Super-Venuses

There are a lot of headlines about exo-planets. It would seem the general public has a modicum of interest in whether there are other planets on distant solar systems, and the continual addition to the collection of known solar systems or at least some selection of planets of distant solar systems keeps the interest up. One of the more prevalent news stories is about how some telescope or some astronomer has reported some super-Earth hundreds of light years from us.

There is a selection effect, in that small planets are harder to detect than a larger planet would be in the same orbit. Larger planets induce more wobble in their parent star and cause a bigger reduction in light when they fly in front of the star. So far, not many planets of the same size as the Earth have been found, but there are multiple super-Earths, which are planets only a small multiple of the mass or diameter of the Earth. This will certainly change, as budgets permit even higher resolution telescopes to be constructed and turned to the search for yet more exo-planets.

There is one Earth-sized planet that seems to be largely neglected, Venus. Venus is almost the same size as the Earth, both in mass and diameter. It is located about 72% of the distance to the sun as Earth, meaning solar radiation is about twice as much. From this alone, Venus should be hotter than the Earth, and it is, but the temperature difference is greatly exaggerated by the fact that Venus has a carbon dioxide atmosphere about a hundred times the mass of Earth’s. This produces a great greenhouse effect, which helps to explain why the equator of Venus has something like 465ºC, rather uniformly because the thick atmosphere spreads the heat.

Venus and Earth would produce identical signals to some alien world watching our star, using the same kinds of instruments as we currently use to find exo-planets. The same mass means that the wobble induced by Venus would be the same as that induced by Earth in the same orbit, and the same diameter means that Venus would block as much of the sun as Earth in the same orbit. So the conclusion from this is that when an astronomer issues a press release indicating they have discovered another super-Earth, or even something similar to Earth, they could just as easily produced a press release saying they discovered a super-Venus, or something similar to Venus. Atmospheres are quite thin compared to the diameter of a rocky planet, so it will be a while before reputable measurements of the atmospheric mass are available, which is what would be needed to directly discriminate between a Venus and an Earth in some distant solar system.

There are two explanations for why the atmospheres of Venus and Earth are so radically different. One might hark back to the formation of the Earth, during the first period of bombardment by asteroids, when a large planetoid is believed to have impacted the Earth, producing the moon. This impact might have blown off much of the atmosphere during the impact, and even more might have been ripped off if the moon started out its life in an orbit very close to the surface of Earth, from which it was torn. Venus has no moon, and it might be quite unlikely that such an impact, with just the right masses, velocities, and miss distance center-to-center, would happen to other planets in other solar systems. If this hypothesis is correct, we should be detecting super-Venuses instead of super-Earths, and soon, Venuses instead of Earths.

Another possible mechanism is that life ate up all the carbon dioxide in the atmosphere of a primitive Earth, producing oxygen in its place, creating the lightweight atmosphere Earth now has. This hypothesis has some difficulties. If there was an Earth with a huge carbon dioxide atmosphere at the present Earth orbital radius, it too would have a greenhouse effect that would raise its temperature above that where life could form. The older sun was somewhat less bright, but not that much less bright so as to allow this form of atmospheric modification to occur.

Just consider for a moment the situation in the galaxy if rocky planets forming in solar systems like the one we inhabit almost invariably have heavy carbon dioxide atmospheres, and there is rarely a situation with the right type of planetesimal collision to strip it down. Using our G2 sun as an example, if there was a Venus-like planet at Earth’s radius, there would not be life because of the high temperature, which means no aliens and no star travel. If there was a Venus-like planet further out toward Mars or even beyond, there would be a range of orbital radii where life could survive. A Venus-like planet in that range, if it somehow originated life, might have the same phenomena happen as happened on Earth: carbon dioxide disappears and oxygen appears. Exactly how so much carbon dioxide goes away might be a further question, but just suppose life is potent enough to have this happen almost completely. But when the carbon dioxide goes away, into rocks or sediments or living creatures, the greenhouse effect diminishes, and the planet gets colder and colder.

We had an ice age on Earth, nicknamed snowball Earth, which did not kill off all life. Quite possibly there was an equatorial area where there was no ice cover. But if Earth had been out at a Mars radius, the equatorial safe zone would likely not exist. The whole Earth would be a snowball, ending the chances of life surviving and evolving. And it would likely stay that way. Thus, one option for the non-existence of aliens traveling to Earth and giving us their business cards is that all the planets out in these distant solar systems are Venuses, too hot for life, or snowballs, too cold for life. Earth, with its fortunate collision four billion years ago, somehow was transformed into a planet where life could both originate and evolve. Perhaps there are other planets like ours, with a moon lingering as evidence of the collision, but the numbers would be drastically less than the count of super-Earths (really super-Venuses) would indicate.