Planet X? How about a small black hole instead?

Planet X has a long and rich history of non-existence. For about 130 years, astronomers debated the existence of one or more additional planets to explain the deviations in the orbits of known planets (mainly Neptune and Uranus). Later, the list of deviations was expanded to cover trans-Neptunian objects. But none of the planet X candidates discovered, including Pluto, have the mass or location to account for the sightings.

The primordial black holes have now been proposed as the last planet X (or planet 9, since Pluto was demoted).

Orbital oddity

The origin of planet X begins with the discovery of Neptune. Neptune was not found by accident: Observations of oddities in Uranus’ orbit were used to calculate Neptune’s location, and it was subsequently found.

It is a game that can be played more than once. Astronomers then noted that the orbits of Uranus and Neptune could be better explained by the existence of another large planet. Follow-up observations have found many objects: Pluto and Charon, Sedna and Eris to name a few. None of these distant bodies are large enough to be Planet X. But some of their orbits may also suggest that a new planet is needed.

Some trans-Neptunian objects have very strange orbits. Many are clustered together and have very elliptical orbits, and there is a subgroup that orbits well outside the plane in which the planets orbit. This is unusual, because the action of gravity and the nature of the disc that formed the planets generally maintain very close to the same orbital plane. Finding sharply tilted orbits suggests that something is pulling the objects out of the plane.

A planet large enough and far enough from the Sun could explain these orbits. Indeed, the researchers calculated a range of different planetary masses and orbits that can account for the behavior of objects like Sedna.

By a happy coincidence, the required masses and distances correspond to an observed gravitational lens anomaly – an excess of events where the gravitational influence of an invisible object distorted the light of distant stars. In other words, there is a mass there, and it is invisible. Could this be a new planet?

A series of unlikely events

So there seems to be a mass, but where does it come from? The planet should be something like 300-1000 AU from the Sun (for reference, Neptune’s orbit is only 30 AU), where there is very little rock or gas. Simply put, a planet cannot form there. One possibility is that the planet formed closer to the Sun and was ping pong there through interactions with one or more of the gas giants. However, to stabilize in a distant orbit, a passing star (or something similar) is needed, which seems unlikely. The remaining option is that our Sun has captured a free planet.

But the wanderers must have been kicked out of their own solar system, which means most of them move at a reasonable pace. Therefore, the chances of capturing such a planet in the required orbit are small, but not impossible.

The new article argues that if we are looking at a low probability event, why not a primordial black hole? Primal black holes may have formed soon after the Big Bang. And, unlike black holes formed from collapsing stars, they could have masses ranging from tiny (10 µg) upwards. This means that there should be a few with the correct mass range. How much is a matter of speculation.

Attracted by the idea, the researchers started throwing d20s: they set the primordial black holes to an arbitrary low number, then concluded that capturing a black hole was about as likely as capturing a wandering planet.

For a penny, for a pound

If Planet X was indeed a black hole, how would we know? Researchers say dark matter annihilation is the thing to look for.

No one knows if there are any primordial black holes. No one knows if dark matter annihilates, and if it does, there is no certainty that it does so in a way that is detectable – dark matter could annihilate itself. to create other forms of dark matter, leaving us, well, in the dark. Thus, a speculative product from the early Universe cannot be detected directly unless another speculative process occurs in the right way. In this case, it is possible to confirm whether a primordial black hole is part of our solar system family.

Underlying the speculation is an interesting coincidence: unexplained gravitational lens events that happen to be the right mass and distance to explain some very strange orbits of trans-Neptunian objects. This coincidence gives the impression of demanding only one explanation, which the researchers are trying to do. This makes their house of cards useful.

Physical Exam Letters, 2020, DOI: 10.1103 / PhysRevLett.125.051103 (About DOIs)