If you think summer is too hot or winter unbearably cold, take solace that in the distant past seasons on our planet might have been much harsher. However, the advent of milder seasons did more than offer comfort, some scientists suggest.
Subdued seasonality might be linked to the emergence of complex life on Earth around 600 million years ago. On alien worlds, extreme seasonal spikes and plunges in temperature could likewise determine whether life teems, scrapes by, or dies.
Seasons arise when the axis of a planet's spin is tilted relative to the plane of the planet's orbit. Recent research has suggested that a loss of axial tilt and its attendant seasonality, which helps moderate global temperatures, could doom extraterrestrial creatures. Scientists are also considering the opposite case: worlds where blazing summers and devastatingly frigid winters make the development of life with any complexity a long shot.
"Axial tilt, or obliquity, is a crucial parameter for climate and the possible habitability of a planet," said René Heller, a postdoctoral research associate at the Leibniz Institute for Astrophysics in Potsdam, Germany. Heller was the lead author on two papers last year on obliquity loss due to tidal interactions on habitable planets around red dwarf stars.
Many phenomena influence obliquity over a planet's history. Major examples include the impacts of large cosmic bodies, as well as the gravitational pulls from companion planets and central stars. Over the course of a year on a tilted planet, varying amounts of warming sunlight strike the northern and southern hemispheres.
The Earth presently has an obliquity of about 23.5 degrees. Along with daily rotations, this moderate obliquity ensures that the temperature differences between the coldest polar and hottest desert regions are not too extreme. [Photos: The Strangest Alien Planets]
Unlike our planet, another world with a low axial tilt of no more than a few degrees would not experience much seasonality. The colder poles would lead to a narrower habitable region, and if coupled with a too-hot equator could render the world a difficult place for complex life. It is an even grimmer picture for high-obliquity planets in a planetary system's "Goldilocks" zone, the orbital band where water can stay in liquid form on a world's surface.
Take the case of an Earth-like planet with an obliquity close to that of Uranus, about 90 degrees. The north pole would point at the central star for a quarter of the year and then directly away for another quarter.
"Your northern pole will be boiled during part of the year while the equator gets little sunlight," said Heller. Meanwhile, "the southern pole freezes in total darkness." Essentially, the conventional notion of a scorching hell dominates one side of the planet, while an ultra-cold hell like that of Dante's Ninth Circle prevails on the other.
Then, to make matters worse, the hells reverse half a year later. "The hemispheres are cyclically sterilized, either by too strong irradiation or by freezing," Heller said.