Oct 02 2010
The recent discovery of Gliese 581g — a potentially Earth-like planet only 20 light years away from us — has become a growing cultural phenomenon. Using the Keck Observatory in Hawaii, Steven Vogt of UC Santa Cruz and his collaborators estimate it’s mass at about 3 – 4 times Earth and it’s distance from its dim, nondescript red dwarf star at 0.15 AU (an AU is one Earth-Sun distance, 93 million miles).
Although its mass and distance from its star are currently all we “know” about this fascinating world, Dr. Vogt was carried away by the moment and asserted that the chances it has life are “100%” ! Of course he meant simple life forms — not sophisticated creatures with big brains. But with that one quote Vogt joined the elite ranks of the “early ebullients” — those modeling the coming, consuming excitement of the new, 1960s-style international space age near 2015. In a few years, almost everyone — including you and me — will momentarily feel the Camelot-like excitement.
21stCenturyWaves.com has previously noted that society’s interest in and attitude toward extraterrestrial life seem to fluctuate with the long, 56-year economic wave — the same force that appears to fundamentally drive great human explorations and macro engineering projects all the way back to Lewis and Clark. For example, during the Great Depression, Americans resonated with the notion of a Martian invasion in Orson Wells production of “War of the Worlds.” However, in 1894 — during the decade just before the ebullient Peary/Panama/T. Rosevelt Maslow Window — Percival Lowell founded his observatory in Arizona to study Mars, and later captivated the world with his claim of intelligent Martians. (See Kepler, Carl Sagan, and the Guzman Prize — Our Century-Long Search for Space Aliens.)
Media interest in Vogt’s emotional statement is consistent with society’s approach to the 2015 Maslow Window — a golden age of prosperity, exploration, and technology — when Earth-like planets, extraterrestrial life, and human expansion into the cosmos are expected to generate global excitement at or beyond the level of the 1960s Moon race.
Using the only two approximate data points we have about Gliese 581g (its mass and star distance), a little physics, and a few reasonable assumptions, let’s take a peek at what may await us on this fascinating “Earth-like” planet only 20 light years away.
How Cool is It?
Assuming an Earth-like atmosphere (albedo of 0.3) the average surface temperature (effective T) of Planet g will be -45 deg C (compared to -19 C for Earth); in effect, Planet g partially compensates for a dim star (1.3 % solar luminosity) by cuddling up to it. An Earth-like greenhouse effect would raise its average surface T to -12 deg C; still below freezing. However Planet g, due to its large mass, is likely to have a thicker atmosphere than Earth and a stronger greenhouse effect.
Because Planet g is only 1/3 of Mercury’s solar distance from its star, it’s assumed the planet is tidally locked like our Moon. Having one side always facing its star would create a warm/cold hemispheric dichotomy. Life forms might survive at low-to-mid latitudes in the twilight zone between the two.
Does Planet g Have a Warm Heart?
Another factor that can affect the surface environment is heat flow from the planet’s interior. Assuming an Earth-like composition — which may be incorrect — we can estimate it. Planetary heat content (from radioactives and heat of formation) is proportional to planet volume (i.e., to its radius (R) cubed), and heat leakage is proportional to planet’s surface area (or R squared); so the heat flow q is proportional to R. Since R goes as the cube root of planet mass, so does q.
Assuming an Earth-like interior, our estimate for Planet g’s heat flow q is 1.5 times Earth’s. This puts it around 0.09 W/m2. For comparison, global estimates of q on other worlds are: Moon = 0.02; and Io, the most volcanically active body, is 2.5. This suggests Planet g will have about 5x the Moon’s heat flow but only about 1/3 that of Io. Given Planet g’s thin lithosphere (see below), this could translate into super-Earth levels of active global volcanism for Planet g. “Geothermal” power should be readily available to the “Gliesans.”
How Does Its Atmosphere Work?
Gliese 581g (i.e., Planet g) will probably have a thicker atmosphere than Earth with a stronger greenhouse effect, but the question is: How will its circulation work? One way to see if it’s Earthlike is to compare the energy received from its star to its internal heat flow (we again assume Earth-like parameters). For Planet g this ratio is about 1.15 that of Earth.
Solar-to-internal energy ratios vary widely across the solar system. For example, Earth’s ratio is almost 4000, indicating an atmosphere driven completely by the Sun, while Jupiter’s is 0.37, characteristic of an outer planet with a large gravitationally-derived heat flow. Despite its dim star, Planet g’s atmosphere will look much more like Earth than Jupiter.
Do Its Continents Drift?
Earth’s cold, rigid surface layer is divided into large mobile plates about 100 km thick that account for most of its quakes and volcanism. Plate Tectonics is also a requirement for planetary habitability because it recycles (though subduction) important elements (e.g., carbon) through the biosphere and maintains Earth’s atmospheric pressure at high levels over billions of years.
Assuming again an Earth-like composition and thermal definitions for its lithosphere, we can estimate Planet g’s lithospheric thickness. Because heat flow is proportional to planet radius, lithospheric thickness is inversely proportional to the cube root of planet mass. So Planet g’s lithosphere is about 2/3 as thick as Earth’s.
A significantly thinner lithosphere — with less inertia and reduced frictional resistance at its base — than Earth implies it should be easier to move Planet g’s plates around. Plate tectonics is almost a sure thing, and the habitation condition should be met, assuming Planet g has water.
Can They Come Here?
Because Gliese 581 is a small star, it did not inflict upon Planet g the catastrophic heat death that awaits Earth in about 800 million years in response to the larger Sun’s increasing luminosity (~10% increase per billion years). Thus, the long-term energy frugality of Gliese 581 (Planet g’s parent star) and Planet g’s snug position near it, suggest that life could have developed and survived many billions of years in response to its generally Earth-like conditions (that I’ve hinted at above), if there is significant water.
The other intriguing aspect of Planet g is its age: estimated at 7 to 11 billion years. On Earth, humans required about 5 billion years to appear, so time was not a limiting factor for Planet g — although tidally locked spin might have precluded the appearance of intelligent beings. This might explain why they have not yet replied to Earth’s radio and TV broadcasts after hearing them for decades!
On the other hand, any intelligent Gliesans would have detected the young Earth eons ago and noted its convenient mass and location in the Goldilocks Habitation Zone (much as Vogt just did with Planet g). And on the scale of our Galaxy, 20 light years is very close. From there it’s not necessary to warp space with Star Trek-like interstellar drives. The early Gliesans could have opted for slow, generation-style star ships for the pleasant, century-plus journey to Earth.
If intelligent Gliesans ever existed, they are already here…
Could that be why Dr. Vogt is “100%” sure?