Mini-Science 2015 Q&A – “An Abundance of Other Worlds: The Exoplanet Zoo”
At the conclusion of each Mini-Science lecture, audience members submit their questions to the evening’s presenter. If there is not enough time to answer them all on the spot, some of the other unanswered questions are sent to the presenter for posting here. Here are questions from Professor Andrew Cumming’s lecture “An Abundance of Other Worlds: The Exoplanet Zoo” (March 18, 2015).
Q: Please explain what an exoplanet is and why there is a value to looking for/at exoplanets other than searching for life?
A: Exoplanets are planets that orbit stars in our Galaxy in the same way that the planets orbit the Sun in our Solar System. We live in a special time when technology has advanced to a stage where we can detect exoplanets. Beyond detecting life on other planets, this is interesting because we want to understand how planets form and evolve. For example, why does the Solar System look the way it does, with terrestrial planets close to the Sun, gas giants further away, and the asteroid and Kuiper belts? We can begin to answer this question now that we have thousands of examples of other planetary systems. It is similar to the way that we can understand our Milky Way Galaxy much better once we are able to study other galaxies and look from the outside rather than the inside. Another reason planets are interesting is the extreme conditions inside them. For example, the hydrogen in Jupiter is at such high pressure it behaves as a metal instead of the gas we are used to. We can study this strange form of matter by examining the properties of gas giant planets.
Q: When a Jupiter sized planet migrates in, does its gas envelope boil away?
A: We don’t know the answer to this question yet, but there are suggestions that evaporation does play a role. Hot jupiters tend to be less massive than gas giant planets further out, suggesting that they lost mass at some point in their lives. They are so close to their host star that their atmospheres are heated to temperatures of two thousand degrees or more, and they are fully exposed to the magnetic eruptions and solar wind of the star. These two effects can drive gas off the surface of the planet, and indeed fast moving gas has been detected in the hot jupiter HD 209458. Another way to lose mass is that huge tides can be raised on the planet by the star, and gas can spill over and fall onto the star.
Q: Would migrating planets change their properties as they move?
A: Yes they certainly could. As planets migrate through the gas disk, they will continue to grow as gas and rocky material falls onto them. Recently a postdoc at McGill, David Tsang, has been working on how the eccentricity of the orbit of a gas giant will change as it migrates. Unlike our Solar System, most gas giants seem to be on elliptical rather than circular orbits, and that might be because the orbit changes as the planet migrates inwards.
Q: What is the technical advance that allows direct visualization of exoplanets?
A: The challenge in directly imaging exoplanets is to be able to see the faint signal from the planet in such close proximity to the star. A key piece of technology is a coronagraph which is used in the telescope to block out the light from the star. Another technique that is used is called angular differential imaging. A patch of sky appears to rotate in the telescope as the night sky passes overhead. This can be used to separate the light associated with the star from the diffraction spikes of the star caused by the telescope optics. Direct imaging is an exciting area at the moment because new instruments are beginning operation, such as the Gemini Planet Imager on the Gemini telescope in Chile, which will discover and image many new young gas giant planets.
Please visit the Mini-Science website for more information about the lecture series.