Saturday, August 27, 2016

The “Earthlike” Planet of Proxima Centauri

The steady improvement of techniques for detecting extrasolar planets has borne fruit—and a low-hanging fruit at that.  The nearest star to the Solar System, Proxima Centauri, has been found to have a planet (imaginatively named Proxima Centauri B) with roughly Earth-like size and even Earth-like temperatures.  This is fascinating news, but what does it really mean?  Is this really another Earth?

The press has been all over this story, and some of what has been written makes sense.  But what reliable knowledge do we have?

First, this is indeed a rocky terrestrial-type planet.  It is the closest known planet to the Solar System, and indeed orbits the star that is the Sun’s nearest neighbor.  Interstellar distances, even for nearest neighbors, are huge: Proxima Centauri (let’s call it Proxy) is 4.24 light years away from us, a dizzying 270,000 Astronomical Units (1 AU is the mean distance of Earth from the Sun).  That’s 64,000 times as far as Jupiter is at its closest to Earth. 

The mass of the planet is estimated to be 1.3 times the mass of Earth.  Its radius and density are unknown.  It orbits once every 11.2 Earth days at an average distance of 0.05 AU from Proxy, following a path whose eccentricity is so far unknown.  Proxy is a faint red Main Sequence star, of spectral class M6, with a mass of about 0.123 times the mass of our Sun and a luminosity only 0.17% of the Sun’s—but almost all of that light (about 86% of it) is infrared (heat) radiation invisible to the human eye.  It is a member of the family of flare stars, undependable neighbors that emit powerful and unpredictable flares. The star’s photosphere (its visible surface) is at a mere 3000 Kelvins, cool enough for “clouds” of refractory metals and oxides to form. 

It is likely that Proxy orbits around the common center of mass of the Alpha Centauri (α Cen) system, but far enough from α Cen that its orbital period must be on the order of a million years.

A planet forced to live in such close proximity to its star suffers a variety of indignities.  The first is that the erratic activity of the star subjects the planet to extreme brightness fluctuations and to bombardment with high fluxes of X-radiation near times of maximum activity.  The second is that tidal friction can quickly despin the planet, causing a rotational lock between the star and planet.  Third, if the planet is too close to its star, the planet may cross the Roche limit and be torn apart by the star’s tidal forces.  Proxy B certainly suffers from the first of these afflictions and certainly does not suffer from the third: if it were inside the Roche limit there would be no planet to detect, only a debris disk; a super asteroid belt.  The intermediate fate, falling into a rotational lock, is unavoidable in such close quarters, but there are several distinct outcomes with very different significance, and for which we presently lack the data to choose between them. 

The simplest possibility, if the orbit of Proxy B is nearly circular, is for it to simply lock directly onto Proxy and always keep the same face toward its star.  With the Sun shining on only one side of the planet, the sub-solar point would be quite hot, and half of the planet would be frozen in eternal night.  Volatile gases would migrate into the darkness and freeze out on the surface, making vast deposits of water ice, carbon dioxide ice, and other gases, and perhaps generating lakes of liquid argon and the heavier inert gases krypton and xenon.  Nitrogen and oxygen, if present, would fall as snow on the night side.  Any slight eccentricity of the planet’s orbit would cause it to rock back and forth once per year (in the case of Proxy B, one year is just 11.2 Earth days). The strong solar tidal forces on the planet would cause the rocking to damp out and the orbit to become more perfectly circular.  This is called a 1:1 spin-orbit resonance, like the Moon around Earth or many satellites of the outer planets around their primaries.

But we have no guarantee that the planet’s orbit is closely circular.  A sobering example is provided by Mercury, a tidally despun planet locked onto its star (the Sun) but with a significant orbital eccentricity.  It actually rotates in a 3:2 spin-orbit resonance: three planetary rotations in two planet years.  At consecutive perihelion passages, opposite points on Mercury’s equator face the Sun.  Thus two regions get alternately scorched—and frozen.  Because of the gravitational stresses, the planet ends up slightly elongated with two bumps on opposite sides of the planet’s equator.  At perihelion passage the angular rate of rotation of Mercury and its angular rate of motion along its orbit are almost exactly equal, so that the “hot pole” tracks the Sun rather closely for many days near perihelion.  Other resonant relationships besides Mercury’s 3:2 resonance are also possible, but they have the potential for disaster: 2:1 and 5:2 and 3:1 resonances are associated with such large orbital eccentricities that they raise the potential for collision with other planets.  Note that 2:1 and 3:1 resonances would have the same spot on the equator being baked on each perihelion passage; 3:2 and 5:2 resonances would have the strongest heating localized alternately in two regions on opposite sides of the planetary equator. 

We don’t yet know the orbit of Proxy B well enough to distinguish between these different states.  But we can see that some of these states would generate extreme temperature and weather behavior that would not be conducive to maintaining a biosphere—and that’s without even considering the effects of wild luminosity and flare activity by the star.

Oh, and one other thing: Proxy B is so close to its star that it is quite near the point at which the tidal forces of its star would disassemble the planet and turn it into an asteroid belt.  Bummer.

But if the planet is in, say, a 3:2 resonance--and all its volatiles don’t go gentle into that good night—the star will remain on the Main Sequence, providing heat to its planets, for another 4000 billion years.  No need to rage against the dying of the light.

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