How would an alien world like this look? That’s the question that
undergraduate art major Joshua Gonzalez attempted to answer. He worked
with Professor Patrick Young’s group to learn how to analyze stellar
spectra to find chemical abundances, and inspired by the scientific
results, he created two digital paintings of possible unusual extrasolar
planets, one being Tau Ceti for his Barrett Honors Thesis. Credit:
Joshua Gonzalez.
The list of potential life-supporting planets just got a little shorter
As the search continues for Earth-size planets orbiting at just the
right distance from their star, a region termed the habitable zone, the
number of potentially life-supporting planets grows. In two decades we
have progressed from having no extrasolar planets to having too many to
search. Narrowing the list of hopefuls requires looking at extrasolar
planets in a new way. Applying a nuanced approach that couples astronomy
and geophysics, Arizona State University researchers report that from
that long list we can cross off cosmic neighbor Tau Ceti.
The Tau Ceti system, popularized in several fictional works,
including Star Trek, has long been used in science fiction, and even
popular news, as a very likely place to have life due to its proximity
to Earth and the star’s sun-like characteristics. Since December 2012
Tau Ceti has become even more appealing, thanks to evidence of possibly
five planets orbiting it, with two of these – Tau Ceti e and f –
potentially residing in the habitable zone.
Using the chemical composition of Tau Ceti, the ASU team modeled the
star’s evolution and calculated its habitable zone. Although their data
confirms that two planets (e and f) may be in the habitable zone it
doesn’t mean life flourishes or even exists there.
“Planet e is in the habitable zone only if we make very generous
assumptions. 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,” explains astrophysicist Michael Pagano, ASU
postdoctoral researcher and lead author of the paper appearing in the
Astrophysical Journal. The collaboration also included ASU
astrophysicists Patrick Young and Amanda Truitt and mineral physicist
Sang-Heon (Dan) Shim.
Based upon the team’s models, planet f has likely been in the
habitable zone much less than 1 billion years. This sounds like a long
time, but it took Earth’s biosphere about 2 billion years to produce
potentially detectable changes in its atmosphere. A planet that entered
the habitable zone only a few hundred million years ago may well be
habitable and even inhabited, but not have detectable biosignatures.
According to Pagano, he and his collaborators didn’t pick Tau Ceti
“hoping, wanting, or thinking” it would be a good candidate to look for
life, but for the idea that these might be truly alien new worlds.
Tau Ceti has a highly unusual composition with respect to its ratio
of magnesium and silicon, which are two of the most important rock
forming minerals on Earth. The ratio of magnesium to silicon in Tau Ceti
is 1.78, which is about 70% more than our sun.
The astrophysicists looked at the data and asked, “What does this mean for the planets?”
Building on the strengths of ASU’s School of Earth and Space
Exploration, which unites earth and space scientists in an effort to
tackle research questions through a holistic approach, Shim was brought
on board for his mineral expertise to provide insights into the possible
nature of the planets themselves.
“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,” explains Shim.
Considering that ferropericlase is much less viscous, or resistant to
flowing, hot, yet solid, mantle rock would flow more easily, possibly
having profound effects on volcanism and tectonics at the planetary
surface, processes which have a significant impact on the habitability
of Earth.
“This is a reminder that geological processes are fundamental in understanding the habitability of planets,” Shim adds.
“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,” says Pagano.
This work was supported by funding from the NASA Astrobiology Institute and NASA Nexus for Exoplanet System Science.
Written by Nikki Cassis
