A duo of scientists at the University of Bern has estimated that the suspected Planet Nine is a smaller version of Uranus and Neptune with a surface temperature of minus 375 degrees Fahrenheit (minus 226 degrees Celsius) and a radius about 3.66 times that of Earth.
Simulated structure of Planet Nine