Friday, June 04, 2004
Watching Venus transit
Nisha sent the Exploratorium website for the benefit of those who will not be able to see the transit of Venus on the 8th of June for geographic reasons. The Exploratarium plans to webcast the transit live. Check for your local timings. In the meanwhile, the
Economist provides a fantastic backgrounder and also several reasons why you should be try and catch the transit.
Astronomers have found scores of planets around other stars, mainly by searching for stars that wobble back-and-forth due to the gravitational pull of an orbiting planet. The problem with this indirect technique is that it does not tell you much about the planet you have discovered. In particular, that planet's diameter and composition remain unknown.
But if the planet's orbit is nearly edge-on, as viewed from Earth, then the planet will transit its parent star. Though even the nearest star is too far away for a planet drifting across its disc to be seen, astronomers can detect the drop in starlight caused by the obstruction. From the size of this drop, they can deduce the diameter of the planet. This was first done in 2000, when a Jupiter-sized planet was discovered around a star named HD 209458.
The next step is to figure out what is in the planet's atmosphere. This can be done by measuring the transit in different colours. Each type of atom or molecule absorbs particular colours. Sodium, for example, is fond of a certain orange-yellow hue. If there is sodium in the atmosphere, then there will be a little extra blockage of that colour during a transit.
This was actually seen for HD 209458, but the amount of sodium observed was less than predicted by models of planetary atmospheres. This could be an important discovery, or it could be the fault of the models. So it would be nice to test those models by observing a planet whose atmosphere is already well known: Venus.
Economist provides a fantastic backgrounder and also several reasons why you should be try and catch the transit.
Astronomers have found scores of planets around other stars, mainly by searching for stars that wobble back-and-forth due to the gravitational pull of an orbiting planet. The problem with this indirect technique is that it does not tell you much about the planet you have discovered. In particular, that planet's diameter and composition remain unknown.
But if the planet's orbit is nearly edge-on, as viewed from Earth, then the planet will transit its parent star. Though even the nearest star is too far away for a planet drifting across its disc to be seen, astronomers can detect the drop in starlight caused by the obstruction. From the size of this drop, they can deduce the diameter of the planet. This was first done in 2000, when a Jupiter-sized planet was discovered around a star named HD 209458.
The next step is to figure out what is in the planet's atmosphere. This can be done by measuring the transit in different colours. Each type of atom or molecule absorbs particular colours. Sodium, for example, is fond of a certain orange-yellow hue. If there is sodium in the atmosphere, then there will be a little extra blockage of that colour during a transit.
This was actually seen for HD 209458, but the amount of sodium observed was less than predicted by models of planetary atmospheres. This could be an important discovery, or it could be the fault of the models. So it would be nice to test those models by observing a planet whose atmosphere is already well known: Venus.
