"Hubble Science"
"Exoplanets: Alien Atmospheres"
Exoplanets are very difficult to detect
because they are tiny little objects orbiting very bright stars.
Other telescopes are designed to be better detectors of exoplanets,
but what Hubble is used for is to find, for some of them, the atmospheric composition
of these exoplanets. It has to be a system that just by chance has to be aligned so
that the planet is orbiting its star along the line of sight of the Hubble Space Telescope.
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For doing a lot of the exoplanet observations, you have to catch what's known as a "transit,"
one the orbit of the exoplanet has to be such that
it's going to go between you and the star it's going around.
And then you have to do the timing. We can't just
do an exoplanet observation whenever we want or whenever it's convenient.
We have to do an exoplanet observation when it's first starting to go into the star.
So they have to know, very accurately, the timing of that. We have to schedule it ahead of time. This
is not something that, you know, Hubble can get around to when it wants to. We have to say,
"No, at this point in time, on this date, you have to be pointed here, and you have to be looking here."
When that planet passes in front of its star, the starlight, some of it is blocked by the planet.
But some of it comes through the outer ridge, the outer rims of the atmosphere of that planet on its
way to the Hubble telescope. Some of that light is absorbed by whatever is in that atmosphere,
and it is absorbed at very particular frequencies that correspond to the atoms and molecules that
are in the atmosphere of that exoplanet. Then when the Hubble telescope receives that light, and we
take it in, usually with a spectrograph, we get the light, we spread it out into its constituent
colors or wavelengths of light, and we can tell which of those wavelengths have been absorbed.
And that tells us, by the pattern of spectroscopy, what are the elements and molecules that are found
in the atmosphere of that planet. Hubble has detected things like sodium and hydrogen, and
even evidence of methane and water vapor, by using transit observations of exoplanets, and measuring,
not only that composition, but also the height of the atmosphere, which can tell us something about
how heavy the atmosphere is, and that tells us something about its composition as well.
Hubble was the pioneer in doing that, and now other observatories
are also using the transit technique to analyze the atmospheres of exoplanets.
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