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Under pressure - extreme atmosphere stripping may limit exoplanets’ habitability

Last Updated on Tuesday, 04 July 2017 11:58
Published on Monday, 03 July 2017 14:52

New models of massive stellar eruptions hint at an extra layer of complexity when considering whether an exoplanet may be habitable or not. Models developed for our own Sun have now been applied to cool stars favoured by exoplanet hunters, in research presented by Dr Christina Kay, of the NASA Goddard Flight Center, on Monday 3rd July at the National Astronomy Meeting at the University of Hull.

 

Coronal mass ejections (CMEs) are huge explosions of plasma and magnetic field that routinely erupt from the Sun and other stars. They are a fundamental factor in so called “space weather”, and are already known to potentially disrupt satellites and other electronic equipment on Earth. However, scientists have shown that the effects of space weather may also have a significant impact on the potential habitability of planets around cool, low mass stars - a popular target in the search for Earth-like exoplanets.

 

Traditionally an exoplanet is considered “habitable” if its orbit corresponds to a temperature where liquid water can exist. Low mass stars are cooler, and therefore should have habitable zones much closer in to the star than in our own solar system, but their CMEs should be much stronger due to their enhanced magnetic fields.

thumb HD 189733bArtist’s impression of HD189733b, showing the planet’s atmosphere being stripped by the radiation from its parent star. Credit: Ron Miller. Click for a larger image

 When a CME impacts a planet, it compresses the planet’s magnetosphere, a protective magnetic bubble shielding the planet. Extreme CMEs can exert enough pressure to shrink a magnetosphere so much that it exposes a planet’s atmosphere, which can then be swept away from the planet. This could in turn leave the planetary surface and any potential developing lifeforms exposed to harmful X-rays from the nearby host star.

 

The team built on recent work done at Boston University, taking information about CMEs in our own solar system and applying it to a cool star system. “We figured that the CMEs would be more powerful and more frequent than solar CMEs, but what was unexpected was where the CMEs ended up” said Christina Kay, who led the research during her PhD work.

 

The team modelled the trajectory of theoretical CMEs from the cool star V374 Pegasi and found that the strong magnetic fields of the star push most CMEs down to the Astrophysical Current Sheet (ACS), the surface corresponding to the minimum magnetic field strength at each distance, where they remain trapped.

 

"While these cool stars may be the most abundant, and seem to offer the best prospects for finding life elsewhere, we find that they can be a lot more dangerous to live around due to their CMEs” said Marc Kornbleuth, a graduate student involved in the project.

 

The results suggest that an exoplanet would need a magnetic field ten to several thousand times that of Earth’s to shield their atmosphere from the cool star’s CMEs. As many as five impacts a day could occur for planets near the ACS, but the rate decreases to one every other day for planets with an inclined orbit.

 

Merav Opher, who advised the work, commented, “This work is pioneering in the sense that we are just now starting to explore space weather effects on exoplanets, which will have to be taken into account when discussing the habitability of planets near very active stars.”

  


Media contacts

 

NAM press office (Monday 3 – Thursday 6 July)

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Robert Massey

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Anita Heward

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Morgan Hollis

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Science contacts

 

Christina Kay

NASA Goddard Space Flight Center

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Images and captions

 

Animation of the trajectory of a CME. The coloured sphere represents the stellar surface with different colours representing the regions above and below the ACS. The blue surface shows the CME front and the white line traces its trajectory as it deflects down to the ACS, where it remains trapped. Credit: C. Kay / NASA Goddard Space Flight Center

 

Artist’s impression of HD189733b, showing the planet’s atmosphere being stripped by the radiation from its parent star. Credit: Ron Miller

 


Further information

 

The new work appears in “Probability of CME impact on exoplanets orbiting M dwarfs and solar-like stars”, Christina Kay, Merav Opher, and Marc Kornbleuth, The Astrophysical Journal 826 (2), 2016. A copy of the paper is available from https://arxiv.org/abs/1605.02683 

 


Notes for editors

 

Running from 2 to 6 July, the RAS National Astronomy Meeting 2017 (NAM 2017, http://nam2017.org) takes place this year at the University of Hull. NAM 2017 will bring together around 500 space scientists and astronomers to discuss the latest research in their respective fields. The conference is principally sponsored by the Royal Astronomical Society and the Science and Technology Facilities Council.

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The Royal Astronomical Society (RAS, www.ras.org.uk), 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 4000 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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STFC's Astronomy and Space Science programme provides support for a wide range of facilities, research groups and individuals in order to investigate some of the highest priority questions in astrophysics, cosmology and solar system science. STFC's astronomy and space science programme is delivered through grant funding for research activities, and also through support of technical activities at STFC's UK Astronomy Technology Centre and RAL Space at the Rutherford Appleton Laboratory. STFC also supports UK astronomy through the international European Southern Observatory. 

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