Ancient worlds from another galaxy next door
An international team of scientists, led by astronomers at Queen Mary University of London, report of two new planets orbiting Kapteyn’s star, one of the oldest stars found near the Sun. One of the newly-discovered planets could be ripe for life as it orbits at the right distance to the star to allow liquid water on its surface. The team report their findings in a letter to the journal Monthly Notices of the Royal Astronomical Society.
Discovered at the end of the 19th century and named after the Dutch astronomer who discovered it (Jacobus Kapteyn), Kapteyn’s star is the second fastest moving star in the sky and belongs to the galactic halo, an extended cloud of stars orbiting our galaxy. With a third of the mass of the sun, this red dwarf star can be seen in the southern constellation of Pictor with an amateur telescope.
The astronomers used new data from the HARPS spectrometer at the ESO La Silla observatory in Chile to measure tiny periodic changes in the motion of the star. Using the Doppler Effect, which shifts the star’s light spectrum depending on its velocity, the scientists can work out some properties of these planets, such as their masses and periods of orbit.
“We were surprised to find planets orbiting Kapteyn’s star. 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,” explains lead author Dr Guillem Anglada-Escude, from QMUL’s School of Physics and Astronomy.
Based on the data collected, the planet Kapetyn b is at least five times as massive as the Earth and it orbits the star every 48 days. This means the planet is warm enough for liquid water to be present on its surface. The second planet, Kapteyn c is quite different: its year lasts for 121 days and astronomers think it’s too cold to support liquid water.
At the moment, only a few properties of the planets are known: approximate masses, orbital periods, and distances to the star. By measuring the atmosphere of these planets with next-generation instruments, scientists will try to find out whether they can bear water.
Typical planetary systems detected by NASA's Kepler mission are hundreds of light years away. In contrast, Kapteyn's star is the 25th nearest star to the sun and is only 13 light years away from Earth.
What makes this discovery different however, is the peculiar story of the star. Kapteyn's star was born in a dwarf galaxy absorbed and disrupted by the early Milky Way. This galactic disruption event put the star in its fast halo orbit. The likely remnant core of the original dwarf galaxy is Omega Centauri, an enigmatic globular cluster 16, 000 light years from earth which contains hundreds of thousands of similarly old suns. This sets the most likely age of the planets at 11.5 billion years; which is 2.5 times older than Earth and 'only' 2 billion years younger than the universe itself (around 13.7 billion years).
Dr Anglada-Escude adds: “It does make you wonder what kind of life could have evolved on those planets over such a long time.”
Professor Richard Nelson, Head of the Astronomy Unit at QMUL, who didn't participate in the research, commented: "This discovery is very exciting. It suggests that many potentially habitable worlds will be found in the next years around nearby stars by ground-based and space-based observatories, such as PLATO. Until we have detected a larger number of them, the properties and possible habitability of the near-most planetary systems will remain mysterious.”
Image, video and captions
High-resolution image: An artist's impression of the red dwarf Kapteyn's star superimposed on a diagram showing its ejection from a nearby dwarf galaxy.
Simulation representing the galactic merging event that put Kapteyn star on its peculiar galactic orbit as a halo star. The simulation has been specifically produced to support this news release. Copyright remains to University of California - Irvine: Victor Robles, James Bullock and Miguel Rocha and University of California - Santa Cruz: Joel Primack
The new work appears in ‘Two planets around Kapteyn's star: a cold and a temperate super-Earth orbiting the nearest halo red-dwarf’ to be published by Oxford University Press in the journal Monthly Notices of the Royal Astronomical Society (MNRAS) in early July. See the preprint of this paper.
The discovery has prompted renowned science fiction writer, Alistair Reynolds to write a short story. Sad Kapteyn describes the arrival of a robotic interstellar probe reaching Kapteyn's planetary system, and a first exploratory survey of its planets in the far future. Alastair Reynolds worked as an astronomer at the European Space Agency, and later he became a full time science-fiction writer. The story is provided for use of this release only on a non-exclusive basis. Copyright remains to Alastair Reynolds 2014.
Notes for editors
Queen Mary University of London is one of the UK's leading research-focused higher education institutions with some 17,840 undergraduate and postgraduate students. A member of the Russell Group, it is amongst the largest of the colleges of the University of London. Queen Mary’s 4,000 staff deliver world class degree programmes and research across 21 academic departments and institutes, within three Faculties: Science and Engineering; Humanities and Social Sciences; and the School of Medicine and Dentistry.
Queen Mary is ranked 11th in the UK according to the Guardian analysis of the 2008 Research Assessment Exercise, and has been described as ‘the biggest star among the research-intensive institutions’ by the Times Higher Education. The College has a strong international reputation, with around 20 per cent of students coming from over 100 countries. Queen Mary has an annual turnover of £300m, research income worth £90m, and generates employment and output worth £600m to the UK economy each year. The College is unique amongst London's universities in being able to offer a completely integrated residential campus, with a 2,000-bed award-winning Student Village on its Mile End campus.
The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3,800 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.
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