Red dwarf stars could strip away planetary protection

Jul 02, 2013
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

(Phys.org) —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 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 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 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 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 . Planets that have relatively strong magnetic fields (like the Earth) deflect these particles, at least within the surrounding region known as the , 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 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 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."

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GSwift7
5 / 5 (5) Jul 02, 2013
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


Yes, there appear to be a large number of competing and complimentary effects that need to be accounted for when determining the potential for habitability of a given planet.

I would suggest that a super earth would be more protected from the above effect than a true earth analogue (due to higher surface gravity, which should hold atmosphere better).

The bigggest problem I have seen regarding red dwarves is solar flares, since red dwarves tend to be more unstable than average. Still, with the vast number of red dwarves out there, if it is even slightly possible to live around a red dwarf, then it is statistically overwhelming that the needed conditions would be met in abundance.
Torbjorn_Larsson_OM
5 / 5 (5) Jul 02, 2013
"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."

Or, as I (and GSwift7) evidently have to note again and again, planets can simply start out with more volatiles, a thicker atmosphere, and/or more mass, a higher surface gravity, to offset initial CMEs and now evidently magnetic fields stripping over time, as well as uneven heating from tidal locks. Or they can happen to orbit the calmer M stars, which IIRC are ~ 50 % of them.

In any case, these effects cuts down the number of habitables, they don't obliterate them as sensationalist articles claim. (" cast doubt on this idea [of commonplace life]".)
GSwift7
3 / 5 (4) Jul 03, 2013
Or, as I (and GSwift7) evidently have to note again and again

and

...as sensationalist articles claim


Yeah, I've noticed exoplanetology has become awash with people trying to jump onto the bandwaggon and get their name on some of the discoveries. Unfortunately, this results in people who are slightly (or in some cases, alarmingly) out of their field of expertise making layperson mistakes in press release quotes.

You can spot the quasi-amatures like the one above because they go beyond simply reporting a finding, and expound on the finding with all kinds of unjustified or incorrect conclusions.
katesisco
1 / 5 (3) Jul 03, 2013
I was thinking that dead magnetars would work also. Supposed to be a lot of them out there. Red dwarfs are supposed to live forever due to the fuel not being layered and being circulated continuously, according to science.
I like the discoveries of Methuselah stars, born in pure gas and still standing.
Q-Star
1 / 5 (4) Jul 03, 2013
Ah Katie, there ya are and so much like yourself. Still the science site troll sure.

I was thinking that dead magnetars would work also.


Maybe ya would be kind enough to describe this phenomena, "dead magnetars".

Supposed to be a lot of them out there.


Not as many as there are cranks out there.

Red dwarfs are supposed to live forever due to the fuel not being layered and being circulated continuously, according to science.


Actually that is almost correct, that should be "according to pseudo-science". How long is forever?

I like the discoveries of Methuselah stars, born in pure gas and still standing.


I like that ya like it,,,, but I do wish ya would like it on one of those new-age pseudo-science sites.

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