Alien life unlikely on planets around scifi favourite star
Sen—Hopes that extraterrestrial life might exist on a Sun-like star that is one of the closest to Earth appear to have been dashed by a new study.
Tau Ceti has long been a favourite location for possible alien life, not just for writers of scifi but for scientists looking for signs of ET too. As well as featuring in many novels, and even an early video game, it played a “star” role in Star Trek and other movies.
A forerunner to SETI, called Project Ozma, picked Tau Ceti as one of two stars, along with Epsilon Eridani, to listen to with a giant radio telescope dish at Green Bank, West Virginia, for alien signals in 1960.
The star, which is bright enough to be seen easily with the unaided eye in a dark sky, lies just under 12 light-years away in the constellation of Cetus, the sea monster or whale. It is four-fifths the mass of the Sun and of similar type.
In recent years, planet hunters have located up to five exoplanets in Tau Ceti’s own solar system. And excitingly, it appeared that two of them lay inside the so-called habitable zone where conditions would allow water to exist in liquid form.
But the possibility that aliens could be living right on our doorstep seems to have been ruled out by new research from Arizona State University (ASU) that used astronomical and geophysical techniques to examine the two planets, dubbed Tau Ceti e and f.
Led by astrophysicist Michael Pagano, the team used the chemical make-up of the home star to work out how it had evolved and the nature of its habitable zone. Their checks confirmed that though planets e and f might be in the habitable zone, the prospects of life are low.
The estimated sizes of the two planets compared to the Earth. Image credit: Planetary Habitability Laboratory, University of Puerto Rico at Arecibo
The research found that the chances of planet e being in the zone are low, and that planet f has not been in it long enough for meaningful life to have developed there. They say that f will have been in the right place for a lot less than a billion years, and only after its home star brightened enough to provide the required warmth.
Pagano said in a statement: “Planet e is in the habitable zone only if we make very generous assumptions. Planet f initially looks more promising, but modelling 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.”
The writer suggested to Pagano that his findings would disappoint legions of science fiction fans.
Pagano told Sen: “Yes, the science shows that Tau Ceti should be a weird and different place for a planet to preside. This means that for the purpose of science, it may be too strange a place for life as we know it to exist.
“This obviously doesn't mean it's not an interesting place to imagine in scifi, because even though we may not put Tau Ceti as high on our list of places to look at for life (by which we mostly mean microbial life, not intelligent life) it still shows that not every planet will be Earth-like and it may be even more exciting to imagine the strangeness of planets so much unlike our own.”
Pagano worked with astrophysicists Patrick Young and Amanda Truitt and mineral physicist Sang-Heon (Dan) Shim, from ASU’s School of Earth and Space Exploration, to analyse the Tau Ceti system. Their research paper is published in the Astrophysical Journal.
Shim checked out the likely composition of the planets, judging from the known chemistry of Tau Ceti. Magnesium and silicon are both important rock forming minerals on Earth which are also found in that star. But the ratio of magnesium to silicon in Tau Ceti is 1.78, which is about 70 per cent more than our Sun.
Shim said in a statement: “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. Tau Ceti’s planets could very well be dominated by the mineral olivine at shallow parts of the mantle and have lower mantles dominated by ferropericlase.”
He explained that this arrangement could have profound implications for volcanism and tectonics at the surface of the star’s planets, both of which would also affect their suitability for life.
“This is a reminder that geological processes are fundamental in understanding the habitability of planets,” Shim added.
The new findings come after UK astronomers discovered, in 2004, that Tau Ceti is surrounded by ten times as much as dust and rocky debris as there is in our own Solar System. This suggested that any life on its planets would suffer many more comet and asteroid impacts than worlds orbiting the Sun.
There is a glimmer of hope for scifi fans. In his comments to Sen, Pagano added: “We are not excluding life on these planets, just saying that because of the possibility for new geophysics it shouldn’t be on the top of observing lists. Life could always find a way. It’s a good motto.”