The discovery of Proxima b, a planet orbiting roughly in the middle of the habitable zone around the star Proxima Centauri, was reported in the August 26, 2016 issue of the journal Nature by a group of astronomers using the High Accuracy Radial-velocity Planet Searcher (HARPS) at the European Southern Observatory at La Silla, Chile. Proxima Centauri has the distinction of being our Sun's nearest stellar neighbor, only 4.25 light years away
Why did they name the new planet Proxima b? The established planet-naming convention used by planet-finding astronomers is to give the newly discovered planet the name of the star followed by lower case letters starting with "b" for the first planet discovered, "c" for the next one found, and so on. The letter "a" is reserved for the star itself. This convention can be confusing, because the order of the letters reflects the order of discovery and not their distance from the parent star.
The orbit of Proxima b is tilted with respect to the Earth, so that it does not make transits across the disc of the star. Therefore, the planet-detection method of NASA's Kepler Mission using blocked starlight cannot be used. Instead, the presence of Proxima b was deduced from the 11.2 day wobble it produced in the radial (line of sight) velocity of the star as the planet orbited. There is also weak evidence suggesting a second planet that would have an orbital period of between 60 and 500 days, but this "signal" is competing with the sizable noise of stellar activity and may not exist. In this column, I want to consider the characteristics of Proxima b and its suitability as the possible site of the first human interstellar colony.
Proxima Centauri is a rather dim red dwarf star, with only 12.3% of the Sun's mass, only 14.1% of its radius, and only 0.17% of its net energy output. As a consequence, its habitable zone must be rather close to the star. Like most small stars, Proxima Centauri is relatively unstable, and it is known to produce frequent large solar flares. Its corona temperature is relatively low, 3,042 K as compared to 5,772 K for our Sun. Most of its emitted light is in the infrared rather than the visible. For this reason, it is so dim that even though it is close, it is not visible in the Earth's sky to the naked eye.
Further, its relative coolness leads to reduced support of its stellar material against the pull of gravity, resulting in a very high density (mass per unit volume). Our Sun has a density 1.41 times that of water, while Proxima Centauri has a much higher density, 56.8 times that of water and 7.23 times that of iron at standard temperature and pressure. While its mass is 129 times greater than that of Jupiter, because of its increased density, its diameter is only 1.5 times that of Jupiter.
The planet Proxima b orbits quite close to Proxima Centauri, at only 5% of the Earth-Sun orbital distance (0.05 AU). Consequently it makes one complete orbit (one Proxima b year) in only 11.2 days. Because the planet is so close to Proxima Centauri, even though the star has a very low energy output, the average surface temperature of the planet is within the range in which water should be liquid on its surface. Does that mean that Proxima b is habitable? Is it another Earth? Is it perhaps suitable for a human colony? Well, there are a few problems …
First, let's consider the force of the tides (see my column AV-63 in the January-1995 issue of Analog). We're familiar with the tidal forces that the Sun and the Moon exert on Earth's oceans. Such forces depend on the mass density of the producing object and on the third power of the object's angular size in the sky. As viewed from Earth, the Sun and Moon have about the same angular size. However, the tidal influence of the Moon is three times greater that that of the Sun because the Moon has three times the Sun's mass density.
Because Proxima b has such a small orbital radius, the star Proxima Centauri, despite its small diameter, will appear to have a 2.86 times larger angular diameter in the sky than does our Sun, as viewed from the Earth. Proxima also has 40 times the density of the Sun. That means that the tidal force of Proxima Centauri on Proxima b will be 939 times greater than that the Sun exerts on the Earth.
For this reason, it is very likely that Proxima b is tide-locked to Proxima Centauri. Like Earth's Moon, Proxima b would always present the same face to Proxima Centauri. This means that Proxima b will have a desert-like hot side and a deep-frozen cold side, with a "twilight zone" band around its mid-region in which the star is always just above or just below the horizon and the temperature lies somewhere between the extremes of the hot and cold sides. It is not clear if a planet could have an Earth-like atmosphere under such circumstances, or whether most of the atmospheric gas would freeze out on the cold dark side of the planet.
However, a tide-locked planet might offer some advantages. Solar energy farms would always be illuminated. Colonists could pick a location with any average temperature they wanted and could hide from the periodic solar flares by keeping dangerous Proxima Centauri just over the horizon and out of harm's way. However, the twilight zone of Proxima b seems unlikely to provide anything resembling an Earth-like environment.
The wavelengths of light that our human eyes can see, the visible spectrum, range from about 400 nanometers (10-9 meters, abbreviated as nm) at the deep violet edge to 700 nm at the deep red edge. The sunlight from Proxima Centauri peaks at 950 nanometers, well into in the infrared, and falls off rapidly in the visible region, with the visible light strongly skewed toward the red. If light from the Sun and Proxima Centauri were equal at the red edge of the visible spectrum, the light from Proxima would be ten times weaker at the violet edge. Nobody could get a sun tan by lying on a beach of Proxima b.
