Thursday, June 05, 2008

The smallest planet ever found? Sorta, kinda, maybe, not really...

Image Credit: D. P. Bennett / The Astrophysical Journal /

Some times you get lucky, sometimes you are unlucky, and sometimes you get lucky and unlucky at the same time. The latter is the case for astronomer David Bennett of the University of Notre Dame's Department of Physics. Dr. Bennett is involved in a search for gravitational microlensing. Microlensing happens when one star (the "lens") passes between the Earth and another star. The gravity of the lens star focuses the light of the more distant star, causing it to appear brighter from the Earth. The amount of brightening and the length of the brightening tells us about the lens star and the background star, and any strange features can tell us about companions (be they other stars or planets) of either the lens or the background star. There are many ongoing searches for these microlensing events.

At the American Astronomical Society meeting on Monday, Dr. Bennett and his collaborators announced that a search they are involved with, the Microlensing Observations in Astrophysics search (or "MOA"), had discovered a microlensing event that showed some strange things, which they interpret as a planet three times the mass of Earth circling a very tiny star (or perhaps even a brown dwarf, or a star too small to shine by its own nuclear reactions). This planet would be the smallest planet known around a star (except for planets known to be circling pulsars, the remains of a supernova explosion). This could be an important discovery, because it would show that even the smallest stars can have planets.

This is a lucky discovery, because microlensing requires luck -- you need an interesting star to either go in front of another star, or to have another star go in front of it. And you have to be looking at the right place at the right time, as there is no warning that something cool is about to happen. And you need automated telescopes that can search the sky on every clear night, that can automatically analyze pictures and look for a star getting brighter in the manner that indicates microlensing, and that can then automatically follow up that star as often as possible. It's not easy; the fact that astronomers can do it is a feat of both teamwork and computing power.

So, why do I say that Dr. Bennett is also unlucky? Look at the graphs at the top of this blog entry. The red and blue points mark when telescopes took pictures of the object, and the different colors of curves show what different types of microlensing events might look like. Some need planets, and some don't. Notice the problem? There are several hours when no pictures were taken, and many different types of microlensing can fit the data Bennett did get. During the hours most critical to interpreting the microlensing event, it was either daytime or the weather was bad.

This doesn't mean all is lost. In a long paper, Dr. Bennett and collaborators go through an extensive amount of models and testing and estimating, and they show that their interpretation, a tiny star with a super-Earth in orbit around it, is the best model that they tested. I am not experienced in interpreting microlensing, so I have to take them at their word, and the paper did pass a review by an independent scientist. But many of the planet people I know are dubious. They want to see more proof (such as the missing points, which it is now impossible to get).

Thankfully, there are some ways using large telescopes and the Hubble Space Telescope to try and get more information on both the lens star (the one with the planet) and the background star. A lot more hard work should be able to help settle whether this small planet is real, or whether there is some other explanation for the observed microlensing event.

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