A planet in the habitable zone has been discovered in our galactic neighborhood. The planet and its star don't look a lot like our own system, but they represent the second closest known opportunity for liquid water at the surface of another world. The star in question is thought to have a truly remarkable history.
Kapteyn's Star is a red dwarf 13 light years from the Sun. While an enormous distance by terrestrial standards, this makes it the 25th closest star system to us out of the hundreds of millions in galaxy. Although intrinsically very faint, its closeness means it can be seen with binoculars near Canopus in the southern skies. The star is named for Jacobus Kapteyn who noticed in 1898 how rapidly it is moving across the sky, its 8 arc seconds exceeded only by Barnard's Star.
Now two “superearths” have been reported in the Monthly Notices of the Royal Astronomical Society (paper not yet online) to orbit it. The discovery was made using the Doppler wobble method and three of the world's largest telescopes.
"We were surprised to find planets orbiting Kapteyn's star," said team leader Guillem Anglada-Escude of Queen Mary University, London. "Previous data showed some moderate excess of variability, so we were looking for very short period planets when the new signals showed up loud and clear."
Kapteyn c has an orbit of 121 days, but the star's low luminosity is expected to make this too cold for water at the surface. However, it is Kapteyn b that is really generating excitement. Its 48 day orbit is shorter than Mercury, but with such a dim star this translates to temperatures suited to liquid water.
Sadly, the prospects of complex life on a world such as this have just taken a hit, with evidence stellar winds would strip the atmosphere. On the plus side, Kapteyn b has five times Earth's mass, bolstering prospects for a more magnetic shield powerful enough to provide protection. Kapteyn's star is thought to be 11.5 billion years old, giving plenty of time for life to evolve if conditions are harsh rather than impossible.
PHL / UPR Arecibo / Aladin Sky Atlas A comparison of Kapteyn b and Earth.
If intelligent life has managed to appear on Kapteyn b it has been offered a very interesting view of the local universe. Kapteyn's Star is part of a group that originated in a dwarf galaxy, probably the one that once surrounded globular cluster Omega Centauri. A dramatic three body interaction expelled it from its home to travel the 16,000 light years to become a retrograde galactic halo object that came within 7 light years of the sun 10,000 years ago.
Globular clusters are home to very old stars, and Kapteyn's Star offers us a rare opportunity for a close-up view of an object that probably dates to just two billion years after the formation of the universe. In keeping with its advanced age Kapteyn's Star has only 14% of the concentration of metals as the sun. One of the major questions in planetary studies is whether such low metal stars are likely to have planets.
“It does make you wonder what kind of life could have evolved on those planets over such a long time," said Anglada-Escude.
PHL / UPR Arecibo / Aladin Sky Atlas The view from Kapteyn b shortly after it was thrown out of the dwarf galaxy in which it was born.