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For the first time, astronomers have detected the light coming from an exoplanet. SciShow Space News explains how they did it, and why it was so difficult in the first place.

Hosted by: Hank Green

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Hank: It's been an exciting week for astronomy enthusiasts like us, because the study of exoplanets has just taken a giant leap forward.  A group of Portuguese researchers announced last week that by carefully looking for patterns in the light from a star called 51 Pegasi, they were able to detect the light from one of its planets called 51 Pegasi B.  We're talking about honest-to-goodness hit the planet and bounced off kind of light, the first direct light we've ever observed from a planet outside the solar system.  

The planet, which people have taken to unofficially calling Bellerophon, is no stranger to fame.  Way back in 1995, it was the first exoplanet found orbiting a sun-like star.  51 Pegasi is 51 light years away in the constellation Pegasus, hence the name, but its yellow g-type main sequence star is just like ours.  And yet, Bellerophon is nothing like Earth.  It's what scientists call a hot Jupiter, huge and gaseous but orbiting really close to its host star.  
Discovering exoplanets like this in the first place is a tricky business because stars are so bright that they tend to outshine any objects around them.  It's like looking into a super powered flashlight.  So we found all of the 2000 or so planets that we know of so far without seeing them directly.  Instead, we've used other methods like watching how a planet's gravity tugs on its star a little causing a shift in the wavelengths of its light.  Looking for these shifts is called the radial velocity method, and it helped us detect hundreds of exoplanets.  The other main way to find exoplanets is by checking to see if a star's light fluctuates in a predictable way.  That might mean that a planet is passing in front of it.  

Both of these methods have been incredibly useful, but they're limited.  Being able to directly measure the light coming off an exoplanet would be exponentially better, because the planet doesn't need to pass right in front of its star in order for us to know it's there, which is actually pretty rare, and plus, its reflected light could actually tell us about the planet.  we could tell just by looking if it had an atmosphere and if it did, if it was chock full of like, nitrogen or methane.  We could even conceivably see what color it is, which is why these new findings, published last week in the Journal of Astronomy and Astrophysics, are so excited.  

Using a 3.6 meter telescope at the La Silla Observatory in Chile, the astronomers collected data over seven different nights and put together 90 different signatures of light or spectra from 51-Pegasi.  The light spectra had most of the usual predictable patterns you would expect from such a star, created by fluctuations in its brightness, but the researchers knew that Bellerophon's light had to be in there as well, creating its own tiny fluctuations.  51-Pegasi was just drowning it out.  The problem was separating that tiny pattern out had never been done successfully, but the team thought that they could do better.  They knew that the light patterns they'd collected were combinations of three different things: the light emitted by the star, the light from the planet, and random noise.  If they could get rid of the star's contribution to that light, the remaining data would just be the planet's light plus some of that noise.  So they put together a sort of template for all of 51-Pegasi's usual spectrum and then just subtracted those points from the data.  

Overall, the spectrum that was left fit the pattern of a planetary signal.  The astronomers were able to give their results a significance of 3 Sigma, which means they're about 99.7% sure that what they're detecting is really the planets.  They also figured out that assuming the signal is correct, Bellerophon has a little less than half of Jupiter's mass, but mainly, they wanted to prove that the technique worked.  

With new, more sensitive instruments like the James Webb space telescope just around the corner, astronomers should eventually be able to use this method to directly measure things like the composition of exoplanets' atmospheres.  For the first time, we'll know what it's like on lots of different kinds of planets and maybe whether any of them might harbor life simply by looking at them.  We've known for years that there were other worlds out there, but for the first time, we can actually see them and study them. 

Thank you for watching this episode of SciShow Space News, which was brought to you by our Patrons on Patreon.  If you want access to extras like a calendar designed by our graphics team and to help us out making this show, just go to

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