The Spitzer Infrared Telescope detects (previously known) planets

On March 22, 2005, press releases announced that the Spitzer Infrared Space Telescope had made a great discovery:

The first sentence of the Spitzer press release was

NASA's Spitzer Space Telescope has for the first time captured the light from two known planets orbiting stars other than our Sun.

But what really happened? In a nutshell,

several years ago, astronomers discovered big planets orbiting around two different stars, using two different indirect techniques
the Spitzer telescope looked at each star in the infrared as the planet passed behind the star ...
... and saw that the total light of (star-plus-planet) decreased slightly when the planet went behind the star

Let's look a bit more closely at these two star systems. The planet around HD 209458 was first noticed back in 1999, when astronomers measured a periodic change in the radial velocity of its host star.

This indicates that the star is moving towards and away from us gently with a period of about 3.5 days; the simplest explanation is that a planet is tugging it as it orbits the star.

The presence of a planet was confirmed when astronomers monitoring the brightness of this star found a very slight decrease in its light at just the time that the planet was passing in front of the star.

Based on the size of the dip, astronomers can calculate the relative sizes of the star and the planet; and, since they know the rough size of the star, we can figure out the size of the planet.

The planet in the other stellar system in the news, TrES-1, was discovered by the "transit" technique. We have measured the small dip in brightness as the planet passes in front of its star from the RIT Observatory:

Note the size of the effect: in both cases, the planet blocked about 1.5 percent of the star's visible light, indicating that

the planet has about 1.5 percent of the star's cross-secion area, and therefore ...
... about 12 percent of the star's radius

Okay, so what's all the hubbub? What the Spitzer telescope did was to watch these two systems over several hours as the planets passed behind the host star.

These light curves show that the planet provides a very small fraction of the total infrared light of the systems: about 0.25 percent of the total. Compare that to the roughly 1.5 percent of the star's visible light which the planets blocked. The dip in brightness that you see in these graphs is what is meant by the press release text

NASA's Spitzer Space Telescope has for the first time captured the light from two known planets orbiting stars other than our Sun.

The "missing" infrared light from the planets is a combination of starlight reflected from the cloudtops, plus light emitted by the warm planet itself. The two groups which made the observations have used modelled the data to estimate the temperatures of the planets in these systems:

HD 209458: about 1100 K
TrES-1: about 1000 K

Since the planets are both very close to their sun-like host stars, these high temperatures are not surprising.

For more information:

The Extrasolar Planet Encyclopedia has lots of information on all the extra-solar planets
This lecture on detecting extrasolar planets from my Phys230 course describes in detail the various methods astronomers can use to find planets around other stars.
Abstract from Deming et al. Nature paper describing Spitzer observations of HD 209458 The paper appears in the 2005 March 23 on-line issue of Nature.
Paper by Charbonneau et al. on Spitzer observations of TrES-1. It is astro-ph 0503457, and will appear in the 2005 June 20 issue of ApJ.