There are times when it seems the Venn diagram of astronomers and science fiction writers is all intersection and no differences. This is one – OK, two – of those times. An event in 1183 CE recorded by Chinese and Japanese astronomers appeared to be a “guest star” as bright as Saturn that disappeared almost as fast as it arrived. No one has been able to explain it, but astronomers now think it was a supernova that left behind a zombie star. A what? And, while other astronomers look for signs of life on conventional exoplanets orbiting ordinary stars, a new group believes the best places to find like are on the moons orbiting so-called ‘rogue planets’ that are untethered to a star. Zombie stars and rogue planets – science, science fiction or band names?
In a report titled “The remnant and origin of the historical supernova 1181AD” in the preprint journal arXiv, co-author Quentin Parker describes how his team at the University of Hong Kong dug through archived data NASA’s Wide-field Infrared Survey Explorer (WISE) for clues to the identity of SN 1181, called a “guest star” by ancient astronomers who had not yet discovered supernovas. They discovered the hottest known rare Wolf Rayet star – extremely hot stars that have lost their outer hydrogen and are fusing helium or heavier elements in the core – in the right location to be SN 1181, and named in Parker’s star after Quentin.
Surrounding Parker’s star was a nebula from an explosive event 1000 years ago – also matching the creation of SN 1181 – which they named Pa 30. There was just one problem – after the supernova, there should have been nothing left … yet there was Parker’s star. The expansion speed of Pa 30 solved the mystery – its extremely slow 1,100 km/s expansion speed meant Parker’s star was a rare kind of supernovae that doesn’t completely detonate its star … leaving behind a zombie star. There’s no word on whether Parker is happy about his star being a zombie, even though it’s the only one in the Milky Way.
As strange as zombie stars are rogue planets -- planetary-mass objects that may have been ejected from a star’s gravity or never orbited a star at all. The Milky Way is estimated to have billions to trillions of rogue planets floating around aimlessly, like galactic pedestrians looking at their phones. As mysterious as rogue planets are, even stranger is the strong likelihood that Jupiter-sized ones have exomoons orbiting them, and those moons could have water and even an atmosphere – key ingredients to support life. Before you can say, “Isn’t it too cold?”, a recent study published in the International Journal of Astrobiology found that these exomoons would receive enough heat and life-starting spark from cosmic rays and tidal heating -- thermal increases caused by tides. Combined, they would provide enough heat to support life.
If you’re skeptical, Tommaso Grassi from University Observatory Munich, Ludwig-Maximilians-University and his team modeled an Earth-mass moon orbiting a Jupiter-mass rogue planet and found a rogue exomoon could maintain liquid water on its surface for life to form -- much less than on Earth, but enough with a rich atmosphere. they’re recommendation is to scan rogue exomoons with infrared and radio astronomy.
Real zombie stars and rogue planets with life-bearing rogue exomoons – the job of science fiction writers just got tougher.