Exoplanets in the 'habitable zone' may be too dangerous for life

Being able to sustain liquid water doesn't necessarily mean they can sustain organisms, say researchers.


Exoplanets in the so-called ‘habitable zone’ might not be the safe, Earth-like oases they’re cracked up to be.

Exoplanets in the habitable zone are an exciting prospect: These planets exist at a distance from their host star at which they could sustain liquid water, and thus could also potentially sustain life. But there’s more to habitability than whether a planet is just the right distance from its star, say astrophysicists from Rice University — dangerous space weather could make the planets too tumultuous to sustain life.

In a paper published Wednesday in The Astrophysical Journal, the researchers used a combination of exoplanet data and a solar-based magnetic field model to tease out how some of these exoplanets might fare under the magnetic might of their host stars.

Rice University graduate student and the study’s lead author, Alison Farrish, said in a statement that the research can fill in gaps in the exoplanet data scientists already have.

“It’s impossible with current technology to determine whether an exoplanet has a protective magnetic field or not, so this paper focuses on what is known as the asterospheric magnetic field,” Farrish said. “This is the interplanetary extension of the stellar magnetic field with which the exoplanet would interact.”

The team focused on two main characteristics: their star’s stellar Rossby number — which determines how active a star is — and their Alfvén surface — which determines at what distance an exoplanet is affected by its star’s magnetic field.

While they might look appealing from Earth, stars are dynamic plasma balls with magnetic fields that reach out in all directions like the strings on a Koosh ball. These fields can spell danger for exoplanets, write the authors.

Just because an exoplanet can sustain water, doesn't mean it can sustain life, researchers say

NASA/JPL-Caltech/T. Pyle

If a star is too active and the planet is too close to its star’s magnetic field — no matter whether it’s within the ‘habitable zone’ or not — it might experience powerful effects that could strip away any magnetosphere the exoplanet has. A planet’s magnetosphere is what protects it from solar radiation — much like Earth’s own magnetic field. As a result, it would experience irradiation of the exoplanet’s surface.

This would be bad news for any potential organisms living on those exoplanets, Farrish’s coauthor and Rice University professor, David Alexander, said in a statement.

“Depending on where it is within the extended magnetic field of the star, it is estimated that some of these habitable zone exoplanets could lose their atmospheres in as little as 100 million years,” Alexander said. “That is a really short time in astronomical terms. The planet may have the right temperature and pressure conditions for habitability, and some simple lifeforms might form, but that’s as far as they’re going to go. The atmosphere would be stripped and the radiation on the surface would be pretty intense.”

The team found that some of astrophysics’ favorite exoplanet systems, including Ross 128, Proxima Centauri and TRAPPIST 1, likely fall into this dangerous magnetic zone. Meaning that while those exoplanets may be ‘habitable’ by definition, they probably aren’t good candidates for a vacation home anytime soon.

While space tourism dreams may take a hit from these new discoveries, Alexander said that they are actually a step forward for the field in terms of narrowing down exactly what makes a planet habitable and what characteristics astrophysicists should look for in the future.

“Our model allows us to nail down some of the key characteristics of a star’s activity with respect to flux emergence and transport over the course of a stellar cycle,” Alexander said. “This allows a direct comparison with observations, which are currently very sparse for stars other than the sun, and a means by which to potentially characterize some of the key physical attributes of the exoplanets through their interaction with the stellar field.”

We employ a flux transport model incorporating observed stellar activity relations to characterize stellar interplanetary fields on cycle timescales for a range of stellar activity defined by the Rossby number. This framework allows us to examine the asterospheric environments of exoplanetary systems and yields references against which exoplanetary observations can be compared. We examine several quantitative measures of star–exoplanet interaction: the ratio of open to total stellar magnetic flux, the location of the stellar Alfvén surface, and the strength of interplanetary magnetic field polarity inversions, all of which influence planetary magnetic environments. For simulations in the range of Rossby numbers considered (0.1–5 RoSun), we find that (1) the fraction of open magnetic flux available to interplanetary space increases with Rossby number, with a maximum of around 40% at stellar minimum for low-activity stars, while the open flux for very active stars (Ro ~ 0.1–0.25 RoSun) is ~1–5%; (2) the mean Alfvén surface radius, R A, varies between 0.7 and 1.3 R A,Sun and is larger for lower stellar activity; and (3) at high activity, the asterospheric current sheet becomes more complex with stronger inversions, possibly resulting in more frequent reconnection events (e.g., magnetic storms) at the planetary magnetosphere. The simulations presented here serve to bound a range of asterospheric magnetic environments within which we can characterize the conditions impacting any exoplanets present. We relate these results to several known exoplanets and discuss how they might be affected by changes in asterospheric magnetic field topologies.