Odd Asteroids Lead Caltech Scientists Towards a New Planet
Chunyang Ding | Mar. 26, 2016
Suppose you blindly tossed six dice into the air and look down to find that not only did all the die land right by each other, all have landed with “one” face up! This would be a remarkable coincidence; it is more likely that the dice were weighted somehow, or that a mischievous friend altered the results while your eyes were still closed. A similar coincidence, with asteroid orbits rather than random dice, has led scientists at Caltech to hypothesize that our solar system has something that has not yet been seen before. Planetary scientists Mike Brown and Konstantin Batygin from the California Institute of Technology published a paper in the Astronomical Journal that analyzes the possible existence of a giant planet in our solar system. This hypothetical planet would be at least 15 times larger than the Earth, and a distance from the sun 20 times farther away than even the planet Neptune.
The research hinges on the discovery of a very peculiar pattern in orbital elements of asteroids. There are many asteroids in the Kuiper Belt, a region beyond the orbit of Neptune with many icy or rocky asteroids. Each asteroid has an orbit that can be defined by only six numbers, which scientists call the orbital elements. While some of those orbital elements are easy to understand — the “semimajor axis” is the average distance between the object and the sun — others are more obscure, with names like the “longitude of the ascending node”. Nonetheless, anything that orbits our sun can be described by these six unique numbers. The latest discovery hinges on how fourteen recently discovered Kuiper Belt asteroids have almost identical values for two of these orbital elements. The odds of this happening by chance are really astronomical — a one in 15,000 chance of happening randomly. Such a coincidence caught the attention of Brown and Batygin, who began hypothesizing on possible influences.
Professor Mike Brown is definitely not a newcomer to the exploration of planets in our solar system. As one of the most prolific planetary scientists in the world, he has discovered the four objects that are currently considered dwarf planets, including Ceres, Makemake (pronounced mock-ee-mock-ee), and Eris, a dwarf planet 24 percent larger than Pluto. It was the discovery of Eris that prompted the International Astronomical Union, or IAU, to redefine “planet” in 2006, demoting Pluto to dwarf planet status. Most of the work that he does is observational, meaning his research tends to focus on describing the patterns seen in the data. His partnership with Batygin, a young theorist who has done significant modeling of our solar system, was a match made in the stars.
Brown and Batygin, shown here at Caltech, have been an unlikely collaboration between an astronomical observer and a theorist. Image Courtesy of Lance Hayashida.
Together, they first explored the probability that these asteroids happened to cluster together randomly, running massive computer simulations of how those asteroids could have orbited the sun under the influence of outer gas giants like Neptune. Surprisingly, they found that the current solar system would almost never randomly produce the asteroids that have been observed.
This is extremely surprising, but it only opened up new questions. If our current solar system is unable to explain the clustering, then what could be out there? The team began simulating all kinds of different planets, with different orbital elements, masses, and other qualities. These simulations are not easy to do; some of the simulations took several months to complete. But eventually, Brown and Batygin discovered some most likely qualities for what they began to call “Planet X”.
Of course, it isn’t easy to propose to the scientific community that you have discovered a planet that 2000 years of astronomers had missed. In order to make such a bold claim, they need to present very strong evidence to show that their model is the best one. One of the ways scientists can do this is to use their model to make new predictions, which can be checked by observation. Brown realized that the object they predicted to exist would also be able to explain some other extremely distant asteroids that had puzzled astronomers since 2003. The astronomy community could not have figured out how these asteroids ended up so far away from all the other objects in the solar system. But Brown, who discovered the first of these asteroids, realized that Planet X might solve the problem. One more mystery of our solar system may have just been solved.
Planet X is particularly convincing because it correctly predicts the orbits of other asteroids. Here, each asteroid is shown as a line orbiting around our sun in the middle, and the lines in blue are predictions that match with observations. Image Courtesy of Mike Brown and Konstantin Batygin
Does this mean that we need to teach grade-school children that the planets of the Solar System are “Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Planet X”? Not exactly. The discovery that Brown and Batygin have made is remarkable, but it will need much more concrete observation before the International Astronomical Union will accept it as a planet. In addition, there are still some open questions. How did this massive planet end up so far away from the sun? What caused it to form? Would it be able to explain even more asteroids? Although 1 in 15,000 is a great probability, it still stands only at 3.8 sigma, a statistical term that quantifies how likely something is to be true — the higher the sigma, the stronger the proof. The standard for most scientists is to have 3 sigma data, but for fundamental ideas, like particle physics, the threshold is even higher at 5 sigma. If Brown and Batygin are able to collect more data and see the same correlation, then it would strengthen their argument further.
One of the ways that this could be confirmed is through direct observation. We have telescopes that are powerful enough to see this planet, if it exists. The Subaru 8-meter telescope, at the top of Mauna Kea in Hawai’i, is already being pointed in the region of the sky that Planet X is hypothesized to exist. Unfortunately, it is a very large part of the sky, and it could take more than five years before conclusive data can be found.
But Brown is optimistic. He has started a blog to keep the public updated, and recently wrote: “We have to go find [Planet X]. We will. I have very little doubt that we will.”
Chunny Ding is a freshman in Saybrook College. Contact him at chunyang.ding@yale.edu.
(Featured Image Courtesy of Caltech)
