OGLE-2005-BLG-390Lb

Although the title may look like gibberish it is actually the catalogue name of the smallest mass extrasolar planet found to date; it is just 5.5 times the mass of the Earth and orbits its star at a distance 2.6AU. Each orbit lasts roughly 3800 days - much longer than it would be in our solar system - because its parent star is only about a fifth the mass of the Sun.

There are several techniques used for finding planets around other stars. The most obvious is to take an image of one. That is incredibly difficult firstly because the system is likely to be too far away to distinguish the planet and parent star and secondly because the parent star is so much brighter. It can be like looking for a glow worm next to a sport stadium's floodlight. A second technique finds planets by looking for the tiny wobble they impart on their star as the planet completes its orbit. This doppler method is easier to detect if the planet is larger and close to its star. A third method requires the plane of the planet's orbit to be lined up with the Earth; every time the planet passes between us and the star the overall amount of light is reduced. This is called the transit method and should be familiar to you if you saw the transit of Venus back in 2004.

OGLE-2005-BLG-390Lb was found using a totally different method; gravitational microlensing. This method relies on the star and planet passing directly infront of a distant bright object. As the closer star passes in front of a distant star it acts as a gravitational lens and magnifies the light of the further object. If you are monitoring the distant object you can plot a light curve - how the brightness varies with time - and you see a shape that looks a bit like a child's drawing of a mountain (see below). When the lensing star has a planet orbiting it, the planet can also add to this effect as it passes by. The light curve below shows the planet as the little 'blip' on the right-hand side of the 'mountain'. Measuring the size of the blip tells you about the mass of the planet that caused it.

OGLE-05-390L b
The light curve of a planet 5.5 times the mass of the Earth CREDIT: PLANET, OGLE


This is exciting because it allows us to detect much smaller planets than is currently possible with the other techniques. The only trouble is that microlensing events are rare as they rely on a planetary system passing in front of another star. They are also one off events, so follow-up may have to wait for future space-based missions. Still, gravitational microlensing events such as this may find the first Earth-mass planet.

Posted in astro blog by Stuart on Wednesday 25th Jan 2006 (22:37 UTC) | Permalink
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