The hypothetical "Planet 9," (also known as Planet X if you believe Pluto is still our ninth planet), which may or may not be hiding on the periphery of our solar system, could be out there and surrounded by a small swarm of potential moons. According to a new study in The Astrophysical Journal, find the moons and you find the planet. If such a body does exist, it would be located in the Kuiper Belt, a frigid region of space beyond Neptune's orbit.
According to astronomer Man Ho Chan from the Education University of Hong Kong, there could be as many as 20 moons surrounding Planet 9, each measuring up to about 62 miles (100 kilometers) in diameter.
Scientists first proposed the existence of Planet 9 in a 2016 issue of The Astronomical Journal. They used the hypothetical planet as a possible explanation for the unusual orbits of a number of extreme trans-Neptunian objects (ETNOs) — asteroids, comets, moons, and dwarf planets located beyond 30 astronomical units from the sun. One astronomical unit is defined as the average distance from the Earth to the Sun, approximately 93,000,000 miles.
Researchers believe that the movement of these ETNOs is best explained by the gravitational pull of an unseen mass. Based on its orbital period of about 250 times that of Earth's, Planet 9 is likely between five and 10 times the size of our Little Blue Dot.
So far, no one has seen any evidence of Planet 9's illumination, despite its purported large size. The sun can't shine brightly enough on the planet to make it visible, so the only way to see it would be if it blocked out the light of a galaxy or star far away in the Milky Way.
But how does this help scientists locate it?
When one body's gravitational energy is absorbed by another, the resulting thermal energy is released as heat in the surface ocean or the interior of the planet or satellite. This is called tidal heating. Any satellite orbiting Planet 9 could experience tidal heating that raises its temperature to somewhere around minus 173 degrees Celsius (minus 280 degrees Fahrenheit). That might sound mighty cold, but it's still a lot warmer than the average temperature in the Kuiper belt, which hovers at around minus 233 degrees Celsius (minutes 370 degrees Fahrenheit).
An example is the volcanically active moon of Jupiter, Io. The extremely molten core forms as a result of the intense tidal heating generated by Io's gravitational tug of war with Jupiter and the other Jovian moons.
If any of Planet 9's satellites do get this hot, as suggested by Chan's paper, then they will likely emit a faint radio signal that can be picked up by telescopes searching for such signals. Time will tell if this is a lead worth pursuing. The hunt for Planet 9 is still on.