We tend to assume alien life
forms will stay on their own
planets, because that’s what
we do. But if they don’t, then the
ISM could be a good place to
look for radio signalsCould life on Earth have originated in interstellar space? “No,
not a chance,” says Lewis Dartnell, an astrobiologist at the
University of Westminster. But that doesn’t stop the ISM
being an important area of study. “There’s a lot of interesting
astrochemistry going on,” says Dartnell.
Amino acids – the building blocks of DNA – have already
been found in meteorites from asteroids that pre-date the
formation of the Earth. “But that’s just the Lego bricks of life.
There’s nothing anywhere near as complex as a fully formed
cell,” Dartnell says.
Life couldn’t have got started under those conditions
without water or energy. “It’s like throwing a bunch of
chopped vegetables on the floor and expecting it to make
soup,” says Duncan Forgan, an astrobiologist from the
University of St Andrews.
However, the fact that they seem so ubiquitous has a lot of
astrobiologists excited about the chances of finding biology
elsewhere in the Universe. If those ingredients end up on a
warm world with water, then the necessary chemistry might
be able to occur.
“My impression is that all this actually tells you is that
carbon as an element is just really good at chemistry and
building up complex molecules,” Dartnell says. “If the cold
vacuum of interstellar space is making amino acids, you can
bet your bottom dollar that same chemistry is going on in the
warm oceans of a new world.”
If life can’t form in the ISM, what are the chances of it
starting out on a planet, then escaping that world and
travelling across interstellar space to seed another? On this
idea – panspermia – Dartnell is equally sceptical. “The idea
has really fallen out of favour,” he says. “The numbers just
don’t add up.”
However, interstellar space could be the place to listen for
signals from alien civilisations. So far we’ve focused on star
systems because we’re largely confined to our own world.
But alien life may not be restricted in the same way.
“Those signals might come from interstellar space as well, if
alien civilisations have spread beyond their home world and are
voyaging between the stars,” says Dartnell.
Interactions between the Sun and the ISM may also play an
important role in shielding us from radiation from the wider
Galaxy, in much the same way the Earth’s magnetism acts as
a force field to keep out dangerous radiation from the Sun.
“It might be the case that having two magnetic fields
protecting us is better than one,” says Forgan.
The Voyager probes, at the boundary between the Sun’s
magnetosphere and deep interstellar space, are uniquely
placed to make measurements of this region.
Now that it is out of the heliosphere, Voyager 1 has noticed
a significant spike in the number of cosmic rays. By
comparing this to the radiation levels from when it was inside,
we can learn to what extent we’re protected by the
heliosphere. That data will help astronomers and
astrobiologists figure out to what extent the double shield
idea is true, and whether that has a knock-on effect for our
notions about how common life might be around other stars.THE ORIGINS
OF LIFE
There’s no way that life could have formed
in interstellar space, but that doesn’t mean
there couldn’t be life there
SCIENCE