What does the Ninth Planet consist?
The most interesting thing in astronomy is, of course, to look into the unknown and discover something new in the deep abyss of space. And when hints of “something new” appear on our cosmic doorstep, global excitement can no longer be hidden, it shudders around the world, looking into all the cracks. We are talking about the notorious “ninth planet”: a hypothetical world that is believed to have a gravitational influence on the outer Solar system, or rather on the frozen asteroid fields far beyond the orbit of Pluto.
In January, Caltech astronomers Mike Brown and Konstantin Batygin announced a discovery: a group of objects in the Kuiper belt — beyond the orbit of Pluto — found a strange orbit. The Kuiper Belt and oddities, in general, often go shoulder to shoulder, but in this case, the movement of small objects hinted at another mysterious object that can gravitationally pull these objects together, giving rise to a strange synchronicity.
Finding planets in the outer Solar system is not an easy task. While we have very powerful observatories that can see the tiniest details in galaxies millions of light-years from Earth, and telescopes that can pinpoint the movements of tiny asteroids bursting through the inner Solar system, the outer Solar system remains a largely mysterious and unexplored region of the local cosmos. If a modest-sized planet orbits far enough away from the Sun, it will be too small and too cold to be noticed by observatories. And if it can’t be detected as part of a sky survey, powerful telescopes won’t know where to aim. These distant planets will be no more than dots in an ocean of stars. After all, space is very large, and planetary discoveries require a combination of skill, precise tools, and even luck.
The composition of the ninth planet according to Mordasini and Linder, from top to bottom: atmosphere-H/He; gas layer-H/He; ice-H20; silicate mantle-MgSiO3; iron core-Fe.
In the case of the ninth planet, it has not yet been directly observed; as with the discovery of Neptune in 1846, it is the movement of other objects in the Solar System that may indicate the presence of something large in this area. Now astronomers are being particularly creative and studying the trajectory of the New Horizons spacecraft in the hope of seeing any unaccounted-for deviations from the planned path through the Kuiper Belt that could also indicate the gravity of the Ninth Planet.
At the same time, scientists from the University of Bern in Switzerland decided to go even further and try to determine the framework of how large and “warm” the planet can be. Their study was published in the journal Astronomy & Astrophysics.
According to the models of Brown and Batygin, the Ninth planet should have a high elliptical orbit, and come no closer than 200 au (200 distances from the Earth to the Sun, 4 times farther than the distance from the Sun to Pluto) and no further than 1200 au. In short, this world is far beyond the boundary of our “classical” Solar System and even further than the most distant object of the Solar System known to date, the dwarf planet Eris (it is located 100 au). Eris was also discovered by Brown in 2005, and this discovery subsequently led to Pluto’s demotion in rank.
After the planet was not found on infrared surveys, Bern astronomers Christoph Mordasini and graduate student Esther Linder intend to decipher additional characteristics of the Ninth Planet using known planetary evolution models that are applied to planets orbiting other exoplanet stars.
Brown and Batygin estimated the mass of the Ninth Planet based on the gravitational influence that it is supposed to have. The planet must be 10 times more massive than Earth, which makes it a kind of “mini-Uranus” – a place with a solid core and a cold, dense layer of gas.
Despite the fact that the Ninth Planet has not yet been shown on infrared surveys (like NASA’s “WISE”), scientists have already determined the upper limit of the physical size of the Ninth Planet and learned its approximate mass, distance from the Sun and a possible model of planet formation. Based on these data, Mordasini and Linder formed an idea of the temperature and size of the planet.
According to their calculations, the Ninth planet should have a radius of 3.7 Earth’s and the temperature of the upper atmosphere in -226 degrees Celsius. These figures were derived based on the estimated orbit of the Ninth Planet around our Sun and the age of the Solar System; the hypothetical world should have formed in the protoplanetary disk of our Sun, which began to condense into planets about 4.6 billion years ago.
At such a great distance from the Sun, it may come as a surprise to us that the Ninth Planet is, of course, cold, but still warmer than predicted by solar heating alone. As planets form, the energy from their cores can keep the subsurface molten for billions of years. This heat is slowly dissipated and can be observed with highly sensitive infrared telescopes.
The temperature of the ninth planet at 47 kelvins (-226 degrees Celsius) means that “the radiation of the planet prevails over the cooling of the core, otherwise the temperature would be only 10 kelvins,” writes Linder. “Her internal strength is about 1,000 times more than her absorbed strength.” This means that the reflected sunlight will be negligible compared to the internal heating that this world produces, making its infrared signal much more powerful than if we were looking for reflected sunlight in the optical wavelength range. This is obvious to astronomers looking for icy objects far from the Sun, but in the case of the Ninth Planet, which may be the hottest object on the fringes of the Solar System, it is difficult to call something with a temperature of 47 degrees above absolute zero “warm”. “Heat” is a relative term.
Based on the few clues about the nature of the Ninth Planet, it is interesting to see how this hypothetical world will take shape. “With our study, the proposed planet 9 is no longer just a point mass, it takes on a shape, physical properties,” says Mordasini.
Currently, astronomers are using observations and models of Brown and Batygin to track the possible location of the Ninth Planet, but with the infrared data that is still available to us, it will be very difficult to isolate the world.
What does the Ninth Planet look like? We may have to wait until a Large Synoptic Survey Telescope is built near Cerro Tololo in Chile. Only then will we be able to prove that this world definitely exists, and understand whether it is really a small gaseous planet or something completely different. Meanwhile, theoretical studies like these help us not only track the location of the Ninth Planet, but also open up a tantalizing opportunity for us to look at what the Ninth Planet looks like and what it consists of.
Yet at the heart of this study is a hypothetical planet that formed from the protoplanetary disk of our Sun, like our other planets. But there remains the possibility that the Ninth Planet was captured from another star system (such a scenario could explain the high eccentricity of the predicted orbit). Until we actually see this planet, we won’t be able to know for sure whether it was born in our Solar System or not.