Sorry, Proxima Centauri Is Probably a Hellhole, Too

proximab
The surface of Proxima b as imagined in this artist’s impression. Sadly, the reality probably doesn’t include an atmosphere (ESO/M. Kornmesser)

The funny thing about habitable zones is that they’re not necessarily habitable. In fact, depending on the star, some of them are likely downright horrible.

Take, for example, the “habitable zone exoplanet” orbiting our neighboring star Proxima Centauri. When the discovery of Proxima b was announced last year, the world erupted with excitement. After all, astronomers had detected an Earth-sized world right on our galactic doorstep, a mere four light-years away.

Immediately there was discussion about Proxima b’s habitable potential (could there be aliens?) and the possibility of the world becoming an interstellar target (might we one day go there on vacation?).

Alas, for the moment, these exo-dreams are pure fantasy as the only things we know about this world are its mass and its orbital period around the star. We have no clue about the composition of this exoplanet’s atmosphere — or even if it has an atmosphere at all. And, according to new research published in The Astrophysical Journal Letters, Proxima b would probably be a very unlikely place to find extraterrestrial life and you’d be ill advised to invest in a vacation home there.

Like TRAPPIST-1 — that other star system that contains “habitable, but probably not so habitable” exoplanets — Proxima Centauri is a red dwarf star. By their nature, red dwarfs are small and cooler than our sun. Their habitable zones are therefore very compact; to receive enough heating energy to keep water in a liquid state on their surfaces, any “habitable” red dwarf exoplanets would need to snuggle up really close to their star. Liquid water (as we all know) is essential for life. So, if you want to find life as we know it (not that weird Titan life), studying habitable zone planets would be a good place to start. And as red dwarfs are abundant in our galaxy, seeking out habitable zone planets in red dwarf star systems would, at first, seem like an even better place to start.

Except, probably not.

Red dwarfs are angry. They erupt with powerful flares, have powerful stellar winds and their habitable zones are awash with intense ultraviolet radiation. And, like TRAPPIST-1, Proxima Centauri probably wouldn’t be a great place to live.

But the researchers decided to test this hypothesis by throwing Earth in at the deep end.

“We decided to take the only habitable planet we know of so far — Earth — and put it where Proxima b is,” said Katherine Garcia-Sage, a space scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., and lead author of the study.

The big advantage for Earth is that it possesses a powerful global magnetic field that can deflect our sun’s solar wind and coronal mass ejections with a minimum of effort. But put Earth in a habitable zone orbit around Proxima Centauri and bad stuff starts to happen, fast.

At this location, the intensity of extreme ultraviolet radiation becomes a problem. Using data from NASA’s Chandra X-ray Observatory, the researchers could gauge the star’s activity and how much radiation would hit Proxima b. According to their calculations, the exoplanet receives hundreds of times more extreme ultraviolet radiation than Earth receives from our sun and, even if we assume Proxima b has an “Earth-like” magnetosphere, it will lose its atmosphere very quickly.

As ultraviolet radiation will ionize the exoplanet’s atmosphere, electrons (that are negatively charged) will be readily stripped from light atoms (hydrogen) and eventually the heavier atoms too (like oxygen and nitrogen). As the electrons are lost to space, a powerful “charge separation” is created and the positively charged ions that are left behind in the atmosphere will be dragged with the electrons, causing them to also be lost to space. Granted, the global magnetic field will have an effect on the rate of atmosphere loss, but the researchers estimate that this process will drain an atmosphere from Proxima b 10,000 times faster than what happens on Earth.

“This was a simple calculation based on average activity from the host star,” added Garcia-Sage. “It doesn’t consider variations like extreme heating in the star’s atmosphere or violent stellar disturbances to the exoplanet’s magnetic field — things we’d expect provide even more ionizing radiation and atmospheric escape.”

In the worst-case scenario, where the outer atmospheric temperatures are highest and the planet exhibits an “open” field line configuration, Proxima b would lose the equivalent of the whole of Earth’s atmosphere in just 100 million years. If the atmospheric temperatures are cool and a “closed” magnetic field line configuration is assumed, it will take 2 billion years for the atmosphere to be completely lost to space. Either way you look at it, unless the atmosphere is being continuously replaced (perhaps by very active volcanism), Proxima b will have very little chance to see life evolve.

“Things can get interesting if an exoplanet holds on to its atmosphere, but Proxima b’s atmospheric loss rates here are so high that habitability is implausible,” said Jeremy Drake, of the Harvard-Smithsonian Center for Astrophysics and study co-author. “This questions the habitability of planets around such red dwarfs in general.”

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TRAPPIST-1: The ‘Habitable’ Star System That’s Probably a Hellhole

trappist-1-star
Red dwarfs can be angry little stars (NASA/GSFC/S. Wiessinger)

There are few places that elicit such vivid thoughts of exotic habitable exoplanets than TRAPPIST-1 — a star system located less than 40 light-years from Earth. Alas, according to two recent studies, the planetary system surrounding the tiny red dwarf star may actually be horrible.

