Are Exomoons
prime suspects in our search for extraterrestrial life? Astronomers
have their fingers crossed that within the haul of data collected by
NASA's Kepler mission,
which has already detected nearly three thousand possible exoplanets,
hide the signatures of the very first exomoons. "The first exomoons that
we find will be large – maybe Mars- or even Earth-sized – and therefore
intrinsically more likely to be habitable than small moons," says René
Heller, a postdoctoral research associate at the Leibniz Institute for
Astrophysics in Potsdam, Germany. "With Kepler finding many more giant
planets than terrestrial planets in stellar habitable zones, it's really
important that we try to figure out what conditions might be like on
the moons of these giants to gauge if they can host extraterrestrial
life."
In a series of recent papers, Heller and his colleague Rory Barnes from the University of Washington and the NASA Astrobiology Institute
tackled some of the big-picture problems to habitability posed by the
relationship between exomoons and their host planets. Heller and Barnes
have proposed a circumplanetary "habitable edge," similar to the
well-established circumstellar "habitable zone." This zone is the
temperature band around a star within which water neither boils off or
freezes away on a planet's surface – not too hot, not too cold, thus
earning it the nickname "the Goldilocks zone."
The discovery of alien moons will open up an exciting new frontier in
the continuing hunt for habitable worlds outside the Solar System. With
the confirmation of exomoons likely right around the corner,
researchers have begun addressing the unique and un-Earthly factors that
might affect their habitability.
Because exomoons orbit a larger planetary body, they have an
additional set of constraints on their potential livability than planets
themselves. Examples include eclipses by their host planet, as well as
reflected sunlight and heat emissions. Most of all,
gravitationally-induced tidal heating by a host planet can dramatically
impact a moon's climate and geology. In essence, compared to planets,
exomoons have additional sources of energy that can alter their "energy
budgets," which, if too high, can turn a temperate, potential paradise
into a scorched wasteland.
"What discriminates the habitability of a satellite from the
habitability of a planet in general is that it has different
contributions to its energy budget," said René Heller, a postdoctoral
research associate at the Leibniz Institute for Astrophysics in Potsdam,
Germany.
An example of a circumstellar irradiation habitable zone, the orbital
band around a star where water can exist in liquid form on a planet or
exomoon's surface. The habitable edge is rather different. It is defined
as the innermost circumplanetary orbit in which an exomoon will not
undergo what is known as a runaway greenhouse effect. "To be habitable, moons must orbit their planets outside of the habitable edge," said Heller.
A runaway greenhouse effect occurs when a planet’s or moon's climate
warms inexorably due to positive feedback loops. An example is thought
to have taken place right next door, so to speak, to the other planet
most like Earth that we know of: Venus.
There, the heat from a young, brightening Sun could have increasingly
evaporated a primordial ocean. This evaporative process put ever more
heat-trapping water vapor in the atmosphere, which led to more
evaporation, and so on, eventually drying the planet out as the water
was broken apart into hydrogen and oxygen by the Sun's ultraviolet
radiation. The atmospheric hydrogen on Venus escaped into space, and
without hydrogen, no more water could form. *Moons situated in fairly
distant orbits from their planets should be safely beyond the habitable
edge wherein this desiccation takes place.
"Typically, and especially in the solar system, stellar illumination
is by far the greatest source of energy on a moon," said Heller. "In
wide planetary orbits, moons will be fed almost entirely by stellar
input. But if a satellite orbits its host planet very closely, then the
planet's stellar reflection, its own thermal emission, eclipses and
tidal heating in the moon can become substantial."
The cumulative effects of the non-tidal heating effects are small,
but could be the difference between an exomoon being inside or outside
the habitable edge.
A figure belwo shows the different kinds of illumination that an
exomoon can receive from both its star and its host planet during four
phases of an orbital period. Note that the image is not to scale and
that penumbras – partial shadows – are ignored for conceptual ease.
Here on Earth, we get a little extra energy from the Moon in the form
of moonlight, which is reflected light from the Sun.
Moons, though, get bathed in a lot more sunlight from their planetary
neighbors; Earth shines almost 50 times as brightly in the lunar sky as
the Moon does in our night sky. In addition to reflected sunlight,
planets also emit absorbed sunlight as thermal radiation onto their
exomoons.
This "planetshine" can add a not-insubstantial amount of energy to an
exomoon's overall intake. Imagine a gas giant planet orbiting a
Sun-like star at about the same distance that Earth orbits our Sun. For a
moon with a relatively close orbit around this planet, like Io’s orbit
around Jupiter, Heller calculates that the moon could absorb an
additional seven or so watts per square meter of power. (Earth absorbs
about 240 watts per square meter from the Sun)
Eclipses can potentially offset some of the extra energy input from
planetshine. For eclipses, Heller calculated that lost stellar
illumination for an exomoon in a close orbit (similar to the closest
found in our solar system) is up to 6.4 percent.
Interestingly, because most moons including ours are tidally locked
to their planet – that is, one side of the moon constantly faces the
planet – eclipses, as well as planetshine, would only darken and lighten
one hemisphere. This phenomenon could modify the climate, as well as
the behavior of life forms, in ways not seen on Earth.
