This diagram illustrates how astronomers using NASA's Spitzer Space
Telescope can capture the elusive spectra of hot-Jupiter planets. Spectra
are an object's light spread apart into its basic components, or
wavelengths. By dissecting light in this way, scientists can sort through
it and uncover clues about the composition of the object giving off the
light.
To obtain a spectrum for an object, one first needs to capture its light.
Hot-Jupiter planets are so close to their stars that even the most
powerful telescopes can't distinguish their light from the light of their
much brighter stars.
But, there are a few planetary systems that allow astronomers to measure
the light from just the planet by using a clever technique. Such
"transiting" systems are oriented in such a way that, from our vantage
point, the planets' orbits are seen edge-on and cross directly in front of
and behind their stars.
In this technique, known as the secondary eclipse method, changes in the
total infrared light from a star system are measured as its planet
transits behind the star, vanishing from our Earthly point of view. The
dip in observed light can then be attributed to the planet alone.
To capture a spectrum of the planet, Spitzer must observe the system
twice. It takes a spectrum of the star together with the planet (first
panel), then, as the planet disappears from view, a spectrum of just the
star (second panel). By subtracting the star's spectrum from the combined
spectrum of the star plus the planet, it is able to get the spectrum for
just the planet (third panel).
This ground-breaking technique was used by Spitzer to obtain the
first-ever spectra of two planets beyond our solar system, HD 209458b and
HD 189733b. The results suggest that the hot planets are socked in with
dry clouds high up in the planet's stratospheres. In addition, HD 209458b
showed hints of silicates, indicating those high clouds might be made of
very fine sand-like particles.