For anyone who knows a thing or two about red dwarfs, this may not come as a surprise. Although they are much smaller than our sun, red dwarfs can pack a powerful space weather punch for any world that orbits too close. And, by their nature, any habitable zone surrounding a red dwarf would have to be really compact, a small detail that would bury any “habitable” exoplanet in a terrible onslaught of ultraviolet radiation and a blowtorch of stellar winds. These factors would make the space weather environment around TRAPPIST-1 extreme to say the least.

“The concept of a habitable zone is based on planets being in orbits where liquid water could exist,” said Manasvi Lingam, a Harvard University researcher who led a Center for Astrophysics (CfA) study, published in the International Journal of Astrobiology. “This is only one factor, however, in determining whether a planet is hospitable for life.”

The habitable zone around any star is the distance at which a small rocky world can orbit and receive just the right amount of heating to maintain liquid water on its hypothetical surface. Orbit too close and the water vaporizes; too far and it freezes. As life needs liquid water to evolve, seeking out exoplanets in their star’s habitable zone is a good place to start.

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The peaceful surface of a TRAPPIST-1 habitable zone exoplanet as imagined in this artist’s rendering (NASA/JPL-Caltech)

For the sun-Earth system, we live in the middle of the habitable zone, at a distance of one astronomical unit (1 AU). For a world orbiting a red dwarf like TRAPPIST-1, its orbital distance would be a fraction of that — i.e. three worlds orbit TRAPPIST-1 in the star’s habitable zone at between 2.8% and 4.5% the distance the Earth orbits the sun. This is because red dwarfs are very dim and produce meager heating — for a world to receive the same degree of heating that our planet enjoys, a red dwarf world would need to snuggle up really close to its star.

But just because TRAPPIST-1 is dim, it doesn’t mean it holds back on ultraviolet radiation. And, according to this study, the three “habitable” exoplanets in the TRAPPIST-1 system are likely anything but — they would receive disproportionate quantities of damaging ultraviolet radiation.

“Because of the onslaught by the star’s radiation, our results suggest the atmosphere on planets in the TRAPPIST-1 system would largely be destroyed,” said co-author Avi Loeb, who also works at Harvard. “This would hurt the chances of life forming or persisting.”

Life as we know it needs an atmosphere, so the erosion by UV radiation seems like a significant downer for the possible evolution of complex life.

That’s not the only bad news for our extraterrestrial life dreams around TRAPPIST-1, however. Another study carried out by the CfA and the University of Massachusetts in Lowell (and published in The Astrophysical Journal Letters) found more problems. Like the sun, TRAPPIST-1 generates stellar winds that blast energetic particles into space. As these worlds orbit the star so close, they would be sitting right next to the proverbial nozzle of a stellar blowtorch — models suggest they experience 1,000 to 100,000 times stellar wind pressure than the solar wind exerts on Earth.

And, again, that’s not good news if a planet wants to hold onto its atmosphere.

“The Earth’s magnetic field acts like a shield against the potentially damaging effects of the solar wind,” said Cecilia Garraffo of the CfA and study lead. “If Earth were much closer to the sun and subjected to the onslaught of particles like the TRAPPIST-1 star delivers, our planetary shield would fail pretty quickly.”

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The TRAPPIST-1 exoplanet family. TRAPPIST-1 e, f and g are located in the system’s habitable zone (NASA/JPL-Caltech)

So it looks like TRAPPIST-1 e, f and g really take a pounding from their angry little star, but the researchers point out that it doesn’t mean we should forget red dwarfs as potential life-giving places. It’s just that life would have many more challenges to endure than we do on our comparatively peaceful place in the galaxy.

“We’re definitely not saying people should give up searching for life around red dwarf stars,” said co-author Jeremy Drake, also from CfA. “But our work and the work of our colleagues shows we should also target as many stars as possible that are more like the sun.”

Smallest ‘Super-Earth’ Discovered With an Atmosphere — but It’s No Oasis

MPIA

For the first time, astronomers have detected an atmosphere around a small (and likely) rocky exoplanet orbiting a star only 39 light-years away. Although atmospheres have been detected on larger alien worlds, this is the smallest world to date that has been found sporting atmospheric gases.

Alas, Gliese (GJ) 1132b isn’t a place we’d necessarily call “habitable”; it orbits its red dwarf a little too close to have an atmosphere anything like Earth’s, so you’d have to be very optimistic if you expect to find life (as we know it) camping there. But this is still a huge discovery that is creating a lot of excitement — especially as this exo-atmosphere has apparently evolved intact so close to a star.

The atmosphere was discovered by an international team of astronomers using the 2.2 meter ESO/MPG telescope at La Silla Observatory in Chile. As the exoplanet orbited in front of the star from our perspective (known as a “transit”), the researchers were able to deduce the physical size of the world by the fraction of starlight it blocked. The exoplanet is around 40 percent bigger than Earth (and 60 percent more massive) making it a so-called “super-Earth.”

