Is this what Planet 9 looks like?

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Possible interior structure of the hypothetical Planet 9(Credit: University of Bern)

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In January, Caltech professor Mike Brown and assistant professor Konstanin Batygin claimed to have found evidence of a ninth planet in the outer Solar System. But if it's there, what is it like and why hasn't it been spotted yet? A possible answer comes from a pair of astrophysicists at the University of Bern, who used models developed for studying exoplanets to determine the structure of the hypothetical Planet 9.

The putative Planet 9 was suggested by the Caltech team as a way of explaining certain deep space orbital anomalies. Based on mathematical modeling and computer simulations derived from the orbits of six Kuiper Belt objects in the outer Solar System, the astronomers calculated that the most likely cause of the odd orbits of the objects was a previously unknown planet in a highly elliptical orbit roughly 700 AU (104 billion km) from the Sun. Unfortunately, Brown and Batygin's orbital calculations don't tell us much about what Planet 9, if it's there, is like.

At the University of Bern, Professor Christoph Mordasini and PhD student Esther Linder used techniques designed to model the formation and evolution of young exoplanets. Working on the assumption that Planet 9 is a smaller version of Uranus or Neptune, they were able to deduce the planet's possible structure, including its size, brightness, temperature, and what sort of telescope would be most likely to find it.

Esther Linder and Christoph Mordasini(Credit: University of Bern)

According to Mordasini and Linder, Planet 9 is probably a gas giant with a core of iron, silicates, and ice surrounded by an atmosphere of hydrogen and helium. It's about 10 times the mass of the Earth and 3.7 times the radius, which works out to a diameter of 29,296 mi (47,144 km), and has a temperature of 47 K (-226º C, -375º F). This would have been about 10º K (minus 263º C, minus 441º F), but as gravity compresses Planet 9, it produces 1,000 times more energy than it receives from the Sun.

This means that not only is Planet 9 a very cold place, but at such a great distance from the Sun it's brighter in infrared than in visible light. Since Planet 9 is so far away, the Sun looks like just another star and the planet is extremely difficult to see from Earth with even the best telescopes. According to the Bern team, it may be hard to detect any planet under 20 Earth masses at that range, but they believe that the new generation of instruments, such as the Large Synoptic Survey Telescope in Chile may be able to finally answer whether there is a Planet 9.

The Bern team's results were published in Astronomy & Astrophysics.

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