Red dwarf stars could strip away planetary protection
Red dwarf stars are the commonest type of stars, making up about 75% of the stars in our Galaxy. They are much smaller and much less massive than our Sun and for that reason a lot dimmer. If planets are found around these stars, then given the number of red dwarfs, life could then be commonplace. But a group of scientists led by Dr Aline Vidotto of the University of St Andrews has cast doubt on this idea. Their work suggests that the magnetic fields of red dwarfs could squash down those found around planets like the Earth, leaving any life vulnerable to radiation from space. Dr Vidotto will present her work on Tuesday 2 July at the National Astronomy Meeting in St Andrews, Scotland.
Because of their faintness, even small planets in orbit around red dwarf stars block out a significant amount of light if they pass between the star and the Earth. The low masses of these stars also mean that the gravitational pull of an Earth-sized planet is enough to make its star wobble as the planet moves around it. This motion leads to a back and forth shift in lines in the spectrum of the star that can be detected with telescopes on Earth.
Red dwarf stars are cooler than the Sun, so the so-called habitable or 'Goldilocks' zone where life could develop is much closer in than in our own Solar System. Planets in the Goldilocks zone are at just the right temperature for liquid water to be found on their surfaces. All this makes red dwarfs prime targets in the search for Earth-like planets elsewhere in the Galaxy. But there are other important factors that make planets good places to live such as a reasonably thick atmosphere.
Over billions of years, the impact of charged particles from space can erode a planetary atmosphere. Planets that have relatively strong magnetic fields (like the Earth) deflect these particles, at least within the surrounding region known as the magnetosphere, adding in a layer of protection for their atmospheres and making them more suitable for the creation and development of life.
A large proportion of the particles hitting a planet originate from the 'stellar wind' flowing off its host star. The pressure of these particles pushes against the magnetosphere of a planet, so whenever the stellar wind is strong, it compresses this magnetic shield. In the case of the Earth, the magnetosphere normally extends out to about 70000 km.
Especially when they are relatively young, red dwarf stars have powerful magnetic fields of their own, with about a dozen of these being seen directly in recent years. These may have a very different effect on orbiting planets. Aline and her team have found that the extreme pressure from these fields may be strong enough to compress planetary magnetospheres enough that their atmospheres are stripped away completely over time, effectively rendering these worlds uninhabitable.
The new work shows that if the Earth was in orbit at the inner edge of the Goldilocks zone of a young red dwarf star, equivalent to the way it orbits the Sun, its magnetosphere would extend no more than 35000 km and could even be crushed into the surface of the planet. To be benign environments for the development of life, Earth-like planets around red dwarfs will need very strong magnetic fields or be significantly further away from their stars, in which case they might be too cold for liquid water.
As stars age, their magnetic fields weaken, offering some respite for any planets in orbit around red dwarfs. The pace at which this happens will be a critical factor in how well the planetary atmospheres survive, but one way of refining the search for these objects will be to measure the speed of rotation of their stars, which also declines with age.
"Our work suggests that red dwarf stars with rotation periods larger than about one to a few months will have magnetic fields that won't significantly squash the magnetosphere of an Earth-analogue planet orbiting inside the habitable zone of its host star", says Aline. "Astronomers will have to take this on board in their search for life elsewhere – the conditions for habitability are turning out to be a lot more complex than we thought."
Dr Aline Vidotto
Dr Moira Jardine
Dr Alexander Russell
Dr Robert Massey
Royal Astronomical Society
Ms Emma Shea
Head of Development Communications
Landline numbers in NAM 2013 press room (available from 9 a.m. to 5 p.m. from 1-4 July, 9 a.m. to 3 p.m. 5 July):
Tel: +44 (0)1334 462231, +44 (0)1334 46 2232
Images and captions
An artist's impression of how Mars lost most of its atmosphere after the loss of its magnetic field. Planets around red dwarfs may suffer a similar fate. Credit: NASA
Notes for editors
Bringing together more than 600 astronomers and space scientists, the RAS National Astronomy Meeting (NAM 2013) will take place from 1-5 July 2013 at the University of St Andrews, Scotland. The conference is held in conjunction with the UK Solar Physics (UKSP: www.uksolphys.org) and Magnetosphere Ionosphere Solar Terrestrial (MIST: www.mist.ac.uk) meetings. NAM 2013 is principally sponsored by the RAS, STFC and the University of St Andrews and will form part of the ongoing programme to celebrate the University's 600th anniversary.
Meeting arrangements and a full and up to date schedule of the scientific programme can be found on the official website at http://www.nam2013.co.uk
The Royal Astronomical Society (RAS: www.ras.org.uk, Twitter: @royalastrosoc), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organises 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 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.
The Science and Technology Facilities Council (STFC: www.stfc.ac.uk, Twitter: @stfc_matters) is keeping the UK at the forefront of international science and tackling some of the most significant challenges facing society such as meeting our future energy needs, monitoring and understanding climate change, and global security. The Council has a broad science portfolio and works with the academic and industrial communities to share its expertise in materials science, space and ground-based astronomy technologies, laser science, microelectronics, wafer scale manufacturing, particle and nuclear physics, alternative energy production, radio communications and radar. It enables UK researchers to access leading international science facilities for example in the area of astronomy, the European Southern Observatory.
Founded in the 15th century, St Andrews is Scotland's first university and the third oldest in the English speaking world. Teaching began in the community of St Andrews in 1410 and the University was formally constituted by the issue of Papal Bull in 1413. The University is now one of Europe's most research intensive seats of learning – over a quarter of its turnover comes from research grants and contracts. It is one of the top rated universities in Europe for research, teaching quality and student satisfaction and is consistently ranked among the UK's top five in leading independent league tables produced by The Times, The Guardian and the Sunday Times.
The University is currently celebrating its 600th anniversary and pursuing a £100 million fundraising campaign, launched by Patron and alumnus HRH Prince William Duke of Cambridge, including £4 million to fund the creation of an 'Other Worlds' Think Tank and Observatory. The new think tank and Observatory project will extend the University of St Andrews' flagship work on extra-solar planets, and provide a creative environment for problem-focused research, education and continuing public engagement.
For further information go to: www.st-andrews.ac.uk/600/