"Asymmetric illumination on the moon could induce wind and
temperature patterns, both in terms of geography and in time, which are
unknown from planetary climates," Heller noted. "Life on a moon with
regular, frequent eclipses would surely have to adapt their sleep-wake
and hunt-hide rhythms as well, but only those creatures on the
planet-facing hemisphere."
Although the eclipse-related loss of several percentage points of
illumination is not a huge loss of energy, a moon-planet duo might need
to be closer to its star to compensate for this deficit if the moon were
still to be considered habitable from a Goldilocks zone perspective.
However, this situation introduces another hurdle to habitability:
The closer a planet is to its star, the stronger the star's
gravitational pull is on the planet's moons. This extra pull can tug
moons into non-circular, or eccentric orbits about their planets.
Eccentric orbits, in turn, result in varying amounts of gravitational
stress exerted on the moon as it orbits.
These “tidal forces,” as they are called, cause heating due to
friction. The ocean tides we experience on Earth occur partly as a
result of the Moon's gravity tugging more on the water and land nearest
it, which distorts Earth's shape. The effect goes both ways, of course,
but not equally, with planets inducing significantly greater tidal
heating within their much smaller moons.
If an exomoon's orbit takes it too close to its planet, tidal heating
could push the energy budget too high, culminating in a runaway
greenhouse effect. At the extremes, the tidal heating could unleash
massive volcanic activity, leaving the satellite covered in magma and
distinctly inhospitable, like the "pizza moon" Io.
On the other hand, it should be noted, tidal heating might be a
savior for life. Tidal heating could help sustain a subsurface ocean,
like the one suspected to exist within Saturn's moon Europa,
alternatively making an otherwise unwelcoming exomoon outside the
traditional habitable zone potentially livable.
Another factor comes into play as eclipses rob a bit of energy from
an exomoon and require the moon-planet pair to be closer to their star.
To remain gravitationally bound to a planet and not be ripped away by
the star's gravity, a moon must fall within a so-called “Hill radius” –
the planet's sphere of gravitational dominance. This radius shrinks with
greater proximity to the host star. The closer the planet and moon are
to their star, the less space is available outside the habitable edge.
For planets and attendant moons around dim, cool, low-mass stars
called red dwarfs, this dynamic becomes important. The habitable zone
around red dwarf stars is very tight; for a star with a quarter of the
Sun's mass, for instance, the Goldilocks zone is thought to be around
just 13 percent the Sun-Earth distance – in other words, a third of
Mercury's orbital distance from the Sun.
In a red dwarf solar system, not only must a moon then be closer to
its habitable zone planet, but given the planet's necessary proximity to
its star, the moon's orbit will tend to be eccentric. These qualities
increase the chances that the moon will fall within the habitable edge.
Heller calculated that for many red dwarf stars, the odds of them
hosting habitable moons is accordingly slim.
"There is a critical stellar mass limit below which no habitable moon
can exist," Heller said. "Around low-mass stars with masses of about
twenty percent the mass of the Sun, a moon must be so close to its
habitable zone planet to remain gravitationally bound that it is subject
to intense tidal heating and cannot under any circumstances be
habitable."
Many factors beyond habitable edge considerations, of course,
ultimately determine an exomoon's habitability. To be considered broadly
habitable by creatures other than, say, subsurface bacteria, an exomoon
must meet some of the same basic criteria as a habitable, Earth-like
exoplanet: It must have liquid surface water, a long-lived substantial
atmosphere, and a magnetic field to protect it from solar radiation
(and, in the case of exomoons around gas giants like Jupiter, from the
charged particles created in the giant exoplanet's magnetosphere)
To possess these qualities, which scientists say grow likelier with
increasing mass, a habitable exomoon will likely be quite large compared
to those in the solar system – more on the order of the size of Earth
itself. The biggest moon in our Solar System, Jupiter's Ganymede, is
just 2.5 percent of Earth's mass. But previous studies have suggested
that monstrous moons by the solar system's standards are indeed
possible.
The Kepler mission is expected to be able to detect exomoons down to
about 20 percent of the mass of the Earth. The data, which consists of
measuring the extremely small dips in the amount of starlight as their
planets (or moons) block it from our point of view – should reveal a
moon’s mass and orbital parameters as well. Armed with this information –
and now with habitable edge considerations – astronomers can thus hope
to make some ballpark speculations on any soon-to-be-discovered
exomoon’s propensity to support living beings.
Heller hopes that there will be a list of candidate exomoons ready
for observing by next-generation instruments, such as the James Webb
Space Telescope and thirty meter-class ground telescopes. These
observatories, coming online in the next decade, could be able to
characterize exomoon atmospheres and offer tantalizing evidence of
life.
Image credits: Credit: NASA/JPL-Caltech; Artist's concept of a pair
of exomoons orbiting a gas giant. R. Heller, AIP; A ringed gas giant
planet and its moon bathed in the crimson rays of a red dwarf star.
Credit: NASA/ESA/G.Bacon (STScI)
Source: The Daily Galaxy
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