Since photosynthesis using chlorophyll is at its maximum efficiency at twin wavelengths of around 470 and 660 nm and cuts off to zero for wavelengths longer than 700 nm, Earth-like plants could photosynthesize with only a small fraction of Proxima Centauri's sunlight. However, silicon solar cells producing electric power peak in conversion efficiency around 700 nm and drop to zero at 1,100 nm. One can imagine that agriculture on Proxima b might be accomplished by placing large solar energy farms soaking up 700 nm light on the hot side of the planet and growing plants in twilight-zone greenhouses illuminated by electrically-powered LEDs producing the 470 and 660 nm wavelengths.
There is also the question of the pull of gravity on the planet's surface. The mass of Proxima b is uncertain, but its mass is a least 1.3 times that of the Earth and could be much more. For a human colony, one key question will be the magnitude of the planet's surface gravity. That depends on an unknown factor: the mass density of Proxima b. Surface gravity scales as the planetary mass to the 1/3 power times the mass density to the 2/3 power.
Let us assume that Proxima b is spherical, has a mass of say 1.5 Earth-masses, and has the same mass density as Earth, which is 5.97 grams per cubic centimeter. In that case Proxima b's surface gravity would be 1.145 gees or 11.22 meters per second squared. Colonists would weigh 14% more than on Earth, but they could probably adapt to that without many problems. However, Earth has a rather high density because of its iron-nickel core, and Proxima b, like Mars, might lack such a core. As an alternative, let's assume then that Proxima b has the mass density of Mars, or 3.93 grams per cubic centimeter. With that density, Proxima b's surface gravity would be 0.866 gees or 8.50 meters per second squared, making life a bit easier for colonists because they would weigh 14% less than on Earth.
However, our assumption of a spherical planet could be wrong. The strong tidal forces would tend to give the tide-locked planet an ovoid or egg shape, with the long axis pointing at the star. The habitable zone would be in the mid-region of the ovoid where its radius would be smallest. Therefore, the habitable mid-region would be closer to the planet's center, making the surface gravity higher here than in the hot and cold zones. If the radius was 10% smaller there, the gravity would be 20% larger.
Proxima Centauri has frequent solar flares, and there is debate about whether the resulting ionization would strip away any atmosphere that Proxima b might have. It has been argued that if Proxima b has a molten core like the Earth and rotates on its axis every 11.2 days, this might be sufficient to generate a planetary magnetic field that would protect the atmosphere.
Could there be pre-existing life on Proxima b? It is possible, but it seems unlikely. Water, preferably in the form of oceans, may be a prerequisite for the development of life. The Earth with its large oceans is a rather special planet. Earth formed inside the "ice line" of the Solar System, where solar energy is likely to vaporize water and to dry out the pre-planetary material before it can coalesce into a planet. However, an event called "the late heavy bombardment", probably created by an orbital resonance between the giant planets Jupiter and Saturn, occurred in the Solar System after Earth had formed. In that bombardment great quantities of icy material residing well outside the ice line were dislodged from their orbits. A sizable fraction of that material bombarded the inner planets, delivering great quantities of water to the Earth, creating its oceans, and also making the prominent craters of the Moon. It is unlikely that a similar scenario could have deposited large quantities of water on Proxima b, so the presence of oceans and existing life there is questionable. Further, the frequent solar flares from Proxima Centauri may have repeatedly killed off any emerging life forms. Thus, even if Proxima b has an atmosphere, it is not likely to contain free oxygen.
A lack of native life, however, is perhaps a benefit for Proxima b as the location of a possible human colony. There are arguments that the many arbitrary branch points in the evolution of life will make life that has evolved on other planets very different from that found on Earth. Such alien life would be very likely to be toxic to us. As a colony site, a sterile planet might be preferable to a biologically hostile one. Such pre-sterilization might make it easier to transplant some simplified version of Earth's ecosystem to Proxima b.
In summary, the discovery of Proxima b raises the possibility of the planet as a site of a human colony in the closest star system, provided the extremely difficult problem of getting there at all can be solved in the coming centuries. However, the planet would definitely not be Earth-like, and the planetary area that might be habitable would be limited to the "twilight zone" around the middle of the tide-locked planet. Nevertheless, it represents a new opportunity for the expansion of the human race to the stars.
Discovery of Proxima Centauri b:
"A terrestrial planet candidate in a temperate orbit around Proxima Centauri", Gullem Anglada-Escude, et al., Nature 536, 437, 23 August, 2016; arXiv: 1609.03448v1 [astro-ph-EP].
John Cramer's new book: a non-fiction work describing his Transactional Interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available for purchase online as a printed or eBook at: http://www.springer.com/gp/book/9783319246406 .
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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