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To Find a Transiting Planet

Anyone involved in exoplanetary science shares a common dream: a view of a blue and green world returned from an advanced imaging system of the sort that may one day fly aboard Terrestrial Planet Finder or other missions. But as we wait for breakthroughs in space-based hardware, planetary detections keep occurring. And astronomer Greg Laughlin (University of California, Santa Cruz) has a thought on what we might find using today’s technologies.

Laughlin notes that that nine extrasolar planets are known to transit their parent stars (i.e., they pass in front of the star as seen from Earth). “It would be nice to find a transiting planet with a longer period,” he adds. “Preferably, this would be a giant planet with towering thunderstorms and warm, drenching rains, and orbited by a habitable Earth-sized moon that we could detect with HST photometry.”

Nice indeed, for that moon would be our first candidate for a life-bearing world in the terrestrial mode, a real coup for transit studies. Laughlin’s comment appears on the Systemic site, which is devoted to refining the tools of exoplanetary detection through the use of the Systemic Console, a java-based tool that allows users to manipulate planetary information. Now the Systemic team has added a new dimension to this work with Transitsearch.org, a distributed effort that pools the observations of amateur telescopes around the world to discover transiting exoplanets.

A bit of background: most planets thus far discovered have been found using the Doppler radial velocity technique. We do know of nine planets that transit their parent stars, but all nine have orbital periods shorter than five days. In other words, they are orbiting so close to their primary that they offer no serious hope for exobiology. The radial velocity method itself does not reveal the orbital inclination of the planet as seen from Earth, so there is some possibility, for each new exoplanet discovered, that it may make a transit. Short period planets have relatively high transit probabilities, says Laughlin, while long-period ones have much lower odds.

And while finding that ‘habitable zone Jupiter’ may be a long-term goal, Transitsearch hopes to accomplish much in the interim. The team intends to work through the list of candidates in hopes of finding a new transiting planet, using participating observers worldwide to study a single candidate star at a time. Finding the best candidate stars to examine for a transit is an involved process, as Laughlin describes:

…as I scroll down the target list in search of a transit candidate for an observational campaign, I look for a number of attributes. First, the prospective transiting planet needs to be adequately “in play”. That is, the star needs to have a window of phase coverage that has not been adequately checked for transits. In general, all of the planets with periods less than 10 days have been fairly well picked over. This is because the a-priori geometric transit probability increases dramatically as the planetary period decreases, and because it is much easier to definitively rule out or confirm transits for a short-period planet than for a longer-period planet. Exciting exceptions do, however, arise from time to time. For example, I realized one afternoon last summer that HD 188753A (P=3.35 d) had apparently not been checked for transits. Also, the red dwarf, GL 581, with a 5.3 day period has not yet been checked (and is hence currently being worked up for a transitsearch campaign).

Coming up in March: a transit window of several days for the star HD 99492, a K-class dwarf with roughly 80 percent the mass of the Sun. The planet orbiting this star has a period of 17.04 days and a mass about 1/10th that of Jupiter. If this planet does make a transit, it should be detectable.

If you own a telescope equipped with a CCD detector and want to get involved, be sure to follow updates on both Systemic and Transitsearch.org (and remember that those without telescopes can still use the Systemic Console to make a meaningful contribution to exoplanetary research). What Transitsearch.org finds will be confirmed by the UC-Carnegie Planet Search Program, and the publication of discoveries will credit the contributing members of the Transitsearch network as co-authors.

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