Searches for planets outside our own solar system (exoplanets) have racked up over one hundred discoveries since the first one a little more than a decade ago. Almost all of those have been discovered using indirect methods such as measuring the spectroscopic radial velocity (RV) shift of a star due to the gravitational influence of an unseen planet. Only a few have been discovered using the transit method where the presence of a planet is revealed in a slight dimming of the star's light. While it is less likely to find a transiting planet in any given sample of stars, the reward is potentially greater.
Once a transiting planet has been found, additional data on radial velocity of the star can be obtained as well as spectroscopic data on the star. These data yield detailed information on the planet as well as the star beyond what is possible with RV data alone.
The greatest difficulty with transit searches is the low density of candidates, and the rich number of ways transit imposters can sneak in.
The low density problem can be solved fairly easily by using a modest survey telescope. Such a scope can be dedicated to searching the sky for stars that betray the presence of a transiting planet by periodically getting get a little dimmer for a short period of time. Software can do a lot to cull out many of the fakers - variable stars, eclipsing binary star systems for example. But inevitably there will be a large number of possible candidates that simply must be observed more carefully to find the real transits.
Making a case to get precious big telescope time to see if any of a few hundred stars might have a transiting planet is a tough sell to most time allocation committees (TACs).
Except if you own the telescope outright, and it is easy to get to and it is equipped with a sensitive CCD. Many amateur astronomers have just such equipment sitting in their backyard, no TAC in sight.
A few years ago, Dr. Peter McCullough had the vision to put those backyard telescopes to work sifting star-exoplanet candidates. He built and deployed a modest telescope to gather the survey data, and with several colleagues assembled the needed sifting software. And then he recruited the first four amateurs to do the second stage work.
Dr. McCullough's program is called the XO Project, and I am proud to be a member. I am even prouder to announce that the team has bagged its first exoplanet, a Jupiter sized world orbiting a star quite like our own Sun.
Paper submitted to the Astrophysical Journal.
Very high resolution (64MB native, 4.3MB compressed) image of Corona Borealis containing XO-1.
Donna Weaver Space Telescope science Institute, Baltimore, Md.
(Phone: 410-338-4493; E-mail: dweaver@stsci.edu )
Peter McCullough Space Telescope Science Institute, Baltimore, Md.
(Phone: 410-338-5068; E-mail: pmcc@stsci.edu )
PRESS RELEASE NO.: STScI-PR06-22
ASTRONOMERS USE INNOVATIVE TECHNIQUE TO FIND 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, Md., includes four amateur astronomers from North America and Europe.
Using modest telescopes to search for extrasolar planets allows for a productive collaboration between professional and amateur astronomers that could accelerate the planet quest.
"This discovery suggests that a fleet of modest telescopes and the help of amateur astronomers can search for transiting extrasolar planets many times faster than we are now," McCullough said. The finding has been accepted for publication in the Astrophysical Journal.
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.
"To replicate the XO prototype telescope would cost $60,000," McCullough explained. "We have spent far more than that on software, in particular on designing and operating the system and extracting this planet from the data."
McCullough's team found the planet, dubbed X0-1b, by noticing slight dips in the star's light output when the planet passed in front of the star, called a transit. The light from the star, called XO-1, dips by approximately 2 percent when the planet XO-1b passes in front of it. The observation also revealed that X0-1b is in a tight four-day orbit around its parent star.
Although astronomers have detected more than 180 extrasolar planets, X0-1b 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 planet's characteristics, such as its density.
The team confirmed the planet's existence by using the Harlan J. Smith Telescope and the Hobby-Eberly Telescope at the University of Texas?s 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. "Of the planets that pass in front of their stars, XO-1b is the most similar to Jupiter yet known, and the star XO-1 is the most similar to the Sun," McCullough said, although he was quick to add, "but XO-1b is much, much closer to its star than Jupiter is to the Sun."
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 in McCullough's three-year search for transiting extrasolar planets. The planet quest is underwritten by a grant from NASA's Origins program.
McCullough's 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 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, McCullough and his team select a few dozen promising leads. He passes these stars 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, his team of amateur astronomers studied a few dozen promising candidate stars identified by McCullough and his team. The star X0-1 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. McCullough's team then turned to the McDonald Observatory in Texas to obtain the object's mass and verify it as a planet. He received the news of the telescope's observation at 12:06 a.m. Feb. 16, 2006, from Chris Johns-Krull, a friend and colleague at Rice University.
"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." The discovery also has special familial significance for the astronomer. "My father's mentor was Harlan J. Smith, the man whose ambition and hard work produced the telescope that we used to acquire the verifying data."
McCullough 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 object'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."
Images and additional information about this research are available on the Web at: http://hubblesite.org/news/2006/22 http://mcdonaldobservatory.org/news/ http://media.rice.edu/media/Default.asp http://www.bu.edu/phpbin/news/releases/browse.php http://www.ifa.hawaii.edu/info/press-releases/
The Space Telescope Science Institute in Baltimore is operated for NASA by the Association of Universities for Research in Astronomy, Inc., Washington.
EMBARGOED UNTIL: 1:00 pm (EDT) May 18, 2006 PHOTO NO.: STScI-PRC06-22b TRAWLING FOR EXTRASOLAR PLANETS
Astronomers used this inexpensive telescope to find an extrasolar planet transiting the face of a Sun-like star 600 light-years from Earth. The telescope, which looks like a pair of binoculars, consists of two 200-millimeter telephoto camera lenses. The lenses are attached to electronic devices that measured slight dips in light output from the star, indicating that an object was crossing in front of it. The telescope is on the summit of the Haleakala volcano in Hawaii.
Research team members are P.R. McCullough, J.E. Stys, and J.A. Valenti (Space Telescope Science Institute); C.M. Johns-Krull (Rice University); K.A. Janes (Boston University); J.N. Heasley (University of Hawaii); B.A. Bye and C. Dodd (University of Illinois); S.W. Fleming (University of Florida); A. Pinnick (Boston University); R. Bissinger (Racoon Run Observatory); B.L. Gary (Hereford Arizona Observatory); P.J. Howell (Boston University); and T. Vanmunster (CBA Belgium Observatory).