Low density planet found

A team of astronomers using the Hungarian Automated Telescope (HAT) network have identified what may be the least dense planet that we know of. According to their measurements, the planet - with the catalogue name HAT--P-1b - has an average density of about 0.3 that of water (at 1 atm and room temperature). That makes it less than half the density of the planet Saturn and is similar to the density of a sponge cake. That is very dilute indeed.

The HAT network (named HATNet which makes me think of old ladies and hair nets) consists of six small automated telescopes; four at the F. L. Whipple Observatory in Arizona and two at the Submillimeter Array on Hawaii. Each of these observes an 8 by 8 degree patch of sky looking for changes in brightness of the stars it is observing. Occasionally, just occasionally, one of the stars will have a dip in its brightness because a planet comes between us and the star during its orbit. (You may have been lucky enough to see the effect of a transiting planet for yourself with the transit of Venus a couple of years ago.)

ADS 16402B
Image of stars ADS 16420A and ADS 16420B taken with the F.L. Whipple Observatory 1.2m telescope. CREDIT: Bakos et al astro-ph/0609369
In this case, the star in question is one of a binary star system - ADS 16402A and ADS 16402B - in which the stars are separated by about a fortieth of a light year (1550 AU). The star that the planet orbits is about 10th magnitude and a little larger than our Sun. Interestingly, the discoverers investigations have shown it to have extremely similar properties to its binary companion making the calibration of their measurements easier.

So, roughly every four and a half days ADS 16402B gets a little dimmer as the planet passes in front of it and the team were able to measure five of these transit events during their observations. Once they had worked out the time for one orbit (4.5 days) they were able to calculate the distance of the planet is from its star. At the same time, measurements of the reduction in light from the star let them work out how big the star is (1.4 times the size of Jupiter) as larger planets block more of the light from the star. Now, to work out the density they used one last trick; they got the mass of the planet by watching how it makes the star wobble. The size of the wobble is related to the distance between the star and the planet and the mass of the planet. So you can work backwards and get the mass of the planet knowing the separation from the star and the size of the wobble from measurements of the speed of the star. Doing all that showed that the planet has a very low density. Pretty neat eh?.

But a planet as dense (or rather sparse) as a sponge cake throws up some other questions. Why is it so puffed up? Could it be because it is heated up by its star as it is pretty close to it after all? Well, the paper's authors don't reckon so because you would expect other 'hot Jupiter' planets to have the same effect and that isn't seen. They actually reckon that it is being heated from the inside by tidal heating because its orbit isn't perfectly circular. What makes it slightly elliptical in this case is another question and they suggest that there may be another planet in the system that is perturbing the one they found. Whatever the case, it is certainly an interesting bit of detective work.

Posted in astro blog by Stuart on Sunday 17th Sep 2006 (16:02 BST) | Permalink
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