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Mercury Faces the Sun

Week of November 15, 1999

It's funny how things work out sometimes. This week I was going to take a minor break from the typical Bitesize Snack and make a couple of quick announcements. One is about the Leonid meteor shower, which peaks this week on Wednesday night (November 17/18). The Leonids go through a ``storm'' every 33 years, and this may be the year. There was a minor storm last year, with peak rates of about 1000 per hour. This year may be more, or it may be less. I suggest taking a look at the page I put up last year. Most of the links should still be good, and there are plenty of explanations there about how to observe the shower.

But it's the other announcement that's actually funny. On Monday, November 15 (hopefully you are reading this in time!) Mercury will pass directly between the Earth and the Sun. That means that for a short time you can actually see the tiny planet in silhouette against the disk of the Sun. This event is technically called a ``transit'', though you can think of it as an eclipse of the Sun by Mercury. It doesn't happen very often, since the orbits of the Earth and Mercury are at a slight angle to each other. Things have to line up just right to see them. The last one happened on 1993, and the next one won't be until 2003.

The reason this is funny is that another transit was in the news this week, except it wasn't a planet anyone has ever seen before. As little as ten years ago, we didn't know if there were any planets orbiting other stars or not. Sure, we figured there must be, but none had ever been observed. Now, many such planets are known. They have been detected by their gravitational tugging of their star; as they orbit, the star moves in a little circle as the planet makes a bigger one. From watching the star's spectrum, we can detect this motion. However, the planet itself is invisible because it is small and very dim, and the star is very bright. These have been indirect detections, because we never see the planet, just its effects.

That has perhaps changed. Marcy and Butler, two American astronomers who have found most of the extrasolar planets known, made an interesting prediction. They knew that orbiting a star with the unlikely name of BD 2090458, there was planet, and, as far as they could tell, its orbit lined up just right so that from the Earth, we would see it pass in front of the star. That meant that the planet would block some of the star's light when it passed in front. They also knew this planet must be roughly as big as Jupiter, so they could guess just how much the light from the star would dim. They alerted Greg Henry, another astronomer, who had access to a good telescope. Right on cue, BD 209458 dropped in brightness by about 1.7%! From this number, and other properties of the orbit they knew from their other studies, they found that the planet has a mass of about 2/3 of Jupiter, and is a bit bigger in diameter.

This result needs to be confirmed, but is still amazing. Marcy and Butler's technique is extremely sensitive, but there are still some scientists that are not 100% convinced the results are correct; they still wonder if there are intrinsic properties of stars that may make them look like they have planets orbiting them. This result with the extrasolar planetary transit may very well quell those doubts. The next step is to continue to observe the star and see if it continues to dim and brighten according to schedule. If it does, this may very well be the first direct detection of an unambiguous planet outside our solar system. And it comes the same week we see one of our own planets pass in front of our own star. Life is full of funny coincidences.

©2008 Phil Plait. All Rights Reserved.

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