Have We Found Planet 9?

Ever since I learned about the concept of Planet 9 proposed by Mike Brown, an astronomer at the California Institute of Technology, and his colleague Konstantin Batygin nearly ten years ago, I have been captivated. The possibility of another large, yet undiscovered, planet lurking within our Solar System seems too good to be true.

Yet, the hypothesis of Planet 9 is rather compelling: the orbits of an unusual clustering of trans-Neptunian objects (TNOs) appear to be shaped by an unseen gravitational force. Mike and Konstantin have estimated that this could be explained by a planet 5–10 times the mass and 2–4 times the radius of the Earth, located within 400-800 Astronomical units (AU) from the Sun. Here's a great talk by Mike Brown that explains it all: https://www.youtube.com/watch?v=Zw5MDh_wpnI

Since numerous exoplanets of similar mass have been discovered, it reinforces the idea that such a planet would not seem out of place in our Solar System. Yet, despite various deep sky surveys performed throughout the years, Planet 9 remains elusive. This isn't entirely unexpected considering the challenges of detecting such a distant planetary object, which might not reflect much sunlight. Not everyone is convinced, though. Observational bias, and not Planet 9, has been put forward to account for the orbits of the particular TNOs.

It goes without saying that observing the ups and downs of the Planet 9 hypothesis over the years has been fascinating.

Therefore, when the news of a possible discovery of a large TNO emerged a few days ago, I couldn't contain my excitement at the thought that this might be the first piece of evidence for Planet 9. As explained in this research paper, to search for Planet 9, scientists analyzed old far-infrared data from two all-sky surveys—the Infrared Astronomy Satellite (IRAS), a NASA-Netherlands-U.K. satellite launched in 1983; and AKARI, a Japanese satellite launched in 2006. These surveys are separated by 23 years, which should be enough to see the slow orbital motion of the planet across the night sky. After applying selection criteria based on flux, position, and movement to 2 million objects, the scientists identified 13 candidate pairs, ultimately narrowing them down to one promising object. This object is visualized as two dots with matching colors and brightness—a sign that they are the same object.

Have we finally discovered Planet 9?

The simple answer is no. But we might have discovered another TNO instead. Indeed, the object's orbit has been determined to be inclined at about 120° relative to the plane of the Solar System. This tilt doesn't fit with Brown's models of Planet 9, as these require the planet's orbit to be tilted only about 15° to 20° from the Solar System's plane. As Mike Brown recently wrote in his BlueSky account: "...if a planet is discovered beyond Neptune and it does not cause the gravitational effects we claim, it is not the planet we predicted...". So, it doesn't look like we discovered it yet. To make matters worse, calculations show that the existence of both Planet 9 and this object is improbable, as their presence would cause each other's orbits to become unstable. Apparently, it's either one or the other.

Given the challenges we face in detecting these distant, low-light objects, it is good news that the Vera C. Rubin Telescope in Chile is scheduled to begin observing small objects in the Solar System by the end of this year.

We live in exciting times!