Tuesday, 8 September 2015

A Surface Map of an Exoplanet

This is a summary of a paper from 2007. However, it is one of the neatest things I have read about, and I'd like to share.

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.
Over there.
The planet was discovered in 2005, by the dimming of the light of the star. Every 50 hours, the light of the star dims by 2%, indicating that there is a planet 4.5 million km away from the star (less than a tenth the distance Mercury is from the sun), about 10% bigger than Jupiter. In addition to the star dimming by 2% every orbit, it also dims by 0.3%  at another point in the orbit, 180 degrees of rotation later. Why does this happen? When the planet is in front of the star, it blocks light from star from reaching Earth. When it is behind the planet, the star blocks light from the planet from reaching Earth. When the planet is "next to" the star, from our perspective, we receive light from both.

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.

Cartoon of my understanding of how this analysis is done. The planet is a bright side that always faces the star, and a dark side facing away. In the top image, the light beamed from the planet to the Earth comes from both the bright and the dark sides. In the second, it has rotated a bit more and now more of the bright side is beaming to Earth, so the planet's contribution increases. When the planet goes behind the star we can no longer see it. Sizes and distances are surprisingly not to scale.

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.
The planet is close enough to the star that it is tidally locked, meaning that the same face is always pointed at the star, same as how the same face of the moon is always pointed at the Earth. This means that on the planet's surface, the star always appears to be in the same point in the sky, and there is one point on the equator where the sun is directly overhead. Logically, one would expect this to be the hottest. However, they found that the hottest point was actually 30 degrees East of the sub-stellar point, while the coldest was 30 degrees West of the darkest spot on the planet. The temperature ranged from about 1200 Kelvin down to 970 Kelvin. For comparison, the hottest part of Mercury is about 700 Kelvin.

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.

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