Snowball and Greenhouse Atmospheres Around Other Stars

Earth’s climate has been shown by a wide range of climate models to be bistable, which means that it can exist in both a frozen state or a warm, ice-free state with the same amount of solar energy. Earth today resides in a warm state with small ice caps, but geologic evidence suggests that the Neoproterozoic Earth, about 500 Myr ago, may have been frozen all the way down to the equator.

Other terrestrial planets orbiting other stars should behave similarly, although the transition point between climate states may differ for stars that are brighter and dimmer than the sun. In a recent paper published in The Astrophysical Journal, titled “Constraints on climate and habitability for Earth-like exoplanets determined from a general circulation model,” my co-authors and I analyze three-dimensional climate simulations of planets orbiting a range of stars. We show that planets can exist around such stars as a frozen snowball, partial melt with an equatorial waterbelt, temperature conditions everywhere, and a hothouse with gradual water loss. Stars slightly cooler than the sun could maintain habitable conditions for longer periods of time, due to the slower rate of water loss.

Why do we live around a yellow star?

Red dwarf stars outnumber yellow dwarf stars like our sun by over a factor of ten. Observations of exoplanets have also shown that rocky, and potentially habitable, planets are just as common around red dwarfs as yellow dwarfs. But if these much smaller stars are more commonplace, then why do we find ourselves around a yellow star like the sun, instead of a red dwarf?

My co-authors and I attempt to address this question of selection bias in a recent paper titled “Why do we find ourselves around a yellow star instead of a red star?” and published in International Journal of Astrobiology. We take a statistical approach to thinking about the region around all stars where life is most likely to develop. The liquid water habitable zone provides the best observational constraint on where we would expect to find planets that could support conscious observers like us, and this study examines the probability of finding oneself on a planet in the habitable zone of a yellow dwarf star, compared to a red dwarf. The results show that even though red dwarfs are much more numerous, they have a narrower habitable zone than yellow dwarfs, so our existence around a star like the sun is actually to be expected.

This study also considers that red dwarf stars will be even more numerous in the distant future of the universe, due to their much longer lifetimes than other stars. If these red dwarf stars will eventually become the predominant place for conscious observers to develop, then why do we not instead find ourselves around a red dwarf star billions or trillions of years into the future? The statistics for this aspect of the problem suggest that our existence around a yellow dwarf star today, compared to a red dwarf star in the future, might be a slight statistical anomaly—perhaps on the order of finding oneself born ambidextrous or with perfect pitch. But this statistical unlikelihood might also suggest that life is wholly impossibly around red dwarf stars, or else any type of conscious observers that do develop around such stars will be drastically different from our type of conscious life.

A Lottery Bond to Fund the Search for Aliens

Finding sources of funding to search for life in the universe can be tricky, with only a few individual wealthy investors and limited opportunities for government research support. New fundraising ideas are needed in order to sustain the search for extraterrestrial intelligence (SETI) over the coming decades.

In a recent paper titled “Funding the search for extraterrestrial intelligence with a lottery bond,” published in Space Policy, I propose the establishment of a “SETI Lottery Bond” to help defray the costs of operating observing facilities like the Allen Telescope Array. The SETI Lottery Bond would provide a fixed-rate of interest that continues in perpetuity, until the first confirmed discovery of extraterrestrial intelligent life, at which point a subset of shares will be awarded a prize from a lottery pool. Investors can also trade their shares, so that SETI Lottery Bond shares may be passed between generations, teaching the value of intergenerational savings while maintaining hope for the discovery of extraterrestrial life.

Lottery-based savings products can only be offered by financial institutions with the legal authorization to engage in banking and gaming activities. I propose that one or more financial institution could realize a profit through the establishment and management of a SETI Lottery Bond, while simultaneously promoting individual savings habits and assisting the search for extraterrestrial intelligent life.

Climate Cycles on Early Mars

Mars today shows evidence of flowing water in its past from the presence of delta basins, canyons, and surface mineralogy. The remaining water on Mars today seems to be locked up in ice, but at some point in the early history of the solar system, Mars had flowing rivers, lakes, and even oceans.

The problem with this idea of a warm and wet early Mars is that the sun was fainter in the past, thereby providing even less energy than today to help thaw a frozen planet. One possibility is that early Mars had a much thicker greenhouse atmosphere than today, which could have provided enough additional warming to melt the ice. However, many climate models struggle to provide sufficient warming, even with a dense carbon dioxide atmosphere. Another option is that episodes of volcanic eruptions of meteor impacts could have temporarily warmed Mars long enough for water to flow and carve the fluvial features we see today. However, this type of episodic warming may not provide enough rainfall to carve the martian valleys observed today.

In a recent paper published in Earth and Planetary Science Letters, titled “Climate cycling on early Mars caused by the carbonate-silicate cycle,” my co-authors and I propose that climate cycling between warm and glacial states could have occurred on early Mars, driven by the carbonate-silicate cycle as we discuss in a previous study. For early Mars, the accumulation of greenhouse gases may have risen and fallen in episodic cycles, providing punctuated periods of warmth for carving the martian valleys. Our proposed hypothesis combines the notion of enhanced greenhouse warming with episodic warming, which can potentially be tested by future exploration of the martian surface.