Image Credit: Caltech/R. Hurt (IPAC)
Today, instead of looking at planets from beyond our Solar System, we’re going to look at something closer to home. In January two Caltech researchers, Dr. Konstantin Batygin and Dr. Michael Brown, submitted a paper to The Astronomical Journal hypothesizing the existence of a ninth planet in our Solar System (sorry, Pluto!). This planet would have a mass about ten times that of Earth’s and orbit as far as 1200 AU from the Sun—24 times that of Pluto’s largest orbital distance!
Actually, no direct detection was made. So, you might ask, how did they do it?
First, a Geometry Brief
Before we continue, it will help to define a few terms. Many objects in our Solar System follow an elliptical path as they orbit the Sun:
The perihelion is the distance of a planet’s closest approach, while the aphelion is the furthest approach. The line connecting these two points is called the major axis, and the semi-major axis is simply half that distance. Ellipses are also characterized by their eccentricity, which is typically denoted as e. The eccentricity describes how much the ellipse deviates from being perfectly circular. It takes on values greater than zero (perfectly circular) to less than one (parabolic).
The proposed planet (we’ll call it Planet Nine) was inferred from observing objects whose orbits came no closer than 30 AU from the Sun, which is the average orbital distance of Neptune. These objects are termed Trans-Neptunian objects, or TNOs. So, what was it about these orbits that lead the Caltech researchers to suggest a new planet in the first place?
These orbital oddities were noted in a 2014 paper by Trujillo and Sheppard , which described the discovery of a new minor planet with a perihelion of 80 AU, called 2012 VP. They noticed that TNOs with a semi-major axis of over 150 AU were oriented similarly in space. This is strange, considering we would expect these orbits to be randomized by gravitational interactions after their initial formation early in the history of the Solar System.
To explain this, Trujillo and Sheppard purposed that these objects were kept roughly aligned in space by a three-body interaction called the Lidov-Kozai effect. The details of this effect need not concern us, only that it would in principle keep these objects similarly oriented in space. In this scheme, one of the three bodies would actually be another planet. Simulations of this effect could not pin down the exact details of the perturber, but a super-Earth-mass object was plausible.
This is where our two scientists from Caltech come in. They argued that the Lidov-Kozai effect could not account for the observed orbits. It would require multiple planets with very specific orbits and would predict the existence of a second population of objects in an opposing orientation, which is not observed. The Caltech scientists decided to investigate a different scenario involving a large unseen planet.
Now, in order to do this, the researchers had to make sure that none of the outer planets in the Solar System (especially Neptune) were having a significant influence on the TNOs over the last 4 billion years. After running several simulations, they determined that only six were unaffected. Using these six objects, they ran numerical simulations to determine what parameters the hypothesize planet would need to cause the observed orbits. These calculations suggested an object ten times the mass of the Earth with an eccentricity of 0.6 and semi-major axis of 700 AU.
The Future of Planet Nine
This paper presents the strongest evidence we have yet of a distant unseen planet in our Solar System, but is it really out there? Well, maybe. It would help to have more than a handful of TNOs for analysis to see if other bodies exhibit similar orbital anomalies. Personally, I will be waiting for a direct detection. The researchers began their search in early March using the 8.2 meter Subaru telescope at the Mauna Kea Observatory on Hawaii. Also, a group of French researchers have narrowed down the search area by using data from the Cassini spacecraft and computer models (Fienga et al. 2016).
So, what implications would this ninth planet have if it is found? Such a discovery would help improve our understanding of the formation of the Solar System. A planet this far out from the Sun may suggest that either the early Solar System covered a large region of space or the planet formed closer to the Sun and was later ejected.
Bonus Section (Don’t Panic)!
The plot thickens! New research casts doubt on the existence of ‘Planet Nine’. See here.
Batygin, K., & Brown, M. E. 2016, AJ, 151, 22
Fienga, J. Laskar, H. Manche and M. Gastineau, A&A, 587 (2016) L8
Trujillo, C. A., & Sheppard, S. S. 2014, Natur, 507, 471