“Is Our Nearest Star’s Planet Proxima B Habitable?” –Three Top Astrophysicists Ponder

 

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The discovery of Proxima b is the biggest exoplanet discovery since the discovery of exoplanets. The planet is not much bigger than Earth and resides in the “habitable zone” of the Sun’s nearest stellar neighbor, the red-dwarf star Proxima Centauri. This planet may represent humanity’s best chance to search for life among the stars. But is Proxima b habitable? Is it inhabited? These questions are impossible to answer at this time because we know so little about the planet, but three astrophysicists have proposed possible answers.


Two astrophysicists at Harvard believe that NASA’s James Webb Space Telescope (JWST), scheduled to launch in 2018, could provide the answer in record time by sampling the star system’s light. The newly detected Earth-mass planet, Proxima b in the habitable zone of Proxima Centauri could potentially host life – if it has an atmosphere that supports surface liquid water.

 

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“With the light we detect, we can ask if this world looks like a bare rock. If it doesn’t, there might be an atmosphere, and there might also be an ocean, which life requires,” says Avi Loeb, an astrophysicist at Harvard University,, who co-authored a pre-print study on arXiv with Laura Kreidberg, a Harvard astronomer who studies exoplanet atmospheres. “It would only take [11 days’] worth of observing time,” its actual orbiting time, Loeb told Business Insider.

Rory Barnes, As an astrobiologist and astronomer at the University of Washington, and a member of NASA’s Virtual Planetary Lab, has investigated the habitability of planets orbiting red dwarfs for years. The discovery of Proxima b inspired his article Opportunities and Obstacles for Life on Promixa B from which we’ve quoted below. Being Earth-sized and in the habitable zone are just the first two requirements for a planet to support life, and the list of requirements is much longer for planets orbiting red dwarfs than for stars like our Sun.

If Proxima b is in fact habitable, meaning it possesses liquid water or even inhabited, meaning life is currently present, then it will have traversed a very different evolutionary path than Earth. This difference is frustrating, in that it will make initial interpretations challenging, but also exciting, as it offers the chance to learn how Earth-sized planets evolve in our universe. Whether Proxima b is a sterile wasteland or teeming with life, we are now embarking on an unprecedented era of discovery, one that may finally provide an answer that age-old question “Are we alone?” says Barnes.
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An artist’s depiction of Proxima b.(Planetary Habitability Laboratory/University of Puerto Rico at Arecibo)

 

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One of the original concerns for the habitability of planets orbiting red dwarfs was that they would become “tidally locked”, meaning that one hemisphere permanently faces the host star. This state is similar to the rotation of our Moon, in which the same tidal forces that raise waves in our ocean have caused the Moon to show only one face to Earth.

Astronomers think Proxima b is tidally locked like the Moon, but instead of always facing the Earth, one side of Proxima b always faces its star: awash in permanent daylight, the other side trapped in an endless cold night. If Proxima b does have an atmosphere, though, says Harvard’s Loeb, it’d not only circulate warmth from the day side to the night side, but also prevent the planet’s water from boiling off into space.

The image at the top of the page shows what a tidally locked habitable Proxima b might look like, also called an “eyeball Earth.” A ring of habitability could exist between the day and night sides. (Beau.TheConsortium/Wikia).

“We basically asked ourselves,” said Loeb, ‘what would a tidally locked Earth look like if you put it right next to Proxima Centauri? Clouds, wind, and water make that question complicated,” he added, but said you could at least tell if it’s a bare rock or is circulating heat using air. On Earth, at least a third of the heat is redistributed by the ocean and atmosphere.”

Loeb believes the key to ruling out an atmosphere is to focus on infrared light — the same “color” of warm, invisible light that our bodies constantly emit. When a rocky planet is warmed up by a star, it absorbs sunlight and re-emits it as infrared light. Yet rocky planets emit a different kind of infrared light than is given off by stars like Proxima Centauri.

 

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NASA’s James Webb Space Telescope is specially designed to observe infrared light. So instead of trying to photograph a tiny planet in a flood of visible light, JWST may only need to hunt for specific wavelengths of infrared light in the glare.

“When we look at the Moon, it shows different phases illuminated by the sun. If you imagine planet going around the star, we’d see different phases of the planet,” Loeb said. “Past the star, we’d see its day side. In front of the star, we’d see its dark side. As Proxima b moves around the star over 11.2 days … we’d see the temperature or ‘color’ of the planet changing with time.”

If Loeb and Kreidberg’s hypothetical observation reveals that the dark side of Proxima b isn’t as cold as it should be, that would mean an atmosphere may be hugging the planet — and redistributing warmth to the night side.

For decades, astronomers were concerned that such a tidally locked planet would be uninhabitable because they believed the atmosphere would freeze and collapse to the surface on the permanently dark side. That possibility is now viewed as very unlikely because winds in the atmosphere will transport energy around the planet and maintain sufficient warmth on the backside to prevent this freeze out. Thus, as far as atmospheric stability is concerned, tidal locking is not a concern for this planet’s potential habitability.

Although tidal locking is not very dangerous for life, it is possible for tides to provide large amounts of energy to the planet’s atmosphere and interior. This energy is often called “tidal heating” and is a result of the deformation of the planet due to changes in the host star’s gravitational force across the planet’s diameter. For example, if the planet is on an elliptical orbit, when it is closer to the star, it feels stronger gravity than when it is farther away. This variation will cause the shape of the planet to change, and this deformation can cause friction between layers in the planet’s interior, producing heat.

In extreme cases, tidal heating could trigger the onset of a runaway greenhouse like the one that desiccated Venus, independent of starlight. Proxima b is not likely to be in that state, but the tidal heating could still be very strong, causing continual volcanic eruptions as on Jupiter’s moon Io, and/or raising enormous ocean waves. Based on the information we have now, we don’t know the magnitude of tidal heating, but we must be aware of it and explore its implications.

So is Proxima b habitable? The short answer is “It’s complicated,” says U of Washington’s Barnes Our observations are few, and what we do know allow for a dizzying array of possibilities. Did Proxima b migrate halfway across the galaxy? Did it endure a planetary-system-wide instability that launched its sibling planets into deep space and changed its orbit? How did it cope with the early high luminosity of its host star? What is it made of? Did it start out as a Neptune-like planet and then become Earth-like? Has it been relentlessly bombarded with flares and coronal mass ejections? Is it tidally heated into an Io-like (or worse) state? These questions are central to unlocking Proxima’s potential habitability and determining if our nearest galactic neighbor is an inhospitable wasteland, an inhabited planet, or a future home for humanity.

The last point is not as rhetorical as it might seem. Since all life requires an energy source, it stands to reason that, in the long term — by which Barnes mean the loooong term — planets like Proxima b might be the ideal homes for life. Our Sun will burn out in a mere 4 billion years, but Proxima Centauri will burn for 4 trillion more. Moreover, if a “planet c” exists and slightly perturbs b’s orbit, tidal heating could supply modest energy to b’s interior indefinitely, providing the power to maintain a stable atmosphere.

The Daily Galaxy via PaleRedDot.org and Business Insider

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