“I wouldn’t be surprised if Planet Nine has already been imaged in one of the large sky surveys currently underway, but, if not, it will be hard for it to hide from the Vera Rubin Observatory once it starts operations in a few years,” Caltech’s Michael Brown told The Daily Galaxy. Brown, along with Caltech’s Konstantin Batygin, presented the first evidence that there might be a giant planet tracing a bizarre, highly elongated orbit through the outer solar system in 2016.
A Typical Super Earth?
“At five Earth masses, Planet Nine is likely to be very reminiscent of a typical extrasolar super-Earth,” says Batygin, an assistant professor of planetary science and Van Nuys Page Scholar at Caltech. Super-Earths are planets with a mass greater than Earth’s, but substantially less than that of a gas giant. “It is the solar system’s missing link of planet formation. Over the last decade, surveys of extrasolar planets have revealed that similar-sized planets are very common around other sun-like stars. Planet Nine is going to be the closest thing we will find to a window into the properties of a typical planet of our galaxy.”
“My favorite characteristic of the Planet Nine hypothesis is that it is observationally testable,” says Caltech’s Batygin. “The prospect of one day seeing real images of Planet Nine is absolutely electrifying. Although finding Planet Nine astronomically is a great challenge, I’m very optimistic that we will image it within the next decade.”
Corresponding with the three-year anniversary of their announcement hypothesizing the existence of a ninth planet in the solar system, Brown and Batygin published a pair of papers analyzing the evidence for Planet Nine’s existence.
The papers offered new details about the suspected nature and location of the planet, which has been the subject of an intense international search ever since Batygin and Brown’s 2016 announcement.
The first, titled Orbital Clustering in the Distant Solar System, was published in The Astronomical Journal. The Planet Nine hypothesis is founded on evidence suggesting that the clustering of objects in the Kuiper Belt, a field of icy bodies that lies beyond Neptune, is influenced by the gravitational tugs of an unseen planet. It has been an open question as to whether that clustering is indeed occurring, or whether it is an artifact resulting from bias in how and where Kuiper Belt objects are observed.
To assess whether observational bias is behind the apparent clustering, Brown and Batygin developed a method to quantify the amount of bias in each individual observation, then calculated the probability that the clustering is spurious. That probability, they found, is around one in 500.
“Though this analysis does not say anything directly about whether Planet Nine is there, it does indicate that the hypothesis rests upon a solid foundation,” says Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy.
The second paper is titled “The Planet Nine Hypothesis,” and was published in Science Direct. The paper provides thousands of new computer models of the dynamical evolution of the distant solar system and offers updated insight into the nature of Planet Nine, including an estimate that it is smaller and closer to the sun than previously suspected.
Based on the new models, Batygin and Brown–together with Fred Adams at the University of Michigan and Juliette Becker, currently a 51 Pegasi b Postdoctoral Fellow at Caltech–concluded that Planet Nine has a mass of about five times that of the earth and has an orbital semimajor axis in the neighborhood of 400 astronomical units (AU), making it smaller and closer to the sun than previously suspected–and potentially brighter. Each astronomical unit is equivalent to the distance between the center of Earth and the center of the sun, or about 149.6 million kilometers.
Increased Confidence in Planet Nine’s Existence
Over the next two years, Brown and Batygin they developed theoretical models of the planet that explained other known phenomena, such as why some Kuiper Belt objects have a perpendicular orbit with respect to the plane of the solar system. The resulting models increased their confidence in Planet Nine’s existence.
After the initial announcement, astronomers around the world, including Brown and Batygin, began searching for observational evidence of the new planet. Although Brown and Batygin have always accepted the possibility that Planet Nine might not exist, they say that the more they examine the orbital dynamics of the solar system, the stronger the evidence supporting it seems.
The Last Word –A Primordial Black Hole?
In 2019, astronomers and physicists proposed that the long-sought for Planet Nine, an elusive dark body in the outer reaches of our solar system, may actually be a primordial black hole, a theoretical remnant of the Big Bang, which could account for the odd orbits observed in the distant solar system.
“The outskirts of the solar system is our backyard. Finding Planet Nine is like discovering a cousin living in the shed behind your home which you never knew about,” said Harvard astronomer Avi Loeb about the Black Hole Initiative’s (BHI) new method to find a grapefruit-sized black hole with a mass of five to 10 times that of the Earth, planet-mass black holes in the outer solar system, and along with it, determine once-and-for-all the true nature of the hypothetical Planet Nine. “It immediately raises questions: why is it there? How did it obtain its properties? Did it shape the solar system history? Are there more like it?”
In an email to The Daily Galaxy Yale’s Malena Rice, currently involved in the TESS search for Planet Nine. wrote: “If Planet Nine exists in its predicted form, it should be detected over the coming years. The most recent predictions have suggested that Planet Nine is both smaller and more nearby than previously expected, with the net effect that it should be brighter. Right now is a particularly exciting time for Planet Nine— the Transiting Exoplanet Survey Satellite (TESS) spacecraft is currently observing the exact part of the sky where recent models have predicted that Planet Nine is most likely to reside. Planet Nine, if it exists, should be in that dataset and should be recoverable; it’s just a matter of figuring out exactly where to look. Fortunately, the computational methods for this search already exist! The discovery of Planet Nine may be right at our fingertips.”
Yale astrophysicist Gregory Laughlin told The Daily Galaxy: “The latest Brown and Batygin analysis squeezes Planet Nine’s likely sky position down to a banana-like swathe of the sky where the ecliptic plane and the galactic plane intersect. Their calculations suggest that it’s bright enough and near enough so that if it exists it should soon be uncovered. The TESS mission’s forthcoming scan of the ecliptic plane provides an exciting opportunity to possibly make the big discovery.”
“With the Vera Rubin observatory coming online in the near future, I’m hopeful that either Planet Nine will be found directly within this decade, or that — at the very least — the census of distant Kuiper belt objects will expand by an order of magnitude. Indeed, the next few years are going to be very exciting for outer solar system science,” Konstantin Batygin told The Daily Galaxy.