Exomoons and Double Planets

The Universe And All That
Cosmic Focus
This article first appeared in Prime Focus Magazine, May 2020.

I wrote last month about how and why the Moon was defined as a natural satellite by the International Astronomical Union. The IAU does not classify objects simply by size. If it didn’t take other factors into account, then the sheer size of Ganymede, Titan and even Callisto would require consideration as planets.

2009-08-03 Moon by Day

I posed the thought that the Earth-Moon System, which is of course defined as a planet/satellite system, could easily be thought of as a double planet system. If the Moon were orbiting the inner solar system in it’s own right, it would most likely be defined as a planet.

It got me wondering whether any exoplanets had yet been discovered which had natural satellites orbiting them – and how they would be classified.

There are over 4,100 known exoplanets but no exosatellites have been listed yet. That’s not surprising when you consider that Pluto was thought to have no Moons until Charon was discovered in 1978 by the United States Naval Observatory.

Most likely our ability to detect them has not yet developed sufficiently but if exoplanets with large diameter natural satellites do not exist, what implication would that have on the search for extra-terrestrial life? The Moon is believed to have played an integral part in the evolution of living organisms on Earth.

The question of double exoplanets is bound to come up eventually and the IAU will need to define the difference between (a) exoplanets with exosatellites and (b) double exoplanets.

That definition would need to be consistent with the current definition of the Moon as a satellite of Earth. So a double exoplanet might be defined to require, for example, (a) the barycentre to be external to both objects; and/or (b) a satellite/planet size ratio of perhaps no less than 0.5.

In the Earth-Moon system, the barycentre (common centre of gravity) lies about 1,700 km below the Earth’s surface and the Moon’s diameter is only 0.27 of Earth’s.

Whilst accepting the IAU definition of a planet, I reckon any aliens observing our solar system from afar would be saying, “look at that, a double planet!”.

Macarthur Astronomical Society field trip – my Meade LX-90 telescope silhouette.

References: Reference: https://en.wikipedia.org/wiki/Barycenter#Gallery


  1. You make a good and reasonable argument. Given what we know about the abundance of planetary moons in our solar system, and the fact that some asteroids have orbiting partners, I think it is very likely we will discover them around exoplanets once the technology is able.

    Barycentre is not a word one sees very often in writings. I’m glad to see it. Yesterday, I came across something I found very interesting and wanted to share it. I couldn’t think of another person who would have any idea what it was without a lot of explanation on my part. It didn’t seem worth the effort. So I dropped it. Now I can’t remember what it was. Hah! I must be getting old and forgetful.

    Liked by 2 people

    1. It’s probably in the recycling bin, if the brain has such a thing. We need to find a way to Google ourselves for lost information.

      Was this interesting thing in the sky by any chance?


  2. It is interesting, but I’d not previously come across the concept of double planets, or twin planets. Are any known? I became interested in barycenters a few years ago while puzzling out how double (and multiple) star systems work. Wrote about it here: https://skirmisheswithreality.net/2015/10/03/heavenly-choreography/
    I think your intuition must be right about the need for the barycenter to be external to both bodies. Cheers!

    Liked by 1 person

    1. I’m unaware of any being discovered so far but I suspect the probability of double planets existing must be high.

      I like your article. I was considering writing more about barycentres but I think you nailed it there.

      Liked by 1 person

      1. Thanks, I’m sure there are different angles to reveal though… would love to see you write about barycenters.

        My head spins when imagining the seasons and sky phenomena an inhabitant of a dual planet system revolving about, say, a binary stary system, would experience. To say nothing of possible moons. 🙂

        Liked by 1 person

  3. It would be challenging to detect an exoplanet’s satellite with current methods unless it and its planet were very large. Of course, based on what we see in our system, a super planet wouldn’t be the best place to start looking for extraterrestrial life, satellites or no. Hopefully we will eventually be able to identify smaller planets and objects around them with enough clarity to start classifying them.


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