Searching for Extraterrestrial Moons

NASA astronomers are optimistic that they'll eventually be able to detect transiting exomoons
NASA astronomers are optimistic that they’ll eventually be able to detect transiting exomoons

Question: Is it possible to detect moons orbiting distant exoplanets? How would this be accomplished?

Questions from the kids (2013-12-30) – If we use our own solar system as an example, we would expect exoplanets to have bodies similar to our own Moon orbiting them. Exomoons, as we’ll refer to them, would be small in comparison to their host planets, and this fact is going to make it more difficult to detect them at the extreme distances involved.

NASA scientists believe exomoons could be a good place for life to start and thrive in many solar systems
NASA scientists believe exomoons could be a good place for life to start and thrive in many solar systems

Despite this fact, astronomers believe exomoons should be detectable, using the same techniques and for the same reasons exoplanets are detected. Exomoons have mass, which means they’ll interact gravitationally with their host planet and sun, causing the exoplanet to move in a mathematically predictable manner in response to the force of gravity. The exomoon will constantly pull on the planet gravitationally, which changes the amount of time it takes the planet to pass in front of its host sun. If an exomoon lines up with its home sun from our point of view here on Earth, this would cause a resulting collection of dips in measured sunlight, just before or after the much more significant transits of the host planet in front of its star. Astronomers believe they can use this fact in the future, along with any new techniques they develop, to search for and find distant exomoons orbiting their home planets.

This detection technique is the most practical way astronomers have developed in order to search for and find distant exomoons. This method provides astronomers with a more direct technique to use in the search for exomoons and at present is the best way to do the job. Currently, NASA’s Kepler telescope, which is looking for smaller transiting exoplanets, is probably our best chance of finding a distant exomoon orbiting its home planet. The Kepler telescope really isn’t designed to search for and find distant exomoons, which makes the job a truly daunting task using this telescope. If we use the largest moon in our solar system, Jupiter’s Ganymede, as an example, we would find Ganymede’s diameter is only about 40 percent of Earth’s. This means Ganymede would only block about 0.0014 percent of the Sun’s light during any transit, which is around six times less than the amount blocked by an Earth transit.

The human journey to the beginning of space and time could one day discover an exomoon looking like this
The human journey to the beginning of space and time could one day discover an exomoon looking like this

All of this is based upon the data and information astronomers have concerning our own solar system, which could be too general, or just wrong. It could be Earth-sized moons orbit transiting planets as large as Jupiter or Saturn, which would mean Kepler would just be able to detect them, and make it possible to search for and find distant exomoons orbiting their home planets.

The best bet astronomers have of finding exomoons orbiting their home planets light-years away will probably be the James Webb Space Telescope once it comes online. This will be when the human journey to the beginning of space and time has the best chance of searching for and finding exomoons orbiting their home planets.

Read about NASA’s Messenger spacecraft and its mission to Mercury

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