|The Universe And All That…|
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.
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!”.
References: Reference: https://en.wikipedia.org/wiki/Barycenter#Gallery