Last week, Earth woke up to the news that it has a twin orbiting Proxima Centauri, a red dwarf star and the sun’s nearest neighbor, just 4.24 light years away. Proxima b, as this twin planet is called, is a little larger than Earth and orbits its star every 11 days at a distance that is just 1/20th of the Earth to the sun.
But since Proxima Centauri is much less luminous than the sun, Proxima b sits well within the so-called habitable zone in which liquid water could exist on its surface. That’s led to detailed speculation about what the surface of Proxima b might be like and how likely it is that these conditions could support life.
But NASA has a software modeling tool called VPlanet that models conditions on exoplanets based on what is known about them, their parent star, and the processes of planetary evolution. VPlanet then forecasts possible conditions on the surface and predicts whether or not the planet is habitable.
Today Rory Barnes and pals at NASA’s Astrobiology Institute say they’ve let VPlanet crunch the data gathered about Proxima b. This has allowed them to predict possible conditions on Proxima b.
VPlanet begins by breaking down the problem of determining exoplanet habitability into nine separate tasks that each explore a different aspect of planetary evolution. For example, one task looks at the evolution of the parent star to see how changes in luminosity, for example, would have influenced the planet’s immediate environment. Another looks at this star system’s movement through the galaxy, looking for close encounters that might have been influential.
Yet another module examines the transport of heat from the planet’s core to the surface to see how this would have influenced the temperature on the surface. And another simulates the way the planet’s atmosphere might have escaped into space and how much would remain in place today. And so on.
The results of this modeling process make for interesting reading. One way of making predictions involving complex situations is to run a simulation many times to see if they usually end up in the same place. VPlanet uses this technique to model the influence of passing stars on the orbit of Proxima b by randomly selecting reasonable orbital parameters for 10,000 different simulations. In each case, the team stopped the simulation when Proxima b’s orbit became unstable.
This happened in about 15 percent of the trials. That’s important because these kinds of instabilities would have a huge impact on a planet’s habitability.
VPlanet also concluded that if Proxima b formed at its current distance from its star, it must be tidally locked. In other words, it presents the same face to its star continuously, like the moon does to Earth. That would make one side of the planet boiling hot while the other is freezing cold. That would be unsuitable for life.
There is a caveat, however. If the planet’s atmosphere is thick enough to transport heat effectively around the planet, the temperature would be much more agreeable and Proxima b could still be habitable while tidally locked.
Of course, the planet may have migrated to its current distance from somewhere else, in which case it would not be tidally locked.
The state of Proxima b’s atmosphere is another crucial concern. In this regard, VPlanet simulates many potential scenarios starting from the assumption that the planet began life with an abundant water supply, several times that of Earth’s. In these conditions, VPlanet predicts that most of the water breaks down and that the resulting hydrogen escapes into space. “If Proxima b formed with less than ten times Earth’s water content, and/or had a persistent convecting, reducing magma ocean, it is likely desiccated today,” says the team.
There is another possibility—that Proxima b had a hydrogen envelope that escapes into space while preserving water on the surface. In this case, Proxima b would be habitable today, although Barnes and co are unable to put a figure on how likely this is.
In short, VPlanet suggests that Proxima b could currently exist in a wide range of possible states, some of which could support life while others would not. Barnes and co conclude that Proxima b could be Venus-like, desiccated, hot, and unfit for life; it could be Neptune-like, surrounded by an envelope of hydrogen that would make the surface too hot to be habitable.
VPlanet also predicts that Proxima b could be so rich in oxygen that this would have prevented the evolution of life—oxygen, of course, is highly reactive and a powerful oxidizer.
But the most exciting prediction is that Proxima b is Earth-like with liquid water and an atmosphere conducive to life. In that case, Barnes and co say, it would look like a pale purple dot when we finally observe it.
That’s a little surprising given that its parent star is a red dwarf but not so different to Earth’s appearance as a pale blue dot, should anybody on Proxima b be looking this way.
Of course, what astrobiologists need now is more data. Proxima b will surely become the most studied planet outside our solar system. The next generation of space telescopes will throw some much-needed light on the matter. The data they gather will help to whittle down the range of possible evolutionary trajectories Proxima b could have taken.
Chief among these observatories is the James Webb Space Telescope, due to launch in 2018, and the Wide Field Infrared Survey Telescope, due for launch in the mid-2020s. The next generation of ground-based telescopes, which will be more than 30 meters across, should also be able to glimpse Earth’s twin.
These instruments will be able to make spectrographic surveys of Proxima b’s atmosphere, looking for hints of oxygen, methane, carbon dioxide, and hydrogen. That could give a hint of whether conditions there are conducive to life. One day, Earth may even wake up to the news that astronomers have detected ocean glint—starlight reflecting off sea water as the planet rotates. What an exciting moment that would be.
Ref: arxiv.org/abs/1608.06919: The Habitability of Proxima Centauri b I: Evolutionary Scenarios; arxiv.org/abs/1608.08620: The Habitability of Proxima Centauri b II: Environmental States and Observational Discriminants
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