Through precision observations of the infrared light coming from the exoplanet during the 1.6 day transits, the astronomers noticed that the planet looked larger at certain wavelengths of light than others. In short, this means that the planet has an atmosphere that blocks certain infrared wavelengths, but allows other wavelengths to pass straight through. Researchers of the University of Cambridge and the Max Planck Institute for Astronomy then used this information to model certain chemical compositions, leading to the conclusion that the atmosphere could be a thick with methane or water vapor.

Judging by the exoplanet’s close proximity to its star, this could mean that the planet is a water world, with an extremely dense and steamy atmosphere. But this is just one of the possibilities.

“The presence of the atmosphere is a reason for cautious optimism,” writes a Max Planck Institute for Astronomy news release. “M dwarfs are the most common types of star, and show high levels of activity; for some set-ups, this activity (in the shape of flares and particle streams) can be expected to blow away nearby planets’ atmospheres. GJ 1132b provides a hopeful counterexample of an atmosphere that has endured for billion of years (that is, long enough for us to detect it). Given the great number of M dwarf stars, such atmospheres could mean that the preconditions for life are quite common in the universe.”

To definitively work out what chemicals are in GJ 1132b’s atmosphere, we may not be waiting that long. New techniques for deriving high-resolution spectra of exoplanetary atmospheres are in the works and this exoplanet will be high on the list of priorities in the hunt for extraterrestrial biosignatures. (For more on this, you can check out a recent article I wrote for HowStuffWorks.)

Although we’ll not be taking a vacation to GJ 1132b any time soon, the discovery of an atmosphere around such a small alien world will boost hopes that similar sized super-Earths will also host atmospheres, despite living close to red dwarf stars that are known for their flaring activity. If atmospheres can persist, particularly on exoplanets orbiting within a star’s so-called habitable zone, then there really should be cause for optimism that there really might be an “Earth 2.0” out there orbiting one of the many red dwarfs in our galaxy.

About Those ‘Habitable’ Exoplanets (RT America Interview)

On Monday, I appeared on RT America’s live news broadcast to talk exoplanets — particularly the three small (possibly rocky) worlds that orbit the stars Kepler-62 and Kepler-69. It was a lot of fun discussing ‘Goldilocks Zones’ and the possibilities of extraterrestrials. Enjoy!

Discovery News coverage of Kepler-62:

Listening Out for the Magnetospheres of Habitable Exoplanets

Searching for Earth-like exoplanets (© Mark Garlick)
Searching for Earth-like exoplanets (© Mark Garlick*)

Is there a new way to hunt for habitable Earth-like exoplanets? According to a US Naval Research Laboratory researcher there is an obvious, yet ingenious, way of listening for these worlds. Like most Earth-like exoplanet searches, we are looking for characteristics of our own planet. So what do we need to survive on Earth? Obviously we need water and the correct mix of oxygen with other atmospheric gases, but what about the magnetic bubble we live in? The Earth’s magnetosphere protects us from the worst the Sun can throw at us, preventing the atmosphere from being eroded into space and deflecting life-hindering radiation.

Although we have yet to develop sensitive enough radio telescopes, it may be possible in the future to detect the radio waves generated as charged particles in stellar winds interact with Earth-like exoplanetary magnetospheres. If there’s a magnetosphere, there may be a protected atmosphere. If there’s an atmosphere, perhaps there’s life being nurtured below

*This image is copyright Mark A. Garlick and has been used with permission. Please do not use this image in any way whatsoever without first contacting the artist.
Continue reading “Listening Out for the Magnetospheres of Habitable Exoplanets”

Introducing the Exomoon, and Detecting them via Exoplanet Wobble

Can astronomers really detect exomoons?
Can astronomers really detect exomoons?

Exomoon: The natural satellite of an exoplanet.

Before today, I hadn’t heard anything about the possibility of looking for moons orbiting planets in other star systems. Sorry, exomoons orbiting exoplanets in other star systems. But a British astronomer has calculated that it is possible to not only detect exomoons, but it is possible to deduce their distance from the parent exoplanet and their mass.

All this is done by measuring the exoplanet’s “wobble”; a practice more commonly used in the pursuit of the exoplanets themselves. By detecting the wobble of distant stars, the gravitational pull of the exoplanet becomes obvious. The same can be done with exoplanets, possibly revealing the presence of Earth-like exomoons.

Of the 300+ exoplanets discovered, 30 are within the habitable zones of their stars. If these large gas giant exoplanets (usually several times the mass of Jupiter) have an exoplanet system of their own, these exomoons also fall within the habitable zone…

Makes you think, doesn’t it?

For the full article, check out Astronomers Now Looking For Exomoons Around Exoplanets on the Universe Today…