Exoplanets are typically detected either by their gravitational influence on their star, or by the decrease in the star's luminosity as the planet occults it. In both these methods, you do not see the planet itself, you see changes in the star's light caused by the planet. Nevertheless, a lot can be learned about the planet: its size, its orbit...ok mainly those things. But with some very clever teloscopy, Heather Knutson and friends managed to make a map of the surface temperature of an exoplanet.
The star is called HD189733. Stars with numbers instead of Greek or Arabic sounding names are generally part of a star catalogue, where a person or a group of people writes down a list of otherwise boring stars (in this case it is the 189733rd star in the Henry Draper catalogue). This star is in the constellation Cygnus, the swan, also known for hosting the first-discovered black hole. The star is 63 lightyears away (close, by stellar standards), and is about 80% the size of the sun.
|Brightness of the star over time. a. planet goes in front of star. b. star goes in front of planet.|
Knutson made a really detailed observation of the brightness of the star, taking 278,528 images over a 33 hour period. The star is far enough away that it just looks like a blob of bright pixels with no structure, known as a point-spread function. There are some exoplanets that can be directly imaged, but not this one. They monitored the brightness of the star as the planet went and emerged from behind the star. The planet is rotating as it revolves, so each point in time is looking at a different hemisphere that is facing Earth, and by monitoring the different brightnesses at different points in the planet's orbit, they could measure how much brightness the planet was beaming at the Earth.
This is in effect, a proxy measurement of the average temperature of each hemisphere that is pointed at Earth at a given time. This can be processed into a map of the temperature across the surface of the planet!
|The surface temperature across the planet! This image is from the Wikipedia article on this planet, it is a different visualization of the same data that is found in the paper.|
|The heat map looks cool, but it's important to look at the raw data.|
Why is the hottest part of the planet not the place with the most sun? The authors conclude that there is an atmospheric wind on the planet, blowing East at hundreds of meters per second, essentially sweeping the heat over a bit.
In 2012, one of the initial authors and two others improved on these methods, using much more sophisticated techniques, and gained the ability to measure temperature as a function of longitude as well as latitude. They improved the accuracy of the latitude of the hotspot to 21 degrees, and concluded that the hottest point is within 11 degrees of the equator. This was expected, but now it is measured.
Even though the methods are coarse, I think it's awesome that we can map the surfaces of planets around other stars.