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by Pat Brennan
NASA's
Exoplanet Exploration Program
June 18, 2019
from
ExoplanetsNasa Website
This artist's impression shows
a view
of the surface of the planet Proxima b
orbiting the red dwarf star
Proxima Centauri,
the
closest star to our solar system.
The
double star Alpha Centauri AB also appears in the image.
Proxima
b is a little more massive than the Earth
and
orbits in the habitable zone around Proxima Centauri,
where
the temperature might be suitable
for
liquid water to exist on its surface.
Credit:
ESO/M. Kornmesser
The main idea is easy to grasp:
Set Goldilocks loose
in our galaxy and let her choose a planet that's "just right."
For decades, the
Goldilocks zone has been the go-to
shorthand for scientists. More formally known as the "habitable
zone," it's the region around a star where the temperature is just
right for liquid water to pool on the surface of planets with
suitable atmospheres.
What is the
habitable zone?
The
habitable zone,
also known as
the 'Goldilocks zone,'
is the region
around a star where the temperature
is just right
for liquid water to pool
on the surface
of a planet.
As tens, then hundreds, then thousands of planets were confirmed to
circle stars across the Milky Way, the question kept bubbling up:
Might any of these
worlds be capable of supporting some form of life?
And Goldilocks offered a
glimmer of an answer.
All earthly life requires
liquid water; it seems reasonable that unearthly life would too. We
are most likely to
find life where we find liquid water
- on planets in the habitable zone.
These days, however, researchers tell us it might not be that
simple.
While the traditional
definition of the habitable zone is still a useful first
approximation, the dizzying variety of planets and planetary systems
- so far none that resemble our own - is prompting a wholesale
re-evaluation of what it takes for a planet to be
habitable.
Traditional picture of the habitable zone
- not
too hot, not too cold.
Image
credit: NASA
Might rogue planets
hurtling through the galaxy, without a star but perhaps with
their own internal heat, be habitable?
Could Earth-sized
planets locked in a close, habitable-zone orbit around a
red dwarf star, and prone to
sterilizing stellar flares, still harbor life?
What about larger
worlds that might resemble scaled-up versions of our home
planet, and orbit in the habitable zone of their star?
Such depictions of
potentially habitable worlds emerge from the bare bones data on
actual planets - that is, once scientists apply their best
conceptual ideas, plus a little computer modeling.
But some of these worlds
can play havoc with 'traditional ideas' of habitable zones...
"It's very important
to realize, it's not just the location," said Mary Voytek, the
senior scientist for astrobiology at NASA headquarters in
Washington, D.C.
"There are many other
factors that contribute to establishing habitable conditions."
Even well-known worlds
close to home, in our own solar system, hint at far more variety
than the chalk outline of the standard "habitable zone" suggests.
Fire and ice
In our system,
Venus by some measures grazes the
inner edge of our Sun's habitable zone, and
Mars falls just inside the outer boundary.
Neither appears to be
'habitable'...
Broiling, barbecued
Venus with its sulfuric acid clouds is much too hot for 'life as
we know it'...
Freezing, desiccated
Mars and its wisp of a carbon dioxide atmosphere is unpromising
as well.
"The Moon is in
the habitable zone in our solar system," Voytek said.
"Is it habitable?
It can't retain an atmosphere. It 'doesn't'
have water on its surface. It's something in the
habitable zone that isn't."
Or cast the net much
farther out, beyond our system's "ice line"...
Our biggest gas giants
are both orbited by frozen moons concealing oceans beneath the
surface.
Jupiter's
Europa or Saturn's
Enceladus might also harbor some
form of ocean-dwelling life - though well outside the traditional
habitable zone around our Sun.
Do gas giants preside over their own habitable zones, in our solar
system or around other stars?
If so, it would be
governed by tidal forces instead of radiant temperature - the push
and pull of a moon's innards by the gas giant's gravity, causing
tidal heating to keep subsurface oceans in a liquid state.
Open the lens a bit and pull back to take in a view of our entire,
pin-wheeling Milky Way galaxy.
Might the galaxy also
have its own habitable zone, the distance from the galactic
center with enough metal to seed rocky planets, yet free of
killer supernovae or smothering molecular clouds?
The habitable zone
- the distance form a star where
liquid water could exist on the surface of a planet -
varies by star type.
For bigger, hotter stars, it's farther out;
for small, cool stars it is much closer in.
Such concepts are intriguing, and might one day contribute to the
search for habitable worlds, but we don't have enough data so far to
make them useful for studies of
exoplanets - planets around other stars.
So researchers are trying instead to narrow the focus and flesh out
the potential habitability of the planets themselves.
They are tasking our
increasingly sophisticated computer models, originally designed to
model Earth's climate, with simulating exotic (if idealized) worlds
instead.
"We're using our
understanding from Earth to inform our search for life, for
habitability, on other planets," said Nancy Kiang, an
astrobiologist at NASA's Goddard Institute for Space Studies in
New York.
Virtual
explorers
The extraordinary portraits these modelers paint are teaching us
about previously unimagined possibilities, not only for potentially
habitable worlds, but for the dynamic atmospheres of enormous, hot,
gaseous planets, oddballs known as mini-Neptunes, or
rocky, terrestrial planets locked in a deep freeze.
