Black holes are engines of destruction on
a cosmic scale, but they may also be the bringers
of life.
New research on supermassive black holes suggests
that the radiation they emit during feeding
frenzies can create biomolecular building
blocks and even power photosynthesis.
The upshot?
Far more worlds roaming the Milky Way and
beyond could be suitable to life, the researchers
speculated.
For their new study, published May 24 in the
Astrophysical Journal, scientists created
computer models to look at the radiating disks
of gas and dust called active galactic nuclei,
or AGN, that swirl around supermassive black
holes.
Some of the brightest objects in the universe,
AGN form as a black hole's gravity binds matter.
As that matter swirls around a black hole,
it releases incredible amounts of light and
radiation.
Since the early 1980s, scientists have suspected
that this radiation would create a dead zone
around an AGN.
Some researchers even proposed that such an
AGN could explain why we haven't seen any
complex extraterrestrial life towards the
center of the Milky Way.
Our galaxy has a monstrous black hole at its
center, called Sagittarius A*.
Previous studies have found that within 3,200
light-years of a Sagittarius A*-sized AGN,
X-rays and ultraviolet light could strip the
atmospheres from Earth-like planets.
(The Milky Way is nearly 53,000 light-years
across.)
"People have mostly been talking about the
detrimental effects [of black holes]," Manasvi
Lingam, lead author on the study and an astronomer
at Harvard University, told Live Science.
"We wanted to reexamine how detrimental [the
radiation] is … and ask ourselves if there
were any positives."
The researchers' models suggest that worlds
with atmospheres that are thicker than Earth's
or those far enough away from an AGN to retain
their atmospheres might still stand a chance
of hosting life.
At certain distances, there exists a galactic
Goldilocks zone that gets just the right amount
of ultraviolet radiation.
At this level of radiation, the atmosphere
wouldn't be stripped away, but the radiation
could break apart molecules, creating compounds
that are necessary for building proteins,
lipids and DNA — the cornerstones to life,
at least as we know it.
For a black hole the size of Sagittarius A*,
the Goldilocks region would extend approximately
140 light-years from the black hole's center,
where 1 light-year is 93 million miles (150
million kilometers).
The scientists also looked at the effects
of the radiation on photosynthesis, the process
by which most plants utilize the sun's energy
to create sugars.
And AGN emit enormous amounts of that key
ingredient — light.
This would be particularly important for plants
on free-floating planets, which have no nearby
host star to provide a light source.
Astronomers have estimated there could be
around 1 billion such rogue planets drifting
in the Goldilocks zone of a Milky Way-like
galaxy, according to Manasvi.
Calculating the area over which AGN could
power photosynthesis, the scientists found
that large portions of galaxies, particularly
those with supermassive black holes, could
have AGN-powered photosynthesis.
For a galaxy similar to our own, this region
would extend around 1,100 light-years out
from the center of the galaxy.
In small, dense galaxies called ultracompact
dwarfs, more than half of the galaxy could
reside in that photosynthetic zone.
Taking a fresh look at the negative effects
of the ultraviolet and X-ray radiation in
these zones, the scientists in the new study
further found that the adverse consequences
of an AGN neighbor have been exaggerated in
the past.
Bacteria on Earth have created biofilms to
protect themselves from ultraviolet rays,
and life in ultraviolet-heavy areas could
have developed similar techniques.
X-rays and gamma-rays, which AGNs also spew
in enormous quantities, are also readily absorbed
by Earth-like atmospheres and would likely
not have a large influence on life, the researchers
said.
The scientists estimated that the damaging
effects of AGN radiation likely would end
at around 100 light-years out from a Sagittarius
A*-size black hole.
"Looking at what we know about Earth, it does
suggest that maybe the positive effects seem
to be extended over a larger region than the
negative effects," Lingam told Live Science.
"That was definitely surprising."
