It seems that an `Earth-like' exoplanet with gravity only "twice as strong on its surface as Earth's" would mean "more volcanoes and `planetquakes'" and also might have major "effects on the planet's magnetic field, which shields the surface from solar radiation." My comments are bold and in square brackets.
Finding 'Super Earth' is a 'Goldilocks' errand, USA Today, Dan Vergano, February 19, 2006 Astronomers have detected more than 150 planets orbiting nearby stars, raising hopes of finding another Earth, not only for the Star Trek crowd but also among sober-minded scientists and NASA administrators. But there may be more to finding that "goldilocks" planet, just the right size and distance from its star to match Earth, warns one research team. Last month, an international team reported in Nature that it had detected the smallest extrasolar planet orbiting a normal star yet. Just over five times heavier than Earth, OGLE-2005-BLG-390Lb, is 28,000 light years away. It orbits two-and-a-half times further away from its star than Earth does the sun, and enjoys chilly temperatures of -364 degrees because of the dimness of its star. A few earlier discoveries of similarly-sized "extra solar" planets had also occurred, but all those orbit very close to their stars. But the discoveries show that astronomers are closing in on a planet in the "habitable zone" where temperatures are neither too cold or too hot for life, suggest researchers like Princeton's Bohdan Paczynski, one of the discoverer's of the latest planet. But it may not be so easy, suggests University of Minnesota physicist Renata Wentzcovitch and colleagues in the current Science magazine. For "Super Earth" planets only a few times heavier than Earth, the interior chemistry of the planet's core may have a big effect on whether future space tourists will ever want to vacation there. In the study, the team looked at the "Super Earth" orbiting the star Gliese 876, 15 light years away. The researchers analyzed the chemistry of perovskite, an electronically inert mineral made of oxygen, silicon and magnesium, found in the mantle covering the iron cores of planets. On Earth, there is a thin layer of the stuff in the mantle. Through computer simulations, the study team found the extra gravity of a "Super Earth" (twice as strong on its surface as Earth's) would crush these minerals into new forms, ones that would take on the properties of semi-conductors or metals, Wentzcovitch [said]. So what? The result there would be enhanced heat flow from the planet's core to the surface, which means more volcanoes and more "planetquakes." The effects on the planet's magnetic field, which on Earth shields the surface from solar radiation, of increased electrical activity in the mantle are more difficult to figure out, she says. The larger point is there is more to finding another Earth than detecting a planet the same size and same distance from its star, she says. Venus and Earth are very similar, she notes, but have significant differences in their interior chemistry. Venus has a more viscous interior that lead to a planet-sized earthquake hundreds of millions of years ago, she says, and that likely also explains the hellish conditions there, where 800-degree winds are lashed by sulfuric acid rain. ... [This adds significantly to the already known major problems of an `Earth-like' exoplanet with a mass only two or three times greater than Earth:
"Also vitally important is a planet's mass. A planet's habitability depends on its mass in many ways; terrestrial planets significantly smaller or larger than Earth are probably less habitable. Because Its surface gravity is weaker, a less massive Earth twin would lose its atmosphere more quickly, and because of its larger surface-area-to-volume ratio, its interior might cool too much to generate a strong magnetic field. smaller planets also tend to have more dangerously erratic orbits. In contrast, without getting more habitable, a more massive Earth-twin would have a larger initial inventory of water 14 and other volatiles, such as methane and carbon dioxide, and would lose less of them over time. Such a planet might resemble the gas giant Jupiter rather than our terrestrial Earth. In fact, Earth may be almost as big as a terrestrial planet can get. While life needs an atmosphere, too much atmosphere can be bad. For example, high surface pressure would slow the evaporation of water and dry the interiors of continents. It would also increase the viscosity of the air at the surface, making it more difficult for big-brained, mobile creatures like us to breathe. In addition, more surface gravity would create less surface relief, with smaller mountains and shallower seas. Even with more vigorous tectonic churning, rocks could not support mountains as high as those we enjoy. The planet probably would be covered by oceans and too mineral-starved at the surface (and too salty throughout) to support life. .... To add insult to injury, the surface gravity of a terrestrial planet increases with mass more rapidly than you might guess. Intense pressures compress the material deep inside, so that a planet just twice the size of Earth would have about fourteen times its mass and 3.5 times its surface gravity. This higher compression would probably result in a more differentiated planet; gases like water vapor, methane, and carbon dioxide would tend to end up in the atmosphere. Earth has kept dry land throughout its long history, in part, because some of its water has been sequestered in the mantle; in contrast, a more massive planet would probably have degassed more than Earth. there's another problem with larger planets impact threats. To put it simply, they're bigger targets. Asteroids and comets have a really hard time avoiding larger planets, so these planets suffer more frequent, high-speed collisions. While their bigger surfaces distribute the greater impact energy over more area, this doesn't compensate for the larger destructive energy, since surface area increases slowly with mass for terrestrial planets more massive than Earth." (Gonzalez G. & Richards J.W., "The Privileged Planet: How Our Place in the Cosmos is Designed For Discovery," Regnery: Washington DC, 2004, pp.59-60)
This therefore is another fine-tuned parameter that adds support to the extreme rarity (if not uniqueness) of Earth. Indeed, as Wentzcovitch points out, "Venus and Earth are very similar but have significant differences in their interior chemistry" with Venus having "hellish conditions where 800-degree winds are lashed by sulfuric acid rain."
So even if astronomers found an exoplanet with a "Mass 0.815 Earths" (which Venus is) it would not mean that it was truly Earth-like (in the all-important sense of permitting life as we know it). They would have to know a lot more about an exoplanet (including its internal chemistry) before they could say it was truly Earth-like, but it is difficult to see how they could ever know that at such immense distances (e.g. this exoplanet is "28,000 light years away"!). I have added Gonzales and Richards' quote above, and this article, to a new section of my "Problems of Evolution" book outline, PE 6.4.3. "Earth's fitness for life ... Mass"]
Stephen E. Jones, BSc (Biol).
"Problems of Evolution"
"Nay, `Natural Selection' seems capable of application not only to the building up of the smallest and most insignificant organisms, but even of extension beyond the biological domain altogether, so as possibly to have relation to the stable equilibrium of the solar system itself, and even of the whole sidereal universe. Thus, whether this theory be true or false, all lovers of natural science should acknowledge a deep debt of gratitude to Messrs. Darwin and Wallace, on account of its practical utility. But the utility of a theory by no means implies its truth. What do we not owe, for example, to the labours of the Alchemists?" (Mivart S.J., "On the Genesis of Species," Macmillan & Co: London & New York , Second edition, 1871, p.10)
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