A new study by Jacob Haqq-Misra of the Blue Marble Space Institute of Science and his co-authors shows how climatic swings narrow the so-called habitable zone of other solar systems—to the point where planets around some types of stars may be altogether unsuitable for complex life.
The climate swings, called limit cycles, occur in the outer regions of a star’s habitable zone (those orbits in which liquid water could be stable on a planetary surface). The limit cycles lead to long periods of global glaciation alternating with periods of relative warmth. Our own planet has experienced global glaciations as well, called Snowball Earth events, but on Earth the ice cover is thought to have been thin, with some areas of open water. Snowball Earth events caused by limit cycles are expected to be more severe, with ice covers measured in kilometers. And that would make the evolution of complex life rather unlikely.
The research team, which also included James Kasting of Pennsylvania State University, found a close link between carbon dioxide outgassing rates and the occurrence of limit cycles. Planets with rates as high as Earth’s is today may not experience limit cycles at all. But there is a distinct possibility that our planet is anomalous in its high outgassing rates and ability to sustain a stable warm climate. If that is so, then Super-Earths—planets with a few times the mass of Earth—might be more likely places to find life in the outer reaches of a habitable zone, as they should have more carbon dioxide in their atmosphere than smaller (Earth-size) planets.
Limit cycles may have caused some of the global glaciations Earth experienced early in its history, when it was located in the outer reaches of the Sun’s habitable zone (the zone has moved outward over time). However, we have no corroborating evidence that carbon dioxide outgassing was less on early Earth than it is today, and that evidence would be hard to come by.
What implications do these new findings have for the likelihood of finding complex life, even intelligent life, on exoplanets? The outlook is bleak for F stars, both because of limit cycles and the short lifetimes of these stars, which usually last only one or two billion years. That leaves planets around G, K, and M stars as reasonable abodes for life. However, recent research indicates that M stars lose massive amounts of water during their evolution. They’re also exposed to intense radiation, and planets around these stars may be tidally locked, meaning that the same side always faces the star. G stars like our sun become more luminous with time, which means that their habitable zone moves outward. This effect will make Earth uninhabitable for humans in about a billion years. That leaves K stars (orange dwarf stars) as perhaps the most likely places to find complex life, though some G stars might also host planets with a substantial biosphere (as evidenced by Earth). The good news: K stars are fairly common, making up about 12 percent of all main sequence stars (compared to G stars, which make up 7.5 percent). So while the expected number of “cosmic zoos” may just have dropped a bit due to severe climate change on other planets, there still should be plenty of habitable worlds out there.