Today, an international group of researchers from University College of London, University of Sheffield, University of Warwick and Universidad de Valparaiso announced the discovery of rocky debris surrounding a binary star system roughly 1,000 light-years away from Earth. While planets orbiting a double sun are nothing new, to date we've only ever detected gas giants. This latest finding suggests that terrestrial planets could be out there as well, making Tatooine more of a reality than ever before.
SDSS 1557 (the catalog designation for this system) consists of a white dwarf and a brown dwarf orbiting each other ever 2.27 hours. However, when astronomers first observed the system, they believed the white dwarf star to be alone.
Using the Gemini Observatory South telescope and the European Southern Observatory's Very Large Telescope (both in Chile), they collected data that showed the star was surrounded by dust and unexpectedly high in metallicity (which to an astronomer means elements higher than helium). Stars naturally have small percentages of elements such as carbon and oxygen, but more than the standard amounts of silicon and magnesium in SDSS 1557 indicated something else was going on.
It turns out, not only was there a brown dwarf star hiding in the dust signal, there was a steady stream of rocky debris being accreted onto the white dwarf, in a process astronomers amusingly refer to as "stellar pollution." Based on the amount of debris detected since observations began in 2010, it likely came from an asteroid at least 4 km in diameter. That might not sound like much, but observations of other "polluted" white dwarfs indicate accretion of objects on order of hundreds of kilometers in diameter [for reference, the diameter of Ceres, the largest asteroid in our Solar System is over 900 km].
This news is exciting because the process of planet formation is an ongoing mystery. Until 1995, we only had one planetary system to go on: ours. And now we are finding systems all sorts of combinations of terrestrial planets and gas giants and unexpected orbits. The birth of a solar system is chaotic to say the least, but the more examples we find, the more we learn.
White dwarfs are mature, adult stars way past their heyday. So anything we learn about surviving planets and/or planetary debris around them, speaks how planetary systems might evolve over a star's lifetime. The vast majority of all stars end their lives as white dwarfs, which requires them to turn into red giants along the way. The red giant is thought to spell disaster for terrestrial planets as the star generally becomes a thousand times brighter and a hundred times bigger. Any small rocky planets closer in to the star are in danger of sterilization, if not outright vaporization. To date, more than three dozen (single) white dwarfs have been found to be accreting planetary debris indicating the presence of minor planets well into the star's 'golden years.'
Now with SDSS 1557, we see evidence that minor planets may survive in circumbinary systems as well. "With the discovery of asteroid debris in the SDSS 1557 system, we see clear signatures of rocky planet assembly via large asteroids that formed, helping us understand how rocky exoplanets are made in double star systems." said Jay Farihi from UCL's Department of Physics & Astronomy.
The next step is to do follow up observations with the Hubble Space Telescope to study the rocky debris in more detail. Which means, once again, Hubble gotchu.