Oct 02 2020

Speaking of Venus

Published by under Astronomy
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I recently discussed the exciting news of the discovery of phosphine gas in the clouds of Venus. This is exciting because phosphine is a potential marker of life. It should not exist on a small rocky world, and there is no known abiotic source on Venus. Microbes in the clouds above Venus are a plausible source, although scientists are careful to point out this is not proof of life, just a possibility. This finding has renewed interest in exploring our nearest neighbor, and even prior to this discovery NASA was planning another probe to Venus. This probe will likely have its mission altered to follow up on the phosphine discovery.

Venus is also interesting because it likely had a complex history over the last several billion years. A recent computer simulation, in fact, indicates that it may have been hospitable to life on the surface more than a billion years ago. Now the surface of Venus is hot enough to melt lead, making it the hottest planet in the solar system – even hotter than Mercury, which is closer. This is due to the extreme greenhouse effect from its mostly carbon dioxide atmosphere. The clouds above Venus are largely sulphuric acid. But a billion years ago it may have been more similar to Earth.

The dramatic change in Venus, in turn, may be tied to Jupiter. This is further connected to what we are learning about how stellar systems typically evolve, by observing exoplanetary systems. We now have many more data points than just our own solar system. We have confirmed over 4,000 exoplanets to date, with thousands more detected and awaiting confirmation. We can usually tell the mass, volume, and distance from the parent star, and so can construct a basic diagram of each system. However, depending on the method of detection used, we do not typically find every planet in an exosystem. Methods are generally biased towards larger and closer worlds.

One of the things we have discovered is that some planets are so-called hot Jupiters – they are gas giants orbiting very close to their star. About 10% of expolanets are hot Jupiters, and about 1% of systems have at least one hot Jupiter. Again – our detection systems more easily discover large and close planets, so these ratios may not represent reality and may be an overestimation. But a the very least, hot Jupiters are common throughout the galaxy.

One further question about hot Jupiters is, where did they form. Did they form close to their parent star, or did they form farther out and then migrate in? The answer may be both for different worlds, but there are reasons to think that many migrated in. Far away from their parent star early planets would more easily gobble up free hydrogen and helium gas for themselves, and form into gas giants. Computer simulations also show how the interaction between such gas giants and the primordial system would cause it to migrate in, very close to its star. In systems where this happens, these giant worlds would interact with the other closer planets of their system, perhaps kicking them out altogether.

What happened with our system? We can’t be sure, but astronomers have been running lots of computer simulations. One theory is that Jupiter did indeed start migrating in to a closer orbit around the sun. But then it moved back out to its current orbit, which has been stable. Some simulations credit Saturn with stabilizing Jupiter’s orbit at its current location. Without Saturn Jupiter may have become a hot Jupiter, and ruined everything for the inner planets. The new simulation now also suggests that this movement of Jupiter, inward then back out, probably affected Venus’s orbit as well (and also the Earth’s). It probably made Venus’s orbit more circular, for example. It’s unclear exactly how it changed the climate of Venus, but it kicked off the runaway greenhouse effect that gives us the current hot Venus.

Overall we have a tidy and stable solar system. This is not a coincidence – in any given solar system some stable scenario will develop. Stable orbits will persist, while unstable orbits will decay or planets will be ejected. Remember – there are likely more rogue planets in our galaxy than planets orbiting stars. Rogue planets wander, starless, in the fast distance between stars. They originally formed in planetary disks around stars and then were ejected, leaving behind the few planets with stable orbits. But also, giant planets with lots of gravity tend to have a stabilizing effect on the orbits of those remaining planets.

If all this is correct, referring to the history of our own solar system, it could mean that life evolved on the surface of Venus and then as Venus became progressively hotter, some of that life adapted to floating in the clouds above the surface, at an altitude that would be comfortable. It is also possible that life existed at one time on Mars, whose atmosphere went in the opposite direction, becoming too cold and thin. If true, it raises the possibility that life could have been seeded among these three worlds. Perhaps life arose only once, and then spread. Perhaps it developed independently three times. These are questions scientists would love to see answered.

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