Tuesday, May 24, 2005

New planet discovery!

Planets are being discovered around other stars fairly commonly these days, so it is rare that a single new planet is quite interesting.

So what is interesting about the new planet, a Jupiter-sized planet around the unpronounceable star OGLE-2005-BLG-071? It is the method by which the planet was detected.

Most planets have been discovered by measuring the movement of their parent star. A planet's gravity causes the parent star to move slightly -- in the case of Jupiter's orbit around the sun, the sun moves a distance about equal to its own diameter every 11 years. It's amazing that this motion can be detected, but this is the most efficient way of discovering planets, with over 140 announced planets!

Another method of discovering planets is to look for the change in light from a star when a planet passes in front of the star, blocking out some of the star's light. This method is hard, because the total change in the light from a star is only about 1%. Even worse, it takes special geometry for these transits to happen, so only a small fraction of stars with planets will show transits. Even so, a handful of planets have been discovered this way.

Of course, you can always try and take direct pictures of a planet, but this is hard, because planets are over ten thousand times fainter than the stars they orbit! Imagine trying to take a picture of a flashlight located a few inches away from a spotlight -- it's hard! A few potential planets have been found this way, but they have yet to be confirmed.

The planet in OGLE-2005-BLG-071 was discovered by a totally different method, gravitational micro-lensing. Several hundred times a year, a star in our galaxy wanders directly in front of a much more distant star as seen from the Earth. The gravity from the star in front acts like a lens and magnifies the background star, making the background star appear brighter. These "microlensing events" have been observed for years and are well-understood.

Earlier this year, one of the searches for these microlenses, the OGLE survey, detected a star getting brighter in the manner that suggested a microlensing event was occurring. The star was monitored closely. But instead of a "normal" event, where the background star steadily gets brighter, reaches a peak, and then fades, this star showed two separate peaks in brightness separated by three days. When the astronomers calculated what type of star system was needed to explain this light curve, they figured out that there was a parent star and a companion planet with a mass 0.7% that of the star. As a comparison, Jupiter is 0.01% the mass of the sun. However, since we do not see the parent star, only its gravitational effects on the background star, the parent star is probably low mass, perhaps as low as one-tenth the mass of the sun, meaning that this planet could be as small as Jupiter, and at most several times the mass of Jupiter. This compares very well with the masses of planets discovered by other methods.

So, this planet confirms that it is possible to discover planets by yet another method, gravitational lensing. The discoverers are continuing to analyze their data to try and learn more about the planet, like its exact mass, its distance from the parent star, and the distance from us to the planet.

1 comment:

  1. I just attended a lecture by Scott Tremaine FRS, FRSC from the Institute for Advanced Study, Princeton University, here in Toronto at the Koffler Institute as part of the Canadian Institite for Theroretical Astrophysics and he went through the methods you describe thourougly. It was facinating to see the slides and hear these concepts. It sure does make me wonder how many planets we have yet to discover over the next few lifetimes.

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