Free Astronomy Magazine July-August 2019

31 JULY-AUGUST 2019 SPACE CHRONICLES general all white dwarfs, extremely dense” . The white dwarf’s gravity is so strong − about 100,000 times that of the Earth’s − that a typical aster- oid would be ripped apart by gravi- tational forces if it passed too close. Professor Boris Gaensicke, co-author from the Department of Physics, added: “The planetesimal we have discovered is deep into the gravita- tional well of the white dwarf, much closer to it than we would expect to find anything still alive. That is only possible because it must be very dense and/or very likely to have in- ternal strength that holds it to- gether, so we propose that it is composed largely of iron and nickel” . And he explained: “If it was pure iron it could survive where it lives now, but equally it could be a body that is rich in iron but with in- ternal strength to hold it together, which is consistent with the plan- etesimal being a fairly massive frag- ment of a planet core. If correct, the original body was at least hundreds of kilometres in diameter because it is only at that point planets begin to differentiate − like oil on water − and have heavier elements sink to form a metallic core.” The discovery offers a hint as to what planets may reside in other solar systems, and a glimpse into the future of our own. Christopher Manser said: “As stars age they grow into red giants, which ‘clean out’ much of the inner part of their planetary system. In our Solar System, the Sun will ex- pand up to where the Earth cur- rently orbits, and will wipe out Earth, Mercury, and Venus. Mars and beyond will survive and will move further out. The general con- sensus is that 5-6 billion years from now, our Solar System will be a white dwarf in place of the Sun, or- bited by Mars, Jupiter, Saturn, the outer planets, as well as asteroids and comets. Gravitational interac- tions are likely to happen in such remnants of planetary systems, meaning the bigger planets can eas- ily nudge the smaller bodies onto an orbit that takes them close to the white dwarf, where they get shred- ded by its enormous gravity”. ! A rtist’s impression of a planetary fragment orbits the star SDSS J122859.93+104032.9, leaving a tail of gas in its wake. [Mark Garlick]