Exo-Moons and Goblins

Article written by: Gavin Ramsay, resident Astronomer in the Armagh Observatory and Planetarium

The discovery of planets orbiting around other stars has been one of humankinds greatest achievements. The existance of these ‘exo-planets’ can challenge our perception of our place in the universe. Since the first confirmed detection of a planet around another normal star (51 Peg) in 1995, many thousands of planets have been discovered. Indeed, it is likely that most stars will have their own system of planets. It is important in answering the question – is there intelligent life out there?

With the discovery of exo-planets, astronomers then began to wonder whether these distant planets have moons, just like our Moon and Jupiter and Saturn – which currently have 79 and 62 known moons apiece. However, discovering ‘exo-moons’ around exo-planets is an extremely challenging task.

A pair of Columbia University astronomers using NASA’s Hubble Space Telescope and Kepler Space Telescope have assembled compelling evidence for the existence of a moon orbiting a gas-giant planet 8,000 light-years away. The image is an artist’s impression of what the system might look like.  Image Courtesy of Dan Durda and Columbia News

Exo-planets can initially be discovered when they are aligned so that they cause a dip in the host stars brightness by a tiny – but measureable – amount. Exo-planets cause a dip once every time it orbits its host star – the planets ‘year’ or orbital period. Exo-moons can be detected by making many and extremely careful observations of the dip and searching for discrepancies in its profile.

This is what two astronomers from Columbia University in New York, Alex Teachey and David Kipping, have done. The host star, Kepler 1625, lies 4000 light years away in the constellation of Cygnus the Swan. It has at least one planet, named Kepler 1625b, which orbits around its host star every 287 days and has a mass several times that of Jupiter. The astronomers made a series of detailed observations using the Hubble Space Telescope to study the dip when the planet crosses our line of sight with the host star. They concluded that the exo-moon has a mass and radius similar to the planet Neptune.

This discovery raises many interesting questions including how such a planet-moon system could have originated and how stable it is. More observations and computer simulations are planned to help address these questions.

Closer to home, although still rather distant, astronomers from the Carnegie Institute of Science in Washington DC, have found a very distant new member of the Solar System. Dubbed the ‘Goblin’ because it was discovered near Halloween, it never gets closer to the Sun than twice the distance that Pluto gets to the Sun, and is usually much further away.  This discovery was made using the giant Japanese Subaru telescope on Hawaii. It shows that our understanding of the most distant parts of the Solar System is far from complete.

 

For more information on ExoMoons you can visit the sites below:

http://hubblesite.org/news_release/news/2018-45

https://news.columbia.edu/exomoon

For more information on the Goblin you can visit the site below:

https://www.theguardian.com/science/2018/oct/02/dwarf-planet-the-goblin-discovery-planet-nine-oort-cloud