Modeling by several researchers, including Aomawa Shields of
the University of California, Irvine, shows that small, rocky worlds
orbiting red-dwarf stars - like the seven roughly Earth-sized
planets orbiting
TRAPPIST-1 - could, under some
conditions, have stable climates and reduced susceptibility to
deep-freeze.
Shields uses computer models to simulate the effects of host stars
on their planets' likely habitability.
She says such findings
suggest that the traditional habitable zone, by itself,
is a limited way to define the potential habitability
of these distant worlds.
"Our solar system is
not the standard model," Shields told an audience at a recent
American Astronomical Society conference in Seattle.
"It's one of many
possible configurations we're seeing out there. Some push the
boundaries of the traditional habitable zone."
Three of the seven roughly Earth-sized planets
orbiting a red-dwarf star, TRAPPIST-1,
are
considered to be in the habitable zone (green)
- where
liquid water is possible on the surface.
Astronomers say the habitable zone concept is mainly useful
for targeting stars and the planets around them for closer study.
That's a big help in an
era of explosive discovery of these distant planets, but one so far
lacking enough telescopic power to read the atmospheres of smaller,
rocky worlds for signs of habitability.
Such telescopes are being
designed with the help of ideas about the habitable zone.
And new, more powerful
instruments will be lofted into space in the years and decades to
come, starting with the
James Webb Space Telescope,
expected to launch in 2021.
The Webb telescope might be able to scrutinize the atmospheres of
large, mysterious worlds known as "super
Earths" - larger than Earth but smaller than Neptune -
and perhaps even planets in Earth's size-range as well, though that
would likely push the limits of the telescope's capability.
"If something looks
like a terrestrial planet within the habitable zone, what can we
actually say about that?" asks Stephen Kane of the University of
California, Riverside, who is also a member of the NASA
Astrobiology Institute and who specializes in habitable zones.
"The answer, at the
moment, is very little."
He calls the habitable
zone "one of the most misunderstood concepts" in astronomy - and one
that might one day pass out of use as our instruments grow sharp
enough to reveal exoplanet characteristics in detail.
For now, however, it's a key,
"target selection
tool."
"There are many candidates to choose from," he said. "How do we
prioritize our list? The answer is the habitable zone."
An ocean of
worlds
And as we sail this sea of unfamiliar planets, we finally fetch up
on the cosmic shoreline.
The shoreline is
metaphorical, but might be a very real statistical trend that links
all planets - those in our solar system and the exoplanets around
other stars.
It's a dividing line that appears when we compare two factors:
An updated version of the
"shoreline" was revealed in a 2017 paper (The
Cosmic Shoreline: The Evidence that Escape Determines which Planets
HaveAtmospheres, and what this May Mean for Proxima Centauri B)
by Kevin Zahnle of the NASA Ames Research Center
in Moffett Field, California, and David Catling of the
University of Washington Astrobiology Program.
They first considered the planets and
moons of our own solar system,
sketching out a standard graph like most of us drew in grade school.
Source
The vertical axis
shows tick marks for solar radiation, increasing by factors of
10 in the upward direction.
The horizontal axis
shows the "escape velocity" that atmospheric particles must
reach to leave the planet, increasing, in kilometers per second,
to the right by a factor of 1,000.
And along the roughly diagonal line that cuts across the graph -
the cosmic shoreline - were planets and moons with atmospheres.
Add in the exoplanets
with known masses and radii, and the number of dots near the
line increases.
The line was where the
solar radiation was low but escape velocity - really, a measure of
gravity - was high.
And sure enough, gathered
on one side of the line like sunbathers on a summer beach were solar
system bodies, planets and moons, known to have atmospheres:
Earth, Venus, our gas
giants Jupiter and Saturn, and even Saturn's moon, Titan, with
its atmosphere of cold, dense hydrocarbons.
Anything below the line
was more likely to have an atmosphere. Anything above probably
didn't.
And planets right on the
line, or very close to it, might have thin atmospheres if any at all
(think Mars, or Pluto).
Initial indications suggest that exoplanets also should divide
reliably along this line, that is, if the relationship turns out to
be real and not simply a statistical fluke.
"It's certainly a
testable hypothesis," Zahnle said.
"It would be kind of
fun to know if this is actually a (physically based) general
feature or just an accident of plotting two unlike things that
coincidentally line up."
As our space telescopes,
in future years, gain the power to probe the atmospheres of
exoplanets, and if the shoreline idea holds up, the relationship
could become important - a way to choose among the menagerie of
planets to find those most likely to possess atmospheres.
And Goldilocks, now confronted with thousands of planets to choose
from, might more easily find a small, rocky, habitable world. Just
right...
This artist's conception of a planetary lineup
shows
habitable-zone planets with similarities to Earth:
from
left, Kepler-22b, Kepler-69c, Kepler-452b,
Kepler-62f and Kepler-186f.
Last in
line is Earth itself.
Image
credit: NASA/Ames/JPL-Caltech
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