Moons may yield clues to what makes planets habitable

Date:
February 1, 2022
Source:
University of Rochester
Summary:
Earth's moon is vitally important in making Earth the planet we
know today. Because the moon is so important to life on Earth,
scientists conjecture that a moon may be a potentially beneficial
feature in harboring life on other planets. Most planets have moons,
but Earth's moon is distinct in that it is large compared to the
size of Earth.



FULL STORY ========================================================================== Earth's moon is vitally important in making Earth the planet we know
today: the moon controls the length of the day and ocean tides, which
affect the biological cycles of lifeforms on our planet. The moon also contributes to Earth's climate by stabilizing Earth's spin axis, offering
an ideal environment for life to develop and evolve.


========================================================================== Because the moon is so important to life on Earth, scientists conjecture
that a moon may be a potentially beneficial feature in harboring life
on other planets. Most planets have moons, but Earth's moon is distinct
in that it is large compared to the size of Earth; the moon's radius is
larger than a quarter of Earth's radius, a much larger ratio than most
moons to their planets.

Miki Nakajima, an assistant professor of earth and environmental sciences
at the University of Rochester, finds that distinction significant. And
in a new study that she led, published in Nature Communications, she and
her colleagues at the Tokyo Institute of Technology and the University
of Arizona examine moon formations and conclude that only certain types
of planets can form moons that are large in respect to their host planets.

"By understanding moon formations, we have a better constraint on what
to look for when searching for Earth-like planets," Nakajima says. "We
expect that exomoons [moons orbiting planets outside our solar system]
should be everywhere, but so far we haven't confirmed any. Our constraints
will be helpful for future observations." The origin of Earth's moon Many scientists have historically believed Earth's large moon was generated
by a collision between proto-Earth -- Earth at its early stages of
development - - and a large, Mars-sized impactor, approximately 4.5
billion years ago. The collision resulted in the formation of a partially vaporized disk around Earth, which eventually formed into the moon.



==========================================================================
In order to find out whether other planets can form similarly large moons, Nakajima and her colleagues conducted impact simulations on the computer,
with a number of hypothetical Earth-like rocky planets and icy planets
of varying masses. They hoped to identify whether the simulated impacts
would result in partially vaporized disks, like the disk that formed
Earth's moon.

The researchers found that rocky planets larger than six times the
mass of Earth (6M) and icy planets larger than one Earth mass (1M)
produce fully - - rather than partially -- vaporized disks, and these fully-vaporized disks are not capable of forming fractionally large moons.

"We found that if the planet is too massive, these impacts produce
completely vapor disks because impacts between massive planets are
generally more energetic than those between small planets," Nakajima says.

After an impact that results in a vaporized disk, over time, the disk
cools and liquid moonlets -- a moon's building blocks -- emerge. In a fully-vaporized disk, the growing moonlets in the disk experience strong
gas drag from vapor, falling onto the planet very quickly. In contrast,
if the disk is only partially vaporized, moonlets do not feel such strong
gas drag.

"As a result, we conclude that a completely vapor disk is not capable
of forming fractionally large moons," Nakajima says. "Planetary masses
need to be smaller than those thresholds we identified in order to
produce such moons." The search for Earth-like planets The constraints outlined by Nakajima and her colleagues are important for astronomers investigating our universe; researchers have detected thousands of
exoplanets and possible exomoons, but have yet to definitively spot a
moon orbiting a planet outside our solar system.

This research may give them a better idea of where to look.

As Nakajima says: "The exoplanet search has typically been
focused on planets larger than six earth masses. We are proposing
that instead we should look at smaller planets because they
are probably better candidates to host fractionally large moons." ========================================================================== Story Source: Materials provided by University_of_Rochester. Original
written by Lindsey Valich. Note: Content may be edited for style and
length.


========================================================================== Journal Reference:
1. Miki Nakajima, Hidenori Genda, Erik Asphaug, Shigeru Ida. Large
planets
may not form fractionally large moons. Nature Communications,
2022; 13 (1) DOI: 10.1038/s41467-022-28063-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/02/220201144027.htm

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