At least 130 of these are known, but only 9 are found in our own Solar System. Recently, light from two of these outside our Solar System has been detected by the Spitzer Space Telescope. What are they?

Planets. In the past ten years, the number of known planets has grown from the nine in our Solar System (although some people question whether Pluto really qualifies as a planet) to more than 130. Until recently, though, all the extrasolar planets had been detected indirectly.

About ten years ago, astronomers began to find evidence of planets around other stars. Although planets are much less massive than stars, some planets are big enough and close enough to their stars that the orbital motion of the planet causes a measurable periodic "wobble" of the star. By making careful measurements of stars over several years, this "wobble" confirmed the presence of the planets. Most of these planets turned out to be at least as massive as Jupiter but very close to their stars, closer than the orbit of Mercury to our Sun.

In some cases, the orbits of these planets turned out to take them in front of the stars as seen from Earth. These transiting planets blocked a tiny bit of the light from the stars, and this dimming could be detected, but the reflected light from the planets themselves was still far too weak to be measured.

All these measurements were done with visible light (the kind we see with our eyes), however. The launch of NASA's Spitzer Space Telescope provided a powerful new opportunity for measurements in the infrared, a type of light with longer wavelengths than our eyes can see. Infrared light is associated with heat, and the fact that the extrasolar planets are close to their stars means that they are hot objects. In infrared light, the planet's glow relative to that of the star is larger.

Astronomers used the Spitzer Telescope to observe two of the planets that are known to transit their stars. Since these planets go in front of the star for part of their orbit, they also go behind the star for another part of their orbit. In the infrared, the light dims when the planet goes behind the star. By subtracting the brightness when the planets were behind the stars (starlight only) from the brightness when the planets were visible (starlight plus planet light), the observers were able to measure the light from the planets themselves. This is the first time light from a confirmed planet beyond our solar system has been seen.

The star/planet combinations are known by their star names: HD 209458b and TrES-1. In both cases, the planets are Jupiter-size and orbit their stars in just a few days. Their temperatures are scorching, over 700 degrees Celsius or over 1400 degrees Farenheit.

Although neither of these planets resembles Earth in any way, NASA does plan future missions to search for Earth-like planets around other stars.

Some descriptions of these results can be seen at:

http://www.news.cornell.edu/stories/March05/extrasolar.ws.html and

http://www.spitzer.caltech.edu/Media/releases/ssc2005-09/release.shtml

with some images and videos at:
http://www.spitzer.caltech.edu/Media/releases/ssc2005-09/visuals.shtml

This week's question is provided by Dr. Dave Thompson. Dr. Thompson is an astrophysicist who studies gamma rays in the Exploration of the Universe Division. He helped build, test, and analyze data from EGRET on the Compton Gamma Ray Observatory, and he is now helping build part of the Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2007. His particular scientific interest is gamma-ray pulsars.