Q & A – Exoplanets

Mini-Science logo At the conclusion of each Mini-Science lecture, audience members submit their questions to the evening’s presenter, who answers as many as possible on the spot. Three of the unanswered questions are sent to the presenter for posting here. In addition, a quiz is held each week based on material from the lecture. Here are questions and the quiz from Prof. Andrew Cumming’s lecture ‘In Search of New Worlds: The Discovery and Characterization of Exoplanets’ (April 22, 2009).

Q: What causes planets to be different in size, composition, orbit, etc.?

A: We are still far from a complete understanding of the process by which planets form, but we believe that the properties of a planet are largely determined by the protostellar disk of gas and dust from which it formed. For example, if the disk is rich in metals (heavy elements) planets can form more easily and grow to larger sizes. We see this in the observed population of exoplanets, which are much more common around metal rich stars. Rocky planets that form in the outer parts of a protostellar disk grow to larger masses than in the inner parts, and can reach large enough mass to attract gas and form a gas giant. This is the reason that the inner Solar System consists of terrestrial planets whereas the outer Solar System has gas giants. Another important process is that planets can change their location by raising tides on the surrounding disk of gas (just like the tides raised on Earth by the Moon are causing the Moon to recede from the Earth). This process is known as migration, and explains how it is possible to find a hot jupiter orbiting so close to its host star: the jupiter formed further out and then migrated inwards. Finally, after formation of the planets is complete, the mutual gravitational attractions of the planets can lead to dramatic changes in the types of orbits, including ejecting planets from the system. All of these different processes must be considered when we try to understand the properties of the exoplanets that we see.

Q: Can we detect and measure the different types of revolutions of the exoplanets?

A:So far, we haven’t been able to directly measure the rotation rate of an exoplanet. The close-in transiting planets that have been detected are believed to be “tidally-locked”. This means that the rotation rate of the hot jupiter equals its orbital period, so that the planet always shows the same face to the star, in the same way that the Moon always shows the same face towards the Earth.

Q: What is the temperature range of a hot planet and are the temperatures on a hot planet fairly constant?

A: The surface temperature of a “hot jupiter” is typically between one and two thousand degrees. They are heated very strongly by the star because they are orbit so closely. For comparison, in our Solar System, Jupiter has a much lower temperature of between one and two hundred degrees. Because the hot jupiters always show the same face to the star, the heating is always on the same side and drives strong winds around the planet that redistribute the energy from the hot day- side to the cooler night-side. The temperature variation between the day- and night-sides depends on how efficiently the winds can transport the heat. This is a subject of current research being carried out here at McGill by postdoc Ian Dobbs-Dixon, and can be tested by observations of transiting planets with the Spitzer Space Telescope.


Q: How are the masses and radii of exoplanets measured?

A: Masses are determined by measuring the amplitude of the radial velocity of the star. A larger mass planet exerts a larger gravitational pull on the star causing a larger velocity. Radii can be determined only for those planets that transit. Then the amount by which the light from the star dims as the planet passes in front tell us the radius.

Q: Name a surprising property of exoplanets when compared to the planets in our Solar System?


  1. Many of the gas giant exoplanets orbit much closer to their host star than in our Solar System, for example the “hot jupiters” orbit once every 3 days whereas Jupiter takes more than 11 years to orbit the Sun.
  2. The exoplanets tend to have eccentric orbits, whereas the planets in the Solar System are on mostly circular orbits.

Q: What do gas giant planets and neutron stars have in common?

A: Either

  1. they are both round or
  2. they contain matter at very high densities difficult to achieve on Earth, and therefore are great laboratories for extreme physics (e.g. metallic hydrogen in Jupiter).

If you have any related comments or questions, please feel free to post them. We cannot promise a reply to every question but will answer what we can.

One response to “Q & A – Exoplanets”

  1. duvar kağıtları says:

    Q & A – Exoplanets at Faculty of Science very good article thank you.

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