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Ancient planets were snatched from another galaxy

Paul Sutherland, Feature writer
Jun 4, 2014, 7:00 UTC

Sen—The latest discovery to be announced of two new exoplanets comes with a major twist, because they and their home star are thought to have been born in another galaxy. Despite that, this new solar system lies right on our cosmic doorstep.

Astronomers are further excited by the find because one of the planets orbits in its own sun’s habitable zone, where any water essential for life could exist in liquid form.

Kapteyn’s Star, in the southern constellation on Pictor, is one of the closest stars to the Sun, just 13 light-years from Earth. Yet it appears to have originated in a dwarf galaxy that was disrupted and absorbed by our Milky Way billions of years ago.

Its proximity to us was first noted by the Dutch astronomer, Jacobus Kapteyn, in 1898, who spotted that it moved relatively quickly against the sky background. This very high speed, known as “proper motion” covers more than 8 arcseconds per year, which is about 1/250th of the apparent diameter of the Moon.

Only one star is known to have a greater proper motion—Barnard’s Star, in the constellation of Ophiuchus, with 10.3 arcseconds.

Kapteyn’s Star is one of an extended group that belong to the Galactic halo, travelling around the Milky Way’s centre in very elliptical orbits. An international team studied its light with the HARPS spectrometer at the European Southern Observatory’s La Silla observatory in Chile and the HIRES spectrometer at the W.M. Keck Observatory in Hawaii to measure tiny wobbles in the star’s motion that could give away the presence of planets.


An impression of Kapteyn's star and its planets against the streams of stars resulting from the merger of the dwarf galaxy with the Milky Way. Image credit: University of California Irvine / University of California Santa Cruz

The technique allowed the scientists—led by Dr Guillem Anglada-Escude, of Queen Mary University of London, with colleagues Pamela Arriagada, Paul Butler, Steve Shectman, Jeff Crane, and Ian Thompson from the Carnegie Institution for Science in Washington, DC—to work out the size and orbital periods of the planets detected. The worlds have been labelled Kapteyn b and Kapteyn c.

Kapteyn b is at least five times the mass of the Earth and orbits its star every 48 days. Kapteyn c is a more massive super-Earth which takes 121 days to go round the star. It is thought to be too cold to have liquid water. Further studies will be made to check the planets’ atmospheres for water.

Most exoplanets previously found lie many hundreds of light-years away and were detected by NASA’s Kepler space telescope as they passed in transit across the faces of their parent stars. Kapteyn’s star, however, is the 25th closest star to the Sun.

Yet this red dwarf and its planets are thought to be around 11.5 billion years old, or two and a half times the age of our Solar System, and only about two billion years younger than the Universe. They were snatched from a dwarf galaxy whose remains can still be seen in the southern sky as Omega Centauri, a cluster of hundreds of thousands of ancient stars.

Dr Anglada-Escude said: “We were surprised to find planets orbiting Kapteyn’s star. Previous data showed some irregular motion so we were looking for very short period planets when the new signals showed up loud and clear.”

Colleague Dr Arriagada said: “Finding a stable planetary system with a potentially habitable planet orbiting one of the very nearest stars in the sky is mind blowing. This is one more piece of evidence that nearly all stars have planets, and that potentially habitable planets in our Galaxy are as common as grains of sand on a beach.”

The discovery is annoucned in the journal the Monthly Notices of the Royal Astronomical Society.