Meet the ploonets. These are planets that used to be moons.
There are none of these planets in our solar system. But they might exist in other star systems. There, some moons might escape their parent planets’ gravity and start orbiting their parent stars instead. That’s according to new computer simulations. Scientists have dubbed the liberated worlds “ploonets.” And, the scientists say, current telescopes may be able to find the wayward objects.
The scientists’ thinking starts with these facts. There are planets that orbit other stars out in space. And those planets could have moons. Those moons are called exomoons. Exomoons should be common. But efforts to find them have turned up empty so far.
One person who wants to know more about these moons is Mario Sucerquia. He is an astrophysicist at the University of Antioquia in Medellín, Colombia. He and his colleagues used computer models to simulate what would happen to moons in other star systems. The team was particularly interested in moons that orbited hot Jupiters. These are giant gas planets that lie scorchingly close to their stars. They orbit their stars within days — sometimes just a few days make up their year.
Many astronomers think that hot Jupiters weren’t born so close to their stars, though. Instead, the planets moved toward their star from a more distant orbit. That movement messes with any moon the planet might have. What happens is the gravity — the tug between the planet and star — adds energy to the moon’s orbit. The moon then is pushed farther and farther from its planet. Eventually, it escapes its planet’s gravity.
“This process should happen in every planetary system composed of a giant planet in a very close-in orbit,” Sucerquia says. “So ploonets should be very frequent.” His team reported its analysis June 29 at arXiv.org.
The team’s work shows that some ploonets may look the same as ordinary planets. Others, however, might give themselves away. A ploonet, in theory, should stay close to its parent planet. The ploonet’s gravitational tugs might disrupt the parent planet’s orbit. Those tugs could change the timing of when the planet crosses in front of the star. Crossing in front of a star is called a transit. Those deviations in the planet’s transit could be seen by telescopes in space. You could find them by combining new data from planet-hunting telescopes like NASA’s TESS and old data from the now-defunct Kepler, Sucerquia says.
When moons become planets, though, their ploonethood may be fairly short-lived. About half of the ploonets in the researchers’ simulations crashed into either their planet or star within about half a million years. And half of the remaining survivors crashed within a million years. A million years is short in cosmic time. And such short lives might make ploonets more difficult to spot.
Ploonets might help to explain some bizarre features of planets outside the solar system. For example, moon debris from such crashes could lead to giant ring systems around planets. That debris might even be what makes up the 37 rings that encircle exoplanet J1407b, the team says.
Or, the ploonet may look more like a comet. If the ploonet, when it was a moon, had an icy surface or an atmosphere and then escaped its planet and moved closer to its star, the star’s heat would evaporate the ice. That would give the ploonet a tail like a comet’s. If the ploonet starts to evaporate, it could grow a long, light-blocking tail. That tail might explain strangely flickering stars like Tabby’s star, Sucerquia says.
“Those structures [rings and flickers] have been discovered, have been observed,” Sucerquia says. “We just propose a natural mechanism to explain [them].”
While our solar system doesn’t have any hot Jupiters, ploonethood may be possible here, too. Earth’s moon is moving slowly away, at a rate of about 4 centimeters (1.6 inches) per year. When it eventually breaks free, “the moon is a potential ploonet,” Sucerquia says. Don't worry, he notes, that won’t happen for about 5 billion years.
The study is a good first step for thinking about what would happen to exomoons in real planetary systems, says Natalie Hinkel. She is a planetary astrophysicist who works at the Southwest Research Institute in San Antonio, Texas, who was not involved in the new work. “Nobody’s looked at the problem quite like this,” she says.
Plus, ploonet is “a wonderful name,” Hinkel says. “Normally I sort of eye-roll at these made-up names. But this one is a keeper.”
arXiv A website that posts research papers — often before they are formally published — in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance and statistics. Anyone can read a posted paper at no charge.
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.
atmosphere The envelope of gases surrounding Earth or another planet.
colleague Someone who works with another; a co-worker or team member.
comet A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
cosmic An adjective that refers to the cosmos — the universe and everything within it.
data Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
debris Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.
disrupt (n. disruption) To break apart something; interrupt the normal operation of something; or to throw the normal organization (or order) of something into disorder.
evaporate To turn from liquid into vapor.
exomoon A moon that orbits an exoplanet.
exoplanet Short for extrasolar planet, it’s a planet that orbits a star outside our solar system.
gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
Jupiter (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).
mechanism The steps or process by which something happens or “works.” It may be the spring that pops something from one hole into another. It could be the squeezing of the heart muscle that pumps blood throughout the body. It could be the friction (with the road and air) that slows down the speed of a coasting car. Researchers often look for the mechanism behind actions and reactions to understand how something functions.
moon The natural satellite of any planet.
NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, the object must be big enough to have pulled neighboring objects into the planet itself or to have slung them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
simulation (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.
solar system The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
theory (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.
transit (in astronomy) The passing of a planet, asteroid or comet across the face of a star, or of a moon across the face of a planet.