Exomoons

We chat to Andrew Norton, Professor of Astrophysics Education at The Open University.

1. Why are we searching for exomoons?

We know that in our own solar system all the giant planets (Jupiter, Saturn, Uranus, Neptune) have large numbers of moons orbiting them. Even the Earth has a large moon (that we call “the Moon”!) and Mars has two small moons too. So it seems likely that many exoplanets will also be orbited by exomoons – there’s no reason to suppose they will have formed any differently to the planets in our solar system. If we think exomoons are there, then it makes sense to look for them.

2. How do they differ from exoplanets?

By definition an exomoon is a body that orbits an exoplanet. Just as in our solar system, a moon is a body that orbits a planet. Some people consider the Earth-Moon system to be a “double planet” as the bodies are (relatively) similar in size. The moons of Mars (Phobos & Deimos) are most likely captured asteroids and very tiny; whilst the moons of the giant planets range in size from bigger than the planet Mercury to small rocks only tens of km across. Exomoons around exoplanets may be similar in size to the solar system’s moons, or even larger.

3. Are exomoons more habitable than exoplanets?

Not necessarily. Habitability is determined by where the body lies (i.e. in the habitable zone of a star where the temperature is such that liquid water may exist on the body’s surface) and also by the conditions of the exoplanet or exomoon itself. Factors will include the size/mass of the body, which determines its surface gravity and whether or not the body has an atmosphere, and what the composition of that atmosphere may be. Gas giant planets like Jupiter and Saturn are not habitable in terms of life as we know it, even if they were in the habitable zone of a star, because they have no solid surface and the conditions within the gaseous planetary atmosphere are very hostile. However, it is conceivable that an Earth-like exomoon could orbit a Jupiter-like exoplanet, where the exoplanet is in a star’s habitable zone, so the exomoon too may be the right temperature to be potentially habitable.

4. How far away are we from the first official discovery of an exomoon?

The most likely scenario for the detection of an exomoon is via transit-timing variation measurements. In a system where an exoplanet passes in front of a star, from our viewpoint, the exoplanet will give rise to transits – tiny dips in the brightness of the star as the planet blocks some of the light. If a transiting exoplanet is orbited by an exomoon, the exomoon will tug the exoplanet back and forth very slightly as it orbits. This will cause both the time and duration of the observed transit to vary slightly in a periodic manner. The transit timing variations (TTVs) and transit duration variations (TDVs) will likely only be a few seconds, but are potentially measureable. If detected, the size of the TTVs and TDVs will allow both the mass of an exomoon and its orbital distance from the exoplanet to be calculated.

I anticipate that the first hints of detections may come in the next couple of years, and will eventually be confirmed as more observations build up to reinforce the detections.

5. They are light-years away and simply too far to travel to with current technology. Do you think we’ll ever be able to reach them?

Certainly not in the foreseeable future. Space travel over interstellar distances can realistically only be achieved in one of two ways with foreseeable technology. One way is the “generation ship” where you launch a vast spaceship with, say, 1000 people on board. Generations later, the descendants of the original crew might reach a nearby star system and land on any exoplanet or exomoon they find there. Alternatively, a way may be found to put a crew in “hibernation” for a few thousand years and make the trip achievable that way. Ideally you would launch a spaceship and accelerate it up to a speed of, say, 99.99995% the speed of light. Then, thanks to the time dilation effects of special relativity, you could travel a distance of 1000 light years, but only about 1 year would elapse for those on board the spaceship. Unfortunately the technology to reach such high speeds is not known or likely to exist in the foreseeable future.

6. Tell me more about the four states of an exomoon. (habitable’, ‘hot’, ‘snowball’, and ‘transient’)

These are the 4 categories described by Forgan & Yotov in their paper. They are not universally accepted definitions or categories and I think were only invented for the purpose of their recent paper. As I said in my article on the conversation, exomoons in the first class have more than 10% of their surface at a temperature between the freezing and boiling points of water, with only a small fluctuation around the average temperature value. Those in the second class have average temperatures above 100°C at all times, whereas those in the third class are permanently frozen – in both cases less than 10% of the surface is habitable. Exomoons in the fourth, transient class are on average habitable, but the amount of habitable surface area varies widely with time.