The most exciting thing about astronomy is peering into the unknown and discovering something new in the deep cosmic abyss. But when there's hints of that "something new" on our cosmic doorstep, the global excitement becomes tangible.
I am, of course, referring to "Planet Nine", a hypothetical world that seems to be causing a gravitational stir in our outer solar system's frozen badlands way beyond the orbit of Pluto.
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In January, Caltech astronomers Mike Brown and Konstantin Batygin announced their discovery of a group of objects in the Kuiper belt - beyond the orbit of Pluto - had a strange orbit. The Kuiper belt and strangeness often go hand-in-hand, but on this occasion, the motion of these small objects hinted at another mysterious object even further away that may be gravitationally tugging on these KBOs, creating their strange synchronicity.
The search for planets in the outer solar system is a tricky affair. Although we have extremely powerful observatories that can see the fine details in galaxies millions of light-years away and survey telescopes that can pinpoint small asteroids as they dash through the inner solar system, the outer solar system remains one of the most exciting, yet largely unexplored regions in the local cosmos. Should a modestly-sized planet be orbiting far enough away from the sun, it could still be too small and too cold to be noticed by surveys. If it can't be detected by surveys, more powerful telescopes won't know where to look to zoom in on the world - but even then these distant planets would be little more than dots in an ocean of stars. Space, after all, is big and planetary discoveries require a combination of skill, precise instrumentation and luck.
In the case of Planet Nine, its presence hasn't been directly observed yet; like the discovery of Neptune in 1846 it's the motions of other solar system objects that may be signalling its gravitational dominance in the region. Now astronomers are getting even more creative and studying the trajectory of NASA's New Horizons mission in the hope of seeing any unaccounted-for drift off its planned path through the Kuiper belt that may also signal evidence of Planet Nine's gravity.
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In the meantime, scientists at the University of Bern, Switzerland, have jumped one step ahead of these exciting early hints of a new planet and put some limits on how big and how "warm" this thing could actually be. Their study has been accepted for publication in the journal Astronomy & Astrophysics.
From Brown and Batygin's models, Planet Nine should have a highly elliptical orbit, coming no closer than 200 AU (that's 200 times the Earth-sun distance, over 4 times the Pluto-sun distance) and extending to 1,200 AU at its farthest. In short, this would be an extreme world, well beyond the boundary of our "classical" solar system and even beyond the most distant solar system object known to date, the dwarf planet Eris (at nearly 100 AU). Eris was also discovered by Brown in 2005, a discovery that ultimately led to the re-classification of Pluto.
Having not obviously popped up in any infrared surveys, Bern astronomers Christoph Mordasini and Ph.D. student Esther Linder set out to decipher a few more characteristics about Planet Nine by using known planetary evolution models they've applied to the formation of planets orbiting other stars - worlds known as exoplanets. This modeling effort could then be used to sift through survey data, perhaps revealing an object that has gone unnoticed in the night sky.
Brown and Batygin have been able to estimate Planet Nine's mass, based on the gravitational influence it seems to exert. It is likely a significantly-sized planet, around 10 times the mass of Earth, possibly making it "mini-Uranus"-like world - a place with a solid core and a cold, dense layer of gas.
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Knowing that Planet Nine has yet to be seen by infrared surveys (such as NASA's Wide-field Infrared Survey Explorer, or WISE, mission), the researchers already had an upper limit on Planet Nine's physical size and knowing the its approximate mass, distance from the sun and applying planetary formation models, Mordasini and Linder were able to form an idea as to the planet's temperature and size.
By their reckoning, Planet Nine should have a radius 3.7 times that of Earth and an upper atmosphere temperature of -226 degrees Celsius (or 47 Kelvin). They arrived at these numbers by considering Planet Nine's predicted orbit around the sun and the age of our solar system; the hypothetical world would have formed from our sun's protoplanetary disk that began to condense into planets some 4.6 billion years ago.
At these staggering distances from the sun, it may not come as a surprise that Planet Nine's predicted temperature would be extremely cold, but it is still warmer than what would be predicted from being heated by sunlight alone. As planets form, the gravitational energy in their cores can keep their interiors molten hot for billions of years. This heat, however, is slowly dissipated and may be observed by highly sensitive infrared telescopes.
Therefore, Planet Nine's temperature of 47 Kelvin "means that the planet's emission is dominated by the cooling of its core, otherwise the temperature would only be 10 Kelvin," said Linder in a press release. "Its intrinsic power is about 1000 times bigger than its absorbed power." This means that the reflected sunlight would be minuscule compared to the internal heat the world is currently generating, making its infrared signal vastly more powerful than looking for reflected sunlight in optical wavelengths. This may seem like an obvious conclusion to astronomers when seeking out icy objects far from the sun, but it's still a staggering thought that Planet Nine is the hottest thing in the solar system's hinterland despite being only 47 degrees above absolute zero. In astronomy, "heat" is a very relative term.
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Knowing just a few clues about the nature of Planet Nine, it's interesting to see this hypothetical world take shape. "With our study, candidate Planet 9 is now more than a simple point mass, it takes shape having physical properties," said Mordasini.
Currently, astronomers are using Brown and Batygin's observations and models to track down the possible location of Planet Nine, but spotting the world is going to be difficult with the infrared data we currently have available to us.
So what will Planet Nine look like? We'll probably have to wait until the Large Synoptic Survey Telescope near Cerro Tololo in Chile is constructed before we see its faint signal. Only then will we be able to decisively prove that the world is out there and begin to understand whether it's actually a small gaseous planet or something a bit different. In the mean time, theoretical studies such as these help us not only track down the location of Planet Nine, they give us a tantalizing look at what Planet Nine may look like and what it's made of.
NOTE: As mentioned in the comments below, this research is only applicable to a hypothetical planet that formed from our sun's protoplanetary disk, with the same material that formed the rest of the planets. There's the possibility that Planet Nine could be a captured world from another star system (a scenario that may explain the high eccentricity of its predicted orbit). Until we actually observe this planet, whether or not Planet Nine was born in the solar system remains to be seen, but this research should help us understand its origins.
Source: University of Bern