Extrasolar Planets: Bigger and More Mortal
Many of the stars and planets found by the Kepler spacecraft are not earthlike. Also, astronomers have seen planets destroyed by their stars.
Earthlike Planet Pool Shrinks
NASA’s Astrobiology Magazine reported that follow-up measurements of Kepler’s candidate earthlike planets show most of them are larger than thought; a quarter of them are 35% larger or more. “By implication, these new results reduce the number of candidate Earth-size planet analogues detected by Kepler,” one researcher said. Source: National Optical Astronomy Observatory News. Historical note: Johannes Kepler, a creationist, speculated on the habitability of other worlds.
The announcement came just days after PhysOrg asked “How common are earths around small stars?” Someone has found a way for earth-size planets to exist around red dwarfs (the most numerous category of stars), where tidal locking and flares were thought to rule out living conditions. The brief article did not explain how those dangerous conditions could be overcome.
PhysOrg reported the discovery of a “dust trap” around a star that could allow a planet to accrete without migrating into the star. If they could get the material to stick, this is the only way astronomers can account for rocky planets forming:
Computer models suggest that dust grains grow when they collide and stick together. However, when these bigger grains collide again at high speed they are often smashed to pieces and sent back to square one. Even when this does not happen, the models show that the larger grains would quickly move inwards because of friction between the dust and gas and fall onto their parent star, leaving no chance that they could grow even further.
Somehow the dust needs a safe haven where the particles can continue growing until they are big enough to survive on their own. Such “dust traps” have been proposed, but there was no observational proof of their existence up to now.
The article did not discuss the sticking problem: dust grains do not stick together; they, too, are “often smashed to pieces” (see 2/03/04, 5/21/09). Having a safe haven does not imply that anything good is likely to happen there.
A paper in Nature claims that a new model keeps Mercury dry and Earth wet when they accrete from dust – that is, if plenty of tweaks are not overlooked:
The authors’ model underscores the importance of the earliest accretion and solidification steps in determining the future evolution of the rocky planets. However, several crucial caveats need to be considered in applying this model. First, in extrapolating back in time, the faint young star’s radiation level needs to be considered. Second, initial atmospheres might not all be water-rich; the rocky building blocks for some planets might have produced atmospheres rich in methane and hydrogen, instead of steam. In the absence of a steam atmosphere, there would be no outgoing radiation limit to slow solidification and cooling. Third, forming an initial atmosphere above a magma ocean is not a simple process. The removal of volatile gases from magma might require a significant degree of supersaturation and might not occur until late in solidification. If this is so, then solidification would proceed to a high degree before a steam atmosphere formed and occluded heat flux.
Planets Aren’t Forever
Old stars devour their progeny, New Scientist reported. “A survey of ageing stars offers some of the first direct evidence that these cantankerous elders often rip their nearest planets to shreds.” A JPL press release said that stars don’t obliterate their planets—at least, that is, very often. It appears “often” is in the eye of the beholder.
Scientific pronouncements are only tentative. Often further observations call them into question. Then later, more observations seem to confirm the earlier idea. At any given time, the probability that scientists are wrong about what they confidently assert is non-trivial.
If a model works, it does not follow that nature operates according to the model. Models are simplifications of complex processes that are useful for exploring possibilities. Who knows what omitted factors, no matter how inconsequential they might seem to the investigator, are crucial for the conclusions? The less models can be tested against observations, the more they become computer games for the entertainment of humans, not for explaining reality.