Earth-like planets around small stars likely have protective magnetic fields, aiding chance for life

September 29, 2015 by Peter Kelley
This artist's concept depicts a planetary system. Credit: NASA/JPL-Caltech

Earth-like planets orbiting close to small stars probably have magnetic fields that protect them from stellar radiation and help maintain surface conditions that could be conducive to life, according to research from astronomers at the University of Washington.

A planet's emanates from its core and is thought to deflect the charged particles of the stellar wind, protecting the atmosphere from being lost to space. Magnetic fields, born from the cooling of a planet's interior, could also protect life on the surface from harmful radiation, as the Earth's magnetic field protects us.

Low-mass stars are among the most common in the universe. Planets orbiting near such stars are easier for astronomers to target for study because when they transit, or pass in front of, their , they block a larger fraction of the light than if they transited a more massive star. But because such a star is small and dim, its habitable zone—where an orbiting planet gets the heat necessary to maintain life-friendly liquid water on the surface—also lies relatively close in.

And a planet so close to its star is subject to the star's powerful gravitational pull, which could cause it to become tidally locked, with the same side forever facing its host star, as the moon is with the Earth. That same gravitational tug from the star also creates tidally generated heat inside the planet, or . Tidal heating is responsible for driving the most volcanically active body in our solar system, Jupiter's moon Io.

In a paper published Sept. 22 in the journal Astrobiology, lead author Peter Driscoll sought to determine the fate of such worlds across time: "The question I wanted to ask is, around these small stars, where people are going to look for , are these planets going to be roasted by gravitational tides?" He was curious, too, about the effect of tidal heating on magnetic fields across long periods of time.

The research combined models of orbital interactions and heating by Rory Barnes, assistant professor of astronomy, with those of thermal evolution of planetary interiors done by Driscoll, who began this work as a UW postdoctoral fellow and is now a geophysicist at the Carnegie Institution for Science in Washington, D.C.

Their simulations ranged from one stellar mass—stars the size of our sun—down to about one-tenth of that size. By merging their models, they were able, Barnes said, "to produce a more realistic picture of what is happening inside these planets."

Barnes said there has been a general feeling in the astronomical community that tidally locked planets are unlikely to have protective magnetic fields "and therefore are completely at the mercy of their star." This research suggests that assumption false.

Far from being harmful to a planet's magnetic field, tidal heating can actually help it along—and in doing so also help the chance for habitability.

This is because of the somewhat counterintuitive fact that the more tidal heating a planetary mantle experiences, the better it is at dissipating its heat, thereby cooling the core, which in turn helps create the magnetic field.

Barnes said that in computer simulations they were able to generate magnetic fields for the lifetimes of these planets, in most cases. "I was excited to see that tidal heating can actually save a planet in the sense that it allows cooling of the core. That's the dominant way to form magnetic fields."

And since small or low mass stars are particularly active early in their lives—for the first few billion years or so—"magnetic fields can exist precisely when life needs them the most."

Driscoll and Barnes also found through orbital calculations that the tidal heating process is more extreme for planets in the habitable zone around very small stars, or those less than half the mass of the sun.

For planets in eccentric, or noncircular orbits around such low mass stars, they found that these orbits tend to become more circular during the time of extreme tidal heating. Once that circularization takes place, the planet stops experiencing any tidal heating at all.

The research was done through the Virtual Planetary Laboratory, a UW-based interdisciplinary research group funded through the NASA Astrobiology Institute.

"These preliminary results are promising, but we still don't know how they would change for a planet like Venus, where slow planetary cooling is already hindering magnetic field generation," Driscoll said. "In the future, exoplanetary magnetic fields could be observable, so we expect there to be a growing interest in this field going forward."

Explore further: Companion planets can increase old worlds' chance at life

More information: "Tidal Heating of Earth-like Exoplanets around M Stars: Thermal, Magnetic, and Orbital Evolutions." Astrobiology. September 2015, 15(9): 739-760. DOI: 10.1089/ast.2015.1325

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11 comments

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Torbjorn_Larsson_OM
4.7 / 5 (12) Sep 29, 2015
Wow! A series of good news today: extant surface water on Mars, cell size regulation finally understood, M star HZ zone planets - i.e. the majority of them - habitable. Makes you wonder what is next!?
Whydening Gyre
5 / 5 (4) Sep 29, 2015
First contact...?
viko_mx
1 / 5 (6) Sep 29, 2015

You remind me for the comedies produced per kilogram for which is standard practice laughing audience to maintain laughing at the moments which their writers hope to be funny for the viewers. In this way invite the audience to laugh at silly jokes, hoping that laughing is contagious. The analogy with your duty admiration for purely speculative articles would hardly left unnoticed.

JustAnotherGuy
4.5 / 5 (11) Sep 29, 2015
You remind me for the comedies produced per kilogram for which is standard practice laughing audience to maintain laughing at the moments which their writers hope to be funny for the viewers. In this way invite the audience to laugh at silly jokes, hoping that laughing is contagious. The analogy with your duty admiration for purely speculative articles would hardly left unnoticed.

Hahahaha....
Yes, that comment could keep people laughing for a while!
Whydening Gyre
4.8 / 5 (11) Sep 29, 2015

You remind me for the comedies produced per kilogram for which is standard practice laughing audience to maintain laughing at the moments which their writers hope to be funny for the viewers. In this way invite the audience to laugh at silly jokes, hoping that laughing is contagious. The analogy with your duty admiration for purely speculative articles would hardly left unnoticed.


You, Viko, could stand to loosen up and laugh a little, yourself...
Taking yourself so seriously must burn up a huge amount of energy that could be better served in opening your mind...
Returners
5 / 5 (4) Sep 29, 2015
Highly eccentric orbits would be difficulty, but not impossible, for Earthlike life to deal with. We do have some animals on Earth which can deal with temperature extremes from one end of the spectrum to the other, such as Bears, Whales, and Camels. However, not all Earth plants and animals have developed traits which allow them to just survive casually in any seasonal conditions.

Something which migrates on Earth, such as Birds, would have a problem on a planet who's seasons are determined by orbital eccentricity, as the temperature changes everywhere on the planet simultaneously in such a scenario, and migration would provide little benefit in most cases.

We do know that microscopic extremophiles can survive in almost any temperature or chemical scenario, and we know this to be likewise true for viruses, though viruses aren't technically considered alive.

What if we found life based on opposite chirality of amino acids, would it be considered a separate "Dominion"?
Returners
5 / 5 (1) Sep 29, 2015
Like hte "Dominion" category only has one present official member and that is simply "Life".

What does it take to add an additional sub-category to the "Dominion" category? Anti-chirality? Artificial intelligence capable of self-replication? I don't know. It seems to be a question that biologists and philosophers alike need to address both due to our advancing technology and our advancing discovery of distant planets.

Do we just call it "Earth-life" and "Mars-life" and "Geliese-life" and "artificial life" like that?
JustAnotherGuy
3.4 / 5 (5) Sep 29, 2015
Let's see... who left the troll's cage open??
Jonseer
3.7 / 5 (3) Sep 30, 2015
Ganymede does just fine generating its own magnetic field while in an orbit locked to Jupiter.

That being the case it's amazing that any serious astronomer would postulate that planets around low mass stars would not have magnetic fields for that reason.

Yes I know Jupiter is not close to being a star, but its very much a nicely scaled down model for one.
Torbjorn_Larsson_OM
3.7 / 5 (3) Sep 30, 2015
@Returners: "What does it take to add an additional sub-category to the "Dominion" category?"

An independent evolutionary process would have another root of the evolutionary tree, another common origin, i.e. another instance of the biological emergence process.

Even viruses show that all Earth "life" derives from a universal common ancestor. That can't reject that there were could have been other instances of emergence on Earth, but it seems odd if only one tree survived.

So yes, we will name it after the planets if we can show that it emerged independently.

@Jonseer: Good point! Ganymede is the most complicated "planet" in that respect, the only known one with an external field (Jupiter), an induced field (Jupiter into the salty ocean) and a geodynamo (its hot core). The problem is, as the article notes, Venus-like conditions: being heated throughout. That kills core convection, so kills the geodynamo.
cracker_mon
not rated yet Oct 05, 2015
Billions of life forms in the universes and we are the only sentient beings. Simply put, if true, then we are still truly alone. Ironic that we, as the only known Gods, have no idea as to what is going on or what we are supposed to do, if anything. Because we think, we think we should have the ability to understand. If we believe that there is or are Gods to save us, why, you might ask are we, as Gods to other creatures, not saving them. Possibly our particular God has to go to work to make a living and we are tearing up his abode thinking that he has abandoned us.???

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