- 4543 - EXOPLANETS - with large moons? - If we want to find life-supporting worlds, we should focus on small planets with large moons. Every planet-hunting method has some type of bias.
------------------------------------------- 4543
- EXOPLANETS - with
large moons?
-
- We’ve found most exoplanets using the
“transit method”, which is biased toward larger planets. Larger planets closer
to their stars block more light, meaning we detect large planets transiting in
front of their stars more readily than we detect small ones.
-
- That’s a problem because some research says
that life-supporting planets are more likely to be small, like Earth. It’s all
because of moons and streaming instability.
Consider Earth’s Moon. While there’s no consensus on every aspect of the
Moon and its role, there’s evidence that it helps make life on Earth possible
and has helped life sustain itself for so long. As natural satellites go, it’s
massive. Of the approximately 300 (and counting) moons in our Solar System, the
Moon is the fifth largest.
-
- The Moon’s diameter is about one-quarter of
Earth’s diameter, and its mass is about 1.2% of Earth’s. The four natural
satellites in the Solar System that are larger than the Moon orbit the gas
giants Jupiter and Saturn. Those moons are tiny compared to their planets.
-
- This means that the Moon has different
effects on Earth than other moons do on their planets. The Moon stabilizes Earth’s orbital tilt,
which helps keep the climate stable and allows life to flourish and organisms
to adapt. It creates tides, which may have played a role in the formation of
nucleic acids and life. The Moon may even help Earth maintain its protective
magnetosphere. Earth would be a very
different place without its huge Moon.
-
- We should look for small planets if we want
to find life-supporting worlds because small planets are more likely to host
larger moons. Relatively small planets
similar to the size of Earth are more difficult to observe and they have not
been the major focus of the hunt for moons.
These planets are actually better candidates to host moons.
-
- The leading theory for the Moon’s formation
is the “Giant Impact Hypothesis”. It states that when the Earth was very young,
about 4.5 billion years ago, a Mars-sized protoplanet named “Theia” slammed
into Earth. The collision created a rotating torus of molten rock that orbited
the Earth. Some fell back down to Earth, and the rest coalesced into the Moon.
-
- The research questions the role of
“streaming instability” in moon formation. Some scientists think that planet
formation is the same as moon formation. However, while streaming instability
is important for planet formation, it may not be for the formation of large
moons like Earth’s, which help make planets habitable.
-
- Simulations were used to examine the role
of streaming instability in moon formation. Streaming instability describes the
effect that drag has on the accretion of matter in a protoplanetary disk that
leads to planetesimals. Inside a disk, drag rapidly drives solid particles to
concentrate spontaneously into clumps. These clumps can then collapse and form
planetesimals.
-
- The question is, does streaming instability
play the same role in the formation of moons around planets? In our case, the
disk isn’t a protoplanetary disk but a disk of debris resulting from a
collision.
-
- Research has found that this instability can
quickly form 100 km-sized moonlets.
However, these moonlets are not large enough to avoid strong drag, and
they still fall onto Earth quickly.
These moonlets could grow further once the disk cools enough and the
vapor mass fraction of the disk becomes small.
However, by this time a significant amount of the disk mass is lost, and
the remaining disk could make only a small moon.
-
- For a large moon like Earth’s to form, the
collision has to be less energetic than one between much more massive planets.
If “Theia” had been more massive, the heat from the impact would’ve created a
completely vapourized disk. Only a much smaller moon could’ve formed in such a
disk.
-
- Streaming instability may not help large
moons form in vapour-rich disks. Fractionally large moons like Earth’s Moon,
which may be necessary for life, might only form in vapour-poor disks. More
massive planets have more energetic collisions, which creates vapour-rich
disks. Smaller planets have vapour-poor disks where larger moons can form.
-
- So, if we want to find life-supporting
planets, look for small worlds where larger moons are more likely to form.
-
- Astronomers have confirmed the existence of
exoplanets with extremely small orbits around their stars. But what about
exoplanets that get close enough to be devoured by their star, and what if it’s
an Earth-sized exoplanet?
-
- Known as “ultra-short-period” (USP)
exoplanets could eventually experience what’s known as tidal disruption,
resulting in its devourment by its star.
Tidal disruption could be a potential fate of rocky planets. It seems like about 10 percent of sun-like
stars might have engulfed their rocky planets.
-
- Researchers analyzed “TOI-6255 b”, whose
radius is 1.08 and mass is 1.44 of Earth’s and located just over 65.2
light-years from Earth. However, while being Earth-sized holds promise for
life, TOI-6255 b’s 5.7-hour orbit not only make this exoplanet far too hot for
life as we know it to exist, but this also means its orbit takes it dangerously
close to what’s known as Roche limit.
The “Roche Limit” is the
distance a smaller object can orbit a larger object until the larger object’s
gravity tears the smaller object to pieces, along with TOI-6255 b also
experiencing the tidal disruption, which is a common occurrence throughout the
cosmos, including black holes.
-
- This particular planet is doomed for tidal
disruption in 400Myr which is short on cosmic scale (13Gyr). The planet is also
tidally distorted to be football like in shape (10 percent deviation from
sphere), in comparison Earth’s tidal distortion due to the moon is only 1e-7
[0.0000001] level.
-
- USPs are exoplanets whose orbits are less
than one day and whose masses are less than 2x the Earth. While intriguing,
only about 100 USPs have been discovered with a 2014 study estimating approximately
0.5 percent exist around Sun-like stars and a 2019 study discussing their bulk
composition (i.e., mass of its iron core and mantle).
-
- Given their extremely short orbit, these
worlds are likely too hot for life as we know it to exist, and along with USPs
are the familiar “hot Jupiters” who orbit their stars in only a few days and
astronomers estimate their population is in the hundreds. These worlds are Jupiter-sized or larger gas
planets and are also potentially far too hot for life as we know it to exist.
But what is the significance of TOI-6255 b being an Earth-sized planet as
opposed to a Jupiter-sized planet, or larger?
-
- Planets similar to Earth in size are most
likely rocky i.e. mostly made of iron core and silicate mantle. They show us
what terrestrial planets in other planetary systems are made of. Jupiter-sized
planets are most certainly covered by thick hydrogen and helium atmospheres.
Jupiter-sized planets are unlikely to harbor life.
-
- While TOI-6255 b isn’t due for disassembly
for another 400 million years, watching any exoplanet get ripped to shreds by
its host star could provide important insights regarding the planet’s exterior
and interior compositions, thus helping us better understand the similarities
between exoplanets and planets within our own solar system.
-
- These unique worlds and their extremely
tight orbits have challenged our understanding of solar system architecture
throughout our Milky Way Galaxy, as Mercury is the closest planet to our Sun,
and it still takes 88 days to complete one orbit.
-
- One similarity between our solar system and
exoplanetary systems is the Roche limit. However, the study also focuses on
tidal disruption that is physically distorting TOI-6255 b. Tidal disruption
could be a potential fate of rocky planets.
-
- Tidal disruption of planets is minimal in
our solar system. However, the rings of Saturn are thought to originate from
tidal disruption of satellites around Saturn. Tidal forces are strongly
dependent on orbital separation, only objects with the shortest orbital period
experience significant tides.
-
-
August 20, 2024 EXOPLANETS - with
large moons? 4543
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--------------------- --- Sunday, August 25, 2024
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