What if the hypothetical planet Nine had moons? : ScienceAlert

In recent years, evidence suggests that there could be something lurking on the outskirts of the solar system – something big and possibly very dark.

This big dark thing has been named Planet Nine, its presence being inferred from some particularly clustered orbits detected in small objects in the Kuiper Belt of the outer solar system. Something, some scientists say, caused a gravitational disturbance that created these orbits.

Their calculations suggest that whatever it is, the object is between 5 and 10 times the mass of Earth.

However, the outer solar system is very far away and objects within it are very difficult to detect. Planet Nine, if it exists, should orbit the Sun somewhere between 400 and 800 times the distance from Earth to the Sun. So, although scientists have searched for Planet Nine, so far everyone has escaped.

A possible reason for this could be if Planet Nine is a dark object; like, for example, a black hole. Not only would such a black hole emit no light, it would be extremely small – virtually impossible to spot even though it could reflect light.

But astronomer Man Ho Chan of the Hong Kong University of Education in China thinks we might still be able to locate it anyway.

Diagram of a 5 Earth-mass black hole, from a 2019 article speculating on the nature of Planet Nine. A black hole of this mass would be about 9 centimeters (5 inches) in diameter. (Schultz and Unwin, arXiv, 2019)

The smoking gun, he exposes in an article uploaded to the arXiv preprint server, and in press at The Astrophysical Journalcould be a multitude of moons accompanying the mysterious piece of something.

“In this paper, we show that the probability of capturing large Trans-Neptunian Objects (TNOs) by Planet Nine to form a satellite system in the Scattered Disc region (between the Inner Oort Cloud and the Kuiper Belt) is great,” Chan writes in his paper.

“By adopting a reference model of Planet Nine, we show that the tidal effect can heat satellites significantly, which can give sufficient thermal radio flux for observations, even though Planet Nine is a dark object.”

Almost every planet in the solar system has at least one moon. In fact, most have more than one. Mercury and Venus are moonless, and Earth is the only planet with a single satellite. Some non-planetary bodies also have moons. There is Pluto, of course, with its moons. Some asteroids even have moons.

In the mid-outer solar system, moons are pretty much all the rage. Some, like Earth’s Moon, may have been formed from materials from the parent body itself. In many other cases, the planet’s gravity grabbed passing rocks and held them back, like strange little rock-gathering goblins.

Where Planet Nine should be, it turns out, should be ripe for moon-picking: the region between the rock-filled Kuiper Belt and the rock-filled Oort Cloud. This region, known as the scattered disk, should be filled with trans-Neptunian objects; basically, rocks that orbit at an average distance greater than that of Neptune.

Chan calculated the probability that the putative planet could have picked up certain satellites and found that it would be weirder if it wasn’t. According to his calculations, on average, an object of the mass of Planet Nine should capture 20 trans-Neptunian objects as large or larger than 140 kilometers (87 miles) in diameter.

On their own, these chunks of icy rock would not be detectable, but a gravitational interaction with a more massive body could change that, if the moon was quite large; say, over 100 kilometers in diameter.

Satellites captured by a planet tend to have irregular and elliptical orbits. This means that the gravitational pulls on the moon change as it moves closer to and further from the planet, stretching it where the gravitational pull is strongest.

These ever-changing stresses heat the moon from within. And the heat is dissipated in the form of thermal radiation. This should be detectable as a radio signal; and that’s something we can research now, says Chan.

“If P9 is a dark object and has a satellite system, our proposal can now directly observe potential thermal signals emitted by satellites,” he writes.

“Therefore, it would be a timely and effective method to confirm the Planet Nine hypothesis and verify whether Planet Nine is a dark object or not.”

Well, that’s as good a thing to try as any.

The newspaper is in press with The Astrophysical Journaland is accessible on arXiv.

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