A transiting exoplanet

Amateur astronomers around the world will attempt to capture the transits of two exoplanets. If either planet follows an orbit that is nearly edge on to our line of sight, it will block about 1 percent of its star's light. Advanced amateurs can detect this dip with CCD cameras.

S&T: Steven A. Simpson.

This holiday season, advanced amateur astronomers can present the astronomical community with an extraordinary gift: the detection of a new transiting exoplanet.

Since 2000, amateur astronomers have used CCD photometry to detect planets transiting other stars. But so far, professionals have discovered all six known transiting exoplanets. This time will be different. We are recruiting amateurs to discover rather than confirm a new transit.

Between December 26, 2004, and January 3, 2005, observers around the world will look for exoplanets transiting two stars: 8.7-magnitude HD 37605 in Orion and 7.6-magnitude HD 74156 in Hydra. The stars are observable from both Hemispheres. Amateurs can detect these transits by looking for dips in the amount of starlight as the planet moves across the star's disk as seen from Earth. Observers will need a CCD camera and the ability to obtain photometry accurate to 0.01 magnitude or better.

Transitsearch.org and the American Association of Variable Star Observers (AAVSO) are coordinating a global campaign to observe predicted transit windows for HD 37605 and HD 74156. Click on the links above for more information, including observing techniques, coordinates, and sky charts.

Professional astronomers detected the presence of both planets by observing their star's spectra. HD 37605b's mass is at least 2.9 times that of Jupiter, and HD 74156b has an estimated mass of about 1.9 Jupiters. Each planet orbits close to its parent star, with a period of 54 and 52 days, respectively. These periods are shorter than Mercury's around the Sun, but they are more than 10 times longer than the periods of the six known transiting exoplanets, which belong to the class known as "hot Jupiters" because of their close proximity to their host stars.

HD 37605b and HD 74156b are of considerable interest because they fall between the hot Jupiters and frigid gas giants like our own Jupiter and Saturn. Detecting transits of either planet will allow astronomers to study important properties of an entirely new class of planet, including mass, size, density, and atmospheric composition. Transit observations could even yield evidence for rings and moons. If we get lucky, a transit detection would have an enormous scientific payoff.

Transit probabilities

The total probability that HD 37605 will transit its host star is 2.2 percent, with the most likely time coming around December 29, 2004. HD 74156 has a 3.8 percent probability of transiting its star, with the most likely time coming on January 1, 2005. This histogram shows the relative transit probability for each planet. The uncertainties in each planet's orbit leads to the range of dates, which necessitates continuous coverage of the stars.

Source: Gregory Laughlin and Aaron Price.

The long orbital periods mean long transit windows and relatively low transit probabilities: 3.8 percent for HD 74156 and 2.2 percent for HD 37605. But this is exactly why we need amateurs for the project. We don't know exactly when the transits might occur, so we need as much global coverage as possible. Very few professionals have the necessary resources to observe the entire transit windows. Even if we don't detect a transit, good coverage of these windows is scientifically valuable because it will help constrain current orbital models of the systems.

The first amateur astronomers to discover a transiting exoplanet will be listed as coauthors on the discovery paper, which will be published in a professional journal. They will also dramatically highlight the emergence of modern amateurs as critical partners in groundbreaking research.

Gregory Laughlin is an astrophysicist at the University of California, Santa Cruz. Aaron Price is an Astronomical Technical Assistant for the American Association of Variable Star Observers.


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