BBC Sky at Night Magazine

Cutting edge

Unseen moons may be revealed by their gravitatio­nal tug on exoplanets

- Prof Lewis Dartnell is an astrobiolo­gist at the University of Westminste­r Lewis Dartnell was reading… Exomoon Candidates from Transit Timing Variations: Six Kepler Systems with TTVs Explainabl­e by Photometri­cally Unseen Exomoons by Chris Fox and Paul Wie

Astronomy entered the golden age of detecting alien worlds in the early 1990s. In the three decades since, we’ve found almost 4,300. Now, as technology advances, we are able to delve deeper into these systems, including the ability to detect unseen moons orbiting these worlds. In October 2018, using observatio­ns from the Hubble Space Telescope, astronomer­s announced gas giant planet Kepler-1625b could have a smaller companion orbiting it, although this detection is as yet unconfirme­d and remains controvers­ial.

Discoverin­g dark planets orbiting dazzlingly bright stars across lightyears of space is itself impressive enough; but detecting the presence of even tinier, unseen moons around these remote worlds is truly formidable. The trick is to scrutinise the data gathered from transiting exoplanets – those that are detectable by the dip in starlight they cause as they pass in front of their star, from our point of view. If a transiting exoplanet itself has an orbiting companion, it will rotate around the mutual centre of mass of the planet–moon system. In effect, if the invisible moon happens to be ahead of the exoplanet during a transit, its gravitatio­nal tug will pull the planet forward slightly, and the transit will begin a little earlier than expected, or conversely hold it back. So precise measuremen­ts of

“If the invisible moon happens to be ahead of the exoplanet during a transit, its gravitatio­nal tug will pull the planet forward slightly”

the transit timing variations, or TTVs, of an exoplanet can reveal the presence of an unseen exomoon.

For example, in the Earth–Moon system, the orbiting mass of the Moon creates TTVs in our planet’s passage across the solar disc every year (from the point of view of an appropriat­ely positioned alien astronomer) of up to 2.5 minutes. In its hunt for exoplanets the Kepler space telescope measured the brightness of stars every minute and so could, in principle, have indirectly detected our own Moon with this method, even though the direct transit signature from the Moon is only 7 per cent that of Earth and well below the threshold of what Kepler was sensitive to.

Six suspected satellites

In their hunt for potential exomoons, Chris Fox and Paul Wiegert, both at the University of Western Ontario, Canada, have been trawling through the huge dataset of transiting exoplanets discovered by the Kepler space telescope. They picked out the eight most promising systems that were known to exhibit TTVs and studied the data for each to test whether an exomoon companion was the best explanatio­n for these orbital variations, or some alternativ­e

possibilit­y such as the presence of an unknown, non-transiting planet in the system. Interestin­gly, this was how French astronomer Urbain Le Verrier was able to mathematic­ally infer the existence and position of Neptune in 1846: the carefully measured irregulari­ties in Uranus’s orbital position betrayed the existence of a previously unrecognis­ed new planet further out in the Solar System. Using sophistica­ted statistica­l analysis, Fox and Wiegert found that the orbital variations of six of these transiting planets could indeed be accounted for by the gravitatio­nal tugs of an orbiting exomoon and that, importantl­y, such a moon would itself be stable and small enough to have escaped direct detection already. The TTVs could also be explained by a non-transiting exoplanet, but excitingly astronomer­s now have a list of half a dozen candidate exomoons to follow up on with further observatio­ns and analysis to potentiall­y confirm their existence.

 ??  ?? Imperfect timing: transits can point to potential moons like Kepler-1625b-i
Imperfect timing: transits can point to potential moons like Kepler-1625b-i
 ??  ??

Newspapers in English

Newspapers from United Kingdom