Some interesting exoplanet news bites from around the web.
Sarah Seager and James Kasting are quasi-interviewed. Seager discusses the Hydrogen Greenhouse planets posited by Eric Gaidos and Raymond Pierrehumbert, while Kasting points out how difficult such worlds would be to observe. Seager also mentions her recent work on Desert Planets and how Venus missed out on becoming a still habitable planet thanks to the Runaway Greenhouse effect. Venus also doesn’t rotate fast enough to allow strong temperature gradients that might allow cooler climates near the planet’s poles.
Charles Qoi reports on Daniel Angerhausen and colleagues work on “Eyeball Earths” – planets that are tidally locked to their stars, with one face forever facing the Sun and the other forever facing the black of space. Exactly how such planets might evolve – whether their water would form a vast Ice-Cap on the night-side – is being actively researched.
Giovanni Vladilo and colleagues examine the effect of atmospheric pressure on a planets habitability. Thin atmospheres tend to result in oceans boiling away more easily than thick ones – thanks to the reduction in boiling point. Water being a super-star greenhouse gas is less likely to cause trouble when there’s more of other gases, like nitrogen or even carbon dioxide. Best to read the paper to get the best idea of their thesis – the URL is given with the diagram: http://wwwuser.oats.inaf.it/astrobiology/planhab/.
Overall the current trend in thinking is towards habitability – of at least part of a planet – has a wider zone around the stars than formerly believed. The inner edge might be ~0.5 AU (X4 Earth’s insolation), while the outer edge is 1.7 AU for a CO2 greenhouse, but much, much further out if there’s enough nitrogen or hydrogen to beef it up. All the way out to Saturn if the planet’s lifeforms are driven by sunlight, or all the way out if they can subsist on geothermal energy.
A puzzling question is whether such planets can be called inhabitable by oxygen-breathers like us. Desert Planets, again, have something to say about that – they withstand a Runaway Greenhouse closer in, and avoid the negative feeback of too much snow further out. But over a narrower range than planets with more exotic atmospheres.