Boston University as well as four amateur astronomers from North America and Europe.

The study, which has been accepted for publication in the Astrophysical Journal, is co-authored by Dr. Kenneth Janes, an astronomy professor at BU, April Pinnick and Paul Howell, BU graduate students, and other members of the team. Several BU undergrads also assisted with the project.

The finding suggests that using modest telescopes to search for extrasolar planets allows for a productive collaboration between professional and amateur astronomers that could accelerate the planet quest.

McCullough deployed a relatively inexpensive telescope made from commercial equipment to scan the skies for extrasolar planets. Called the XO telescope, it consists of two 200-millimeter telephoto camera lenses and looks like a pair of binoculars. The telescope is on the summit of the Haleakala volcano, in Hawaii.

The team found the planet, dubbed XO-1b,by noticing slight dips in the star’s light output when the planet passed in front of it, called a transit. The light from the star, called XO-1, dips by approximately 2 percent when XO-1b passes in front of it. The observation also revealed that XO-1b is in a tight four-day orbit around its parent star.

“Using data from a telescope in Flagstaff, Arizona that BU operates in partnership with Lowell Observatory, we helped to confirm for Dr. McCullough that XO-1b really is in fact a planet as well as quantify its size,” said Janes. “By looking at how much the star’s brightness decreases as it is blocked by the planet the team was able to determine that XO-1b is somewhat larger in diameter than Jupiter.”

Although astronomers have detected more than 180 extrasolar planets, XO-1b is only the tenth planet discovered using the transit method. It is the second planet found using telephoto lenses. The first, dubbed TrES-1, was reported in 2004. The transit method allows astronomers to determine a planet’s mass and size. Astronomers use this information to deduce the other characteristics, such as density.

The team used the Harlan J. Smith Telescope and the Hobby-Eberly Telescope at the University of Texas’ McDonald Observatory to measure the slight wobble induced by the planet on its parent star. This so-called radial-velocity method allowed the team to calculate a precise mass for the planet, which is slightly less than that of Jupiter (about 0.9 Jupiter masses). The planet also is much larger than its mass would suggest.

The planet-finding technique involves nightly sweeps of the sky using the XO telescope in Hawaii to note the brightness of the stars it encounters. A computer software program, a portion of which was developed by Janes, sifts through many thousands of stars every two months looking for tiny dips in the stars’ light, the signature of a possible planetary transit. The computer comes up with a few hundred possibilities. From those candidates, the team selects a few dozen promising leads. These stars were then passed on to the four amateur astronomers to study the possible transits more carefully.

From September 2003 to September 2005, the XO telescope observed tens of thousands of bright stars. In that time, the team of amateur astronomers studied a few dozen promising candidate stars identified by the team. The star XO-1 was pegged as a promising candidate in June 2005. The amateur astronomers observed it in June and July 2005, confirming that a planet-sized object was eclipsing the star.

The astronomer’s innovative technique of using relatively inexpensive telescopes to look for eclipsing planets favors finding planets orbiting close to their parent stars. The planet also must be large enough to produce a measurable dip in starlight.

The planet is the first discovered through the three-year search for transiting extrasolar planets. The planet quest is underwritten by a grant from NASA’s Origins program.

“It was a wonderful feeling because the team had worked for three years to find this one planet,” McCullough explained. “The discovery represents a few bytes out of nearly a terabyte of data: It’s like trying to distill gold out of seawater.”

“It truly was a collaborative effort,” agreed Janes. “It was an exciting opportunity to work with both professional and amateur astronomers on a team that can say they discovered a planet – something that not many people can say they’ve done.”

The team believes the newly found planet is a perfect candidate for study by the Hubble and Spitzer space telescopes. Hubble can measure precisely the star’s distance and the planet’s size. Spitzer can actually see the infrared radiation from the planet. By timing the disappearance of the planet behind the star, Spitzer also can measure the “ellipticity,” or “out-of-roundness,” of the planet’s orbit. If the orbit is elliptical, then the varying gravitational force would result in extra heating of the planet, expanding its atmosphere and perhaps explaining why the XO-1b’s diameter seems especially large for a body of its calculated mass.

“By timing the planet’s passages across the star, both amateur and professional astronomers might be lucky enough to detect the presence of another planet in the XO-1 system by its gravitational tugs on XO-1b,” McCullough said. “It’s even possible that such a planet could be similar to Earth.”

The Space Telescope Science Institute in Baltimore is operated for NASA by the Association of Universities for Research in Astronomy, Inc., Washington.

Founded in 1839, Boston University is an internationally recognized institution of higher education and research. With more than 30,000 students, it is the fourth largest independent university in the United States. BU contains 17 colleges and schools along with a number of multi-disciplinary centers and institutes which are central to the school’s research and teaching mission.