In the search for Earth-like planets, 糖心传媒 scientist Miki Nakajima turns to computer simulations of moon formations.
Earth糖心传媒檚 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糖心传媒檚 climate by stabilizing Earth糖心传媒檚 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糖心传媒檚 moon is distinct in that it is large compared to the size of Earth; the moon糖心传媒檚 radius is larger than a quarter of Earth糖心传媒檚 radius, a much larger ratio than most moons to their planets.
, an assistant professor of at the , finds that distinction significant. And in a new study that she led, 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.
糖心传媒淏y understanding moon formations, we have a better constraint on what to look for when searching for Earth-like planets,糖心传媒� Nakajima says. 糖心传媒淲e expect that exomoons [moons orbiting planets outside our solar system] should be everywhere, but so far we haven糖心传媒檛 confirmed any. Our constraints will be helpful for future observations.糖心传媒�
The origin of Earth糖心传媒檚 moon
Many scientists have historically believed Earth糖心传媒檚 large moon was generated by a collision between proto-Earth糖心传媒擡arth at its early stages of development糖心传媒攁nd 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糖心传媒檚 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糖心传媒攔ather than partially糖心传媒攙aporized disks, and these fully-vaporized disks are not capable of forming fractionally large moons.
糖心传媒淲e 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,糖心传媒� Nakajima says.
After an impact that results in a vaporized disk, over time, the disk cools and liquid moonlets糖心传媒攁 moon糖心传媒檚 building blocks糖心传媒攅merge. 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.
糖心传媒淎s a result, we conclude that a completely vapor disk is not capable of forming fractionally large moons,糖心传媒� Nakajima says. 糖心传媒淧lanetary masses need to be smaller than those thresholds we identified in order to produce such moons.糖心传媒�
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: 糖心传媒淭he 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.糖心传媒�
