Transit Window Soon to Open for GL 581

by Paul Gilster on March 24, 2006

If the goal is to find terrestrial planets around nearby stars, the transit method is our best bet. Sure, microlensing can deliver powerful results, and is fully capable, we believe, of finding a small, rocky world around a distant star. But microlensing as currently used is limited to stars that are tens of thousands of light years from Earth. In other words, find a terrestrial planet with microlensing and you can’t do much by way of follow-up study.

But transit methods are different. If a star’s system of planets is oriented so that the planets cross in front of the star as seen from Earth, it is possible not only to find the planets but to do spectroscopic analysis and learn something of their composition. All that makes Transitsearch.org an exciting thing to be a part of. As discussed earlier in these pages, it’s a cooperative project that gets amateur astronomers and smaller observatories into the transit hunt, supplying dates and times when transits are thought to occur.

Greg Laughlin (University of California, Santa Cruz), who heads the Transitsearch collaboration, now reminds participants that we have a transit opportunity coming up on March 28. The star is the M-class red dwarf GL 581, which is known to have a Neptune-class planet on a tight 5-day orbit. Laughlin reports that the planet has not been checked for transits, and is thereby a candidate for scrutiny as the observing window opens. Observing a transit of this planet would be a major step forward for exoplanet studies, and would help us refine the tools that will eventually find that Earth-like world.

Here are the coordinates as given by Laughlin:

ICRS 2000.0
RA 15 19 26.8250 DEC -07 43 20.209

Predicted transit midpoint: JD 2453822.79 (2006 Mar 28 06:59 UT)
Predicted central transit duration: 88 minutes.
Predicted transit depth: 1.6%
A-priori transit probability: 3.6%

The full ephemeris table is found on the candidates page at Transitsearch.org site. If you are an amateur astronomer with CCD capability, give serious thought to getting involved in Transitsearch. The effort concentrates on planet-bearing stars with high probabilities of displaying transits, and it needs coverage on a global basis for best results. It’s a remarkable fact that transiting worlds can be observed using commercial telescopes with CCD detectors — what an upgrade for amateur astronomy since the days when, as a star-struck teenager, I used to lug my little 3-inch reflector out into the yard for 1960′s era astronomy!