Possible discovery of exomoon excites astronomers, but what is it?

4 Oct 2018

An artist’s illustration of the gas giant planet Kepler-1625b and a suspected accompanying moon. Image: NASA, ESA and L Hustak (STScI)

Astronomers believe they have found the first compelling evidence for an exomoon, about the size of Neptune and located 8,000 light years away.

Our hunt for exoplanets (planets further into the cosmos) has thrown up some truly strange and fascinating examples in recent years, with confirmed discoveries now numbering in the thousands.

But now, a combination of old data collected by the Kepler Space Telescope and observations using the Hubble Space Telescope may have revealed the first evidence for an ‘exomoon’, a moon orbiting an exoplanet.

We are still discovering dwarf planets within our own solar system, and astronomers have spent the past 30 years trying to locate an exomoon, but to no avail.

In a paper published to Science Advances, two scientists from Columbia University in the US put forward their evidence indicating the existence of a Neptune-sized exomoon orbiting a Jupiter-sized planet 8,000 light years away.

Hints of a moon orbiting the planet called Kepler-1625b were first detected last year, but the pair used Hubble to analyse the region in greater detail. Like its moon, the gas giant planet is unusual because of its large size, seven times greater in mass than Jupiter.

A shocking moment of discovery

To find evidence for the exomoon – dubbed Kepler-1625b-i – the team observed the planet while it was in transit in front of its parent star, causing a dimming of the starlight. Sure enough, the scientists said, they detected deviations and wobbles, indicating the possible presence of a moon.

After the transit ended, Hubble detected a second and much smaller decrease in the star’s brightness approximately 3.5 hours later, consistent with the effect of a moon trailing the planet.

Describing the moment of discovery, scientist David Kipping said: “It was definitely a shocking moment to see that light curve. My heart started beating a little faster and I just kept looking at that signature.”

In addition to this second dip in the light curve, Hubble detected the planet’s transit more than one hour earlier than predicted, bolstering the exomoon theory. This is consistent with a model of the system in which the planet and its moon orbit a common centre of gravity, causing the planet to wobble away from its predicted location.

Alex Teachey, the graduate student who led the discovery, added: “It is an exciting reminder of how little we really know about distant planetary systems and the great spirit of discovery exoplanetary science embodies.”

Colm Gorey was a senior journalist with Silicon Republic