What would sunset look like if you were on the planet HD209458b, a gas giant orbiting a star 150 light years away? According to exoplanetary scientist Frédéric Pont, it looks like this:
Isn’t that pretty? And there’s quite a bit of science in that, too.
First things first: HD209458 is a star pretty similar to our Sun. It was one of the first stars determined to have a planet orbiting it (way back in 1999) — the aforementioned HD209458b, nicknamed Osiris — and it turns out the planet’s orbit is so close to edge-on as seen from Earth that we see that planet passing directly in between us and that star once per orbit. When the planet transits that star the amount of light we see dips a little bit. From that we can get the period of the orbit and the size of the planet (a bigger planet blocks more light).
But we can get more, too. There’s a camera on board Hubble called the Space Telescope Imaging Spectrograph, or STIS. It can take the light from an object and break it up into thousands of separate narrowly sliced colors, called a spectrum. By analyzing that spectrum we can find out an astonishing amount of things about astronomical objects: their temperature, rotation, even their composition!
Shortly after HD209458b was discovered to be a transiting exoplanet, STIS was pointed at the star. The camera took hundreds of very short exposures during a transit in the hope of being able to detect the atmosphere of the planet. Osiris was known to be massive, about 70% as massive as Jupiter, so it most likely has a thick atmosphere. It also orbits so close to its parent star — 6.7 million km (4 million miles), much closer than Mercury orbits the Sun — that the heat from the star puffs the atmosphere up, making it easier to see.
In fact, the spectra did reveal the presence of an atmosphere; the first time the atmosphere of an alien planet was ever observed. Different elements and molecules absorb light at different colors, so in the spectrum there are dark spots where the planet’s air absorbs the light from the star behind it during a transit, and how dark that spot gets tells you how much light is absorbed.
It’s this information Prof. Pont used to create the image above (inspired by investigation and an animation done by Alain Lecavelier des Etangs). By knowing the color of the star itself, and using the way the planet’s atmosphere absorbs light, he created this image of the star using sophisticated computer modeling. Read More