Exoplanet research is evolving rapidly. A handful of years ago, the main focus in the field was simply finding and categorising as many exoplanets as possible to help us understand how common they are and what the population of planets in our Galaxy might look like. Now, researchers are developing the tools and techniques needed to measure exactly what the atmospheres and climates of individual exoplanets are like.
In our own Solar System, there are essentially three kinds of planets: small rocky planets near the Sun, large gas giants a little farther out, and even more distant mid-sized ice giants. One of the biggest surprises that came out of the first major exoplanet surveys was the sheer variety of different kinds of planets that exist. We were discovering exoplanets unlike anything we find in our Solar System.
Though challenging to detect and measure, a handful of Earth-like planets have been catalogued and are being actively studied. Somewhat more foreign are the in-between planets a bit larger than Earth (super-Earths) or a bit smaller than Neptune (sub-Neptunes). These are fascinating objects because they teach us about planetary evolution and how some planets might lose a large portion of their atmosphere and shrink in size.
Jupiter and Saturn are huge gas planets that take more than a decade to orbit the Sun, and astronomers originally thought that all gas giants would have similar orbits. However, the first exoplanets ever detected are what we now call Hot Jupiters. These are giant planets that orbit extremely close to their stars. A “year” for these planets can be just a few Earth-days, or even hours! They are obviously very hot planets and they can have ferocious supersonic winds that circulate hot material from the day side to the night side.
Another strange type of planet is a super-Puff. As you might expect, they get their names because their atmospheres are extremely puffed up. Even though they are very large planets, their mass is relatively low. They are so puffy that their overall density is similar to that of cotton candy!
One of the most dramatic kinds of exoplanet might be the lava planets. These are the small rocky equivalent of a Hot Jupiter. The temperatures on their dayside get so hot that the rocky surface itself melts. Bizarrely, vapourized rock can blow to the nightside where it literally rains rocks!
How do we know what these planets’ atmospheres are like? One of the main observational methods to study planet atmospheres is called transit spectroscopy. When a planet passes in front of its star (called a “transit” event), some of the starlight shines through the planet’s atmosphere, leaving an atmospheric fingerprint in the starlight that is only present during the transit. Astronomers split this mixed starlight into all its individual colours (in a process called spectroscopy) and try to extract the exoplanet atmosphere’s signal.
Another method is called eclipse spectroscopy where the light coming from the face of the planet is detected. This is challenging because the planet emits a small amount of its own light that is mixed with the starlight it reflects, and everything is drowned out by the very bright star next to it. To disentangle the starlight from the “planet light”, we wait until the planet goes behind the star (called an “eclipse”), revealing what the pure starlight alone looks like. Once the planet re-emerges, the change we observe is due to the planet’s contribution.
Using both transit and eclipse spectroscopy, astronomers use these planet “fingerprints” to determine what kinds of molecules and atoms make up the exoplanet’s atmosphere, its temperature, and other key parameters that help us better understand its climate and its habitability. We are not restricted to just looking at the transits and eclipses either. Planets orbiting their star display phases just like our Moon (e.g. crescent, gibbous, full, etc.). By measuring the atmosphere at different phases, we can detect gradual changes and recreate an East-West temperature map. This is about as close as we can currently get to “seeing” details of an exoplanet, although in this case we are “seeing” the temperature rather than a visual surface.
The spectrum measurements gathered from spectroscopic observations can be used in a process called atmospheric retrieval to determine the atmosphere’s structure. We do this by creating a computer model of a planet and atmosphere with a particular mix of chemicals and temperature/pressure profile. We then check how well the artificial spectrum matches the observed spectrum. A powerful computer will rapidly test millions of models, making slight adjustments until the best match is found.
Members of our Institute are making great strides in exoplanet atmospheric science and helping envision these faraway planets as worlds of their own. To learn more, we invite you to read their profiles: