A solar system close enough to Earth to be considered a neighbor has been long put forward as a prime candidate to harbor alien life. A new study now says the chances of finding it there are slim.
Tau Ceti is a sun-like star just 12 light-years away. It has five orbiting planets, and two of them, Tau Ceti e and Tau Ceti f, may be orbiting in the star's habitable zone — that "just right" Goldilocks distance that might allow life to flourish.
A study led by scientists at Arizona State University has however modeled the evolution of the star and its planets and found that, even if the two planets are in fact in Tau Ceti's habitable zone, there are compelling reasons to suspect that neither supports the existence of liquid water on their surface — an established requirement for life.
"Planet e is in the habitable zone only if we make very generous assumptions," said study lead author Michael Pagano. "Planet f initially looks more promising, but modeling the evolution of the star makes it seem probable that it has only moved into the habitable zone recently, as Tau Ceti has gotten more luminous over the course of its life."
In addition, Tau Ceti – although seeming similar to our own sun in many ways – has a chemistry that is quite different, with much more magnesium than silicon.
That unbalanced ratio means any rocky planets forming in orbit would have mantles with high levels of a magnesium and iron-rich mineral known as ferropericlase.
Ferropericlase is extremely dense and does not flow easily. Those are qualities that would severely limit plate tectonic activity and resultant volcanism — processes greatly involved in the evolution of life on Earth.
"With such a high magnesium and silicon ratio, it is possible that the mineralogical make-up of planets around Tau Ceti could be significantly different from that of Earth," said ASU mineral physicist Sang-Heon (Dan) Shim. "This is a reminder that geological processes are fundamental in understanding the habitability of planets."
Although the new study will come as a disappointment to some – Tau Ceti has featured prominently in science fiction works like Star Trek – Pagano said it shouldn't discouraging the ongoing search for extraterrestrial life.
"Tau Ceti has been a popular destination for science fiction writers and everyone's imagination as somewhere there could possibly be life, but even though life around Tau Ceti may be unlikely, it should not be seen as a letdown, but should invigorate our minds to consider what exotic planets likely orbit the star, and the new and unusual planets that may exist in this vast universe," he explained.
This study was published in the Astrophysical Journal.
