Cambridge, MA - A team of astrophysicists and planetary scientists has predicted that Neptune-like planets located near the center of the Milky Way galaxy have been transformed into rocky planets by outbursts generated by the nearby supermassive black hole.
These findings combine computer simulations with data from recent
exoplanet findings, and X-ray and ultraviolet observations of stars and
black holes.
"It's pretty wild to think of black holes shaping the evolutionary
destiny of a planet, but that very well may be the case in the center of
our Galaxy," said Howard Chen of Northwestern University in Evanston,
IL, who led the study.
Howard Chen and collaborators from the Harvard-Smithsonian Center for
Astrophysics (CfA) in Cambridge, Mass., examined the environment around
the closest supermassive black hole to Earth: the four-million-solar
mass black hole known as Sagittarius A*.
It is well known that material falling into the black hole in
occasional feeding frenzies will generate bright flares of X-ray and
ultraviolet radiation. Indeed, X-ray telescopes such as NASA's Chandra
X-ray Observatory and ESA's XMM-Newton have seen evidence for bright
outbursts generated in the past by the black hole ranging from about 6
million years to just over a century ago.
"We wondered what these outbursts from Sagittarius A* would do to any
planets in its vicinity," said John Forbes, a co-author from the CfA.
"Our work shows the black hole could dramatically change a planet's
life."
The authors considered the effects of this high-energy radiation on
planets with masses in between Earth and Neptune that are located less
than 70 light years away from the black hole.
They found that the X-ray and ultraviolet radiation would blast away a
large amount of the thick, gas atmosphere of such planets near the
black hole. In some cases this would leave behind a bare, rocky core.
Such rocky planets would be heavier than the Earth and are what
astronomers call super-Earths.
"These super-Earths are one of the most common types of planet that
astronomers have discovered outside our Solar System," said co-author
Avi Loeb, also of CfA, "Our work shows that in the right environment
they might form in exotic ways."
The researchers think that this black hole impact may be one of the
most common ways for rocky super-Earths to form close to the center of
our Galaxy.
While some of these planets will be located in the habitable zone of
stars like the Sun, the environment they exist within would be
challenging for any life to arise. Supernova explosions and gamma ray
bursts would buffet these super-Earths, which might damage the chemistry
of any atmosphere remaining on these planets. Additional outbursts from
the supermassive black hole could provide a knockout punch and
completely erode the planet's atmosphere.
These planets would also be subjected to the gravitational
disruptions of a passing star that could fling the planet away from its
life-sustaining host star. Such encounters might occur frequently near
the Milky Way's supermassive black hole since the region is so packed
with stars. How crowded is it in the Galactic Center? Within about 70
light years of the center of the Galaxy, astronomers think the average
separation between rocky worlds is between about 75 and 750 billion
kilometers. By comparison the nearest star to the Solar System is 40,000
billion kilometers away.
"It is generally accepted that the innermost regions of the Milky Way
is not favorable for life. Indeed, even though the deck seems stacked
against life in this region, the likelihood of panspermia, where life is
transmitted via interplanetary or interstellar contact, would be much
more common in such a dense environment," said Loeb. "This process might
give life a fighting chance to arise and survive."
There are formidable challenges required to directly detect such
planets. The distance to the Galactic Center (26,000 light years from
Earth), the crowded region, and the blocking of light by intervening
dust and gas all make the observation of such planets very difficult.
However, these challenges may be met by the next generation of
extraordinarily large ground-based telescopes. For example, searches for
transits with future observatories like the European Extremely Large
Telescope might detect evidence for these planets. Another possibility
is searching for stars with unusual patterns of elements in their
atmosphere that have migrated away from the center of the galaxy.
A paper describing these results appeared in the February 22, 2018 issue of The Astrophysical Journal Letters and is available online.
Headquartered
in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics
(CfA) is a collaboration between the Smithsonian Astrophysical
Observatory and the Harvard College Observatory. CfA scientists,
organized into six research divisions, study the origin, evolution and
ultimate fate of the universe.
For more information, contact:
Megan Watzke
Harvard-Smithsonian Center for Astrophysics
+1 617-496-7998
mwatzke@cfa.harvard.edu
Peter Edmonds
Harvard-Smithsonian Center for Astrophysics
+1 617-571-7279
pedmonds@cfa.harvard.edu
Megan Watzke
Harvard-Smithsonian Center for Astrophysics
+1 617-496-7998
mwatzke@cfa.harvard.edu
Peter Edmonds
Harvard-Smithsonian Center for Astrophysics
+1 617-571-7279
pedmonds@cfa.harvard.edu
