Seeing an extrasolar planet

When going into schools, I sometimes ask kids if they know the difference between a planet and a star. This can be a tricky question, but some of them remember that a star gives out its own light whereas a planet doesn't; it just reflects light from a star. Well actually, that isn't quite correct as anything with a temperature above -273 Celsius will give out radiation from radio waves to infra-red.

This week, a paper was published in Nature showing that light had been detected from two planets going around stars other than the Sun. Although the European Southern Observatory's VLT imaged what might have been a giant planet last September, this is the first time light has been directly detected from known planets. The observations were made with the Spitzer Space Telescope which directly observed the warm infra-red glows of two previously detected 'hot Jupiter' planets. The planets are named HD 209458b and TrES-1 (don't you just love the names that astronomers come up with) and the latter was first found by two small aperture (10cm) telescopes last August.

The Spitzer detection technique relies on the fact that the orbits of these planets are suitably aligned so that they pass in front of (transit) and behind (occultation) their stars. The infra-red light was monitored as the planet orbited its parent star. Comparing the difference in infra-red measured when the planet was out of view to when it was visible, it was possible to calculate the contribution from the planet alone. Their temperatures have been calculated as over 800 degrees Celsius, so they deserve the name 'hot Jupiters'.

Planets around other stars - extrasolar planets as they are known - were first discovered in 1995. The last decade has seen a string of discoveries bringing the total to 152 planets in 134 separate planetary systems. So next time somebody asks you how many planets there are, you can tell them that there are certainly more than nine.

Posted in astro blog by Stuart on Friday 25th Mar 2005 (22:07 UTC) | Permalink
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