Much of the
search for life outside of Earth's biological oasis has focused on examining
the conditions on the other planets in our solar system and probing the cosmos
for other Earth-like planets in distant planetary systems.
But one
team of astronomers is approaching the question of life
elsewhere in the universe by looking for life's
potential beginning.
Aparna
Venkatesan, of the University of San Francisco, and Lynn Rothschild, of NASA's
Ames Research Center in Moffett Field, Calif., are using models of star
formation and destruction to determine when in the roughly 13.7 billion-year history
of the universe the biogenic elements – those essential to life as we know it –
might have been pervasive enough to allow life to form.
We can pin
down the emergence of life on Earth to somewhere around 3.5 billion years ago. Venkatesan
and Rothschild want to find out what happens when you broaden the question to
life throughout the universe.
"Can
you blast that open? Could you really start really talking about life in the
universe at 12 billion years? And that's the question that we're talking about,"
Rothschild said.
With basic
estimates of the elements produced by the first several generations of stars,
the pair has so far found that "most of [the essential elements] can be
created fairly quickly in the early universe," Venkatesan said.
Venkatesan
presented their first findings last week at the 214th meeting of the American
Astronomical Society in Pasadena, Calif.
Biogenic
elements
For life as
we know it to form and thrive, four conditions must be met: sufficient amounts
of the so-called biogenic elements, a solvent (on Earth, that solvent is liquid
water), a source of energy, and time "for the elements to build up and
create a home and conditions for life to thrive," Venkatesan explained.
The biogenic
elements include carbon, nitrogen, oxygen, phosphorous, sulfur, iron, and
magnesium.
"Carbon
in particular is very interesting," Venkatesan said. Carbon is "ubiquitous
in the solar system and beyond" and "is extremely versatile
chemically."
These
elements, like all elements present in the universe today, are forged in the
furnaces of stars. But not all stars make each element, and some produce
elements much faster than others.
Low-mass
stars create all the elements on the periodic table through carbon,
but because these stars live long lives, they produce the elements slowly.
Intermediate mass stars tack on nitrogen through oxygen. Finally, the most
massive stars, with their intense ovens, make all the elements up to iron and some
other heavy metals. And because these stellar beasts lead such short,
violent lives, they can churn out elements faster than smaller stars.
The
explosions that end these stars' lives can vary though, and their different
signatures indicate the amounts of metals, such as iron and nickel, involved,
Venkatesan said.
It is
thought that the first stars to form in the early universe were very massive.
These stars would have characteristic compositions that in turn imply that they
would have specific elemental abundances "that they create in their death
throes."
The two
scientists came up with the idea for applying the study of the first stars to
astrobiology when Rothschild came to Venkatesan's department for a talk. While
talking at dinner that night, "we began to realize it might be really fun
to look at just when the first building blocks for life could be out
there," Venkatesan said. "To the best of our knowledge, we didn't
know anyone else out there who was at the time talking about it or thinking
about it."
Wish
list
Rothschild
drew up what she calls her "wish list" of elements that she considers
absolutely essential to life as we know it. Venkatesan then used current
theories of star formation, from the first very massive stars to the stars that
formed later from the seeds sown by the first stars, to model the build up of
each of the biogenic elements.
"The
number one element is carbon," Rothschild said. "And you come up with
that because they're really only two elements that have any real versatility in
terms of being able to create a bunch of compounds that could then form a life,
and one is silicon and one is carbon."
But silicon
gets ruled out because it isn't as prevalent in the universe, nor as chemically
versatile.
"The
reality check is that we're sitting on a big silicate rock, and we're not made
of silicon," Rothschild said.
Rounding
out the list of must-haves are hydrogen, oxygen and nitrogen.
"Nitrogen
seems to be critical. It's found in so many compounds, and that really adds
huge versatility then to the suite," Rothschild said. Nitrogen, for
example, is the backbone of amino acids, which in turn are the building blocks
of proteins and have been detected in interstellar space.
Secondary
and tertiary lists include phosphorus, sulfur, iron and magnesium, "and
all sorts of funky things which are used a lot, but I could more easily
conceive of a system without it," Rothschild said.
Elemental
build-ups
They found
that "nitrogen can actually build up very quickly," Venkatesan said.
But not right at the beginning, because those first massive stars "woefully
under-produce nitrogen." It takes later-generation stars to boost levels
high enough to what scientists think might be needed to make the element
pervasive enough.
Carbon also
"takes a little while to build up," because it needs low- and
intermediate- mass stars, Venkatesan said.
While those
early massive stars would have had trouble producing nitrogen, they "are
fairly efficient at producing iron early on. That is because they completely
blow apart," Venkatesan said.
Overall,
the modeling effort found that iron and magnesium levels would have surged
early on, with carbon taking at least 100 million years to build up.
Though the
critical masses of biogenic elements needed to allow life to form aren't known,
"these amounts will be more than enough," Venkatesan said.
So by
perhaps around 100 million after the universe began, many of these elements would be found in
substantial enough numbers, though the timescale may be more around 500 million
years for carbon and the jury is still out with nitrogen.
Other
ingredients
Better
models and improved knowledge of the physics at work in early stars could
change the picture somewhat, changing the timescales for the buildups of the
elements and the interstellar environment they are born into.
Of course, knowing
which elements need to be present and whether or not they are won't answer the
question of when life might have been able to spring forth. The elements must
also collect in pools in significant enough amounts.
"That
final question is not only which elements, but what concentration do you build
up locally?" Rothschild said.
Once
Rothschild comes up with estimates of the amounts of different elements likely
required, she and Venkatesan can use models that estimate concentrations in
galaxies and solar systems over time and see if they find any likely-looking
spots for life to form.
"All
we need is one place in the universe that has the conditions, the prerequisites,"
Rothschild said.
Solvents,
such as liquid water or methane, will also have to be factored in. Venkatesan
said that in the long term, they hope to use the same methods to figure out
when water might have existed in sufficient quantities.
There is
also the question of whether life could have thrived in the harsh,
ultraviolet-dominated environments of the early
stars. Ultraviolet light is thought to have both beneficial and detrimental
effects on life, but which might have won out in the early universe isn't
known.
Ultimately
the question will become, "can we build up the building blocks" early
on, Venkatesan said. Though answering that question will take some time, it
could have a substantial impact on studies of the early universe, exoplanet
research, and the expectations of how far along alien life might have evolved,
not to mention our view of our place in the universe.
"It's
not going to cure cancer," Rothschild said. "But I think in a way,
it's a very profound question: when can you start talking about life in our
universe?"
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