Quirks & Quarks

Watching a moon rise — in a solar system 8000 light years away

There is compelling evidence for the first moon outside our solar system

There is compelling evidence for the first moon outside our solar system

Artist's impression of planet Kepler 1625B, and its candidate exomoon following behind. (Dan Durda)

There are nearly 200 moons in our solar system, but there has never been one found in another star system. Now astronomers say they may have found one. They have the first compelling evidence for an "exo-moon."

Alex Teachey, a graduate student at Columbia University in New York, and his supervisor  began their search for an exo-moon by studying data from 284 planets identified by the Kepler space telescope. They noticed that one particular planet - Kepler 1625b - had intriguing anomalies. In order to gather more precise information about this planet, they used the more powerful Hubble space telescope.    

The Hubble space telescope. (NASA)

A very full exo-moon

The exo-moon candidate is the size of Neptune — large compared to the moons in our solar system. It orbits a planet the size of Jupiter, which in turn circles a star 8,000 light years away.  Both the moon and its planet are thought to be made of gas, rather than rocky like our moon. The researchers estimate that the moon is about 1.5 per cent the mass of its planet, which is a similar mass ratio to that of our Earth and moon.   

Two lines of evidence

The researchers haven't directly observed the moon (or it's planet for that matter). So their evidence for it's existence is indirect. When the astronomers monitor the planet's 19 hour transit across the face of the star, they see a dimming of the star's light. This is the common method for detecting the presence of planets. But in this case, they saw a second dimming of the host star's brightness about 3.5 hours later. This is consistent with a moon trailing a planet.

The second line of evidence involves a deviation in the timing of the planet's orbit. Planet 1625b began its transit over one hour earlier than predicted, the result of a wobble in its orbit. This is likely caused by the pull of a moon on a planet in the same orbit. 


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