Exoplanets are a hot topic in the news. The Ottawa Centre of the Royal Astronomical Society of Canada (RASC) has invited Professor Nicolas Cohen of the McGill Space Institute to talk about exoplanets at next month’s public meeting.
Brian Carroll interviewed Professor Cohen about his upcoming presentation.
Apt613: The title of your talk is Surfaces and Atmospheres of Exoplanets. What is an exoplanet?
An exoplanet is just a planet orbiting some other star, not the Sun. It’s technically a contraction of extra-solar planet.
Kelly Jordan, meeting chair of the Ottawa RASC, has seen your presentation and says, “It’s an excellent talk for beginners.” She particularly commented on your use of graphs, diagrams and artists’ depictions. How did you tailor your presentation to a layperson audience?
In astronomy you get used to giving public talks. Astronomy is special in that we really do have amateur astronomers. The presence of these astronomical communities means that, from a very early age as a professional, you go and give talks. Because the amateur astronomers, although they mostly like to look at the night sky and see things that they can see with their telescopes, they also like to hear about things that they can’t see with their telescope. Things that need a Hubble telescope or a Keck telescope, something much bigger.
I’ve been giving public talks since I was a graduate student.
How do we learn about the atmosphere of an exoplanet?
The challenge is that your exoplanets are about 10 to 100, sometimes 1000, light years away. They orbit a very, very bright star. They’re orbiting a normal star, but that star is about 10 billion times brighter than this planet you care about. So how to you learn anything about the planet?
One of the tricks we’ve figured out in the last 10-15 years is, if you’re lucky and planet passes directly in front of the star, then it actually filters a little bit of the starlight on its way to you. Essentially the planetary atmosphere imprints information about the molecules on the starlight. You see something that looks like starlight, but it’s got these molecular features. Which doesn’t make sense, because you don’t expect molecules in these stars. That’s called transit spectroscopy. That’s an example of a trick we’ve used to study the atmosphere of planets even though they’re really far away and faint.
How do you learn about the surface of an exoplanet?
Even your instincts tell you that the surface is harder to characterize. That’s because it’s underneath the atmosphere. When you look at a planet from far away, you’re looking at it from top down. You first see the atmosphere, and then if you’re lucky you can see through the atmosphere and see the surface.
Just to give you an idea of one of the ways you study the surface the planet. You have Carl Sagan’s proverbial Pale Blue Dot (Earth as imaged by Voyager 1). If you manage somehow to block out the starlight and see this faint blue dot that’s 10 billion times fainter than the star that’s beside it, that’s Earth.
Or some Earth-like planet.
If you’ve ever owned a globe and spun it around, the colour of that blue dot is not always the same. It changes as the object spins. When the Pacific Ocean is in view, the Pale Blue Dot looks a little bit bluer and darker. When Africa and Eurasia are in view, it looks a little bit redder and brighter. You can actually extract information from those colour variations. They’re very subtle colour variations, but if you do some math, you can extract lots of cool stuff. You can learn what kinds of surfaces are on the planet, the colours of the surfaces and where the surface is distributed. You can make a really crude map of the planet, and figure our whether it has oceans and continents.
I can understand it with respect to planets in our solar system. Before the mission to Pluto, I saw a picture of Pluto and it was in these HUGE pixels, but they had different shadings. But you’re talking about planets that are 10, 100, 1000 light years away. That’s really small.
Yes it’s a single pixel. Pluto’s an excellent example. The old school pictures of Pluto were made exactly in this way. Pluto was just a pixel. But we used two ways to map Pluto. We’ve used both these methods to map exoplanets.
One we call rotational mapping. The colours of Pluto change as it spins. Different colours come in and out of view.
The other way you can map it is when Charon, its moon, passes in front of it. Now it occults part of Pluto. Now Pluto gets dimmer because part of it is being obscured by Charon. The colours change as it gets obscured by Charon. Maybe Charon first obscures a red part, and then a yellow part, and then a green part. That’s called occultation mapping. We’ve used that for exoplanets as well.
Surfaces and Atmospheres of Exoplanets will be presented during the next meeting of the Ottawa Centre of the RASC at the Canada Aviation and Space Museum, Friday, February 3rd at 7:30PM. For more information see the Ottawa RASC website. The meeting is open to the public. Admission to the meeting is free. Parking at the museum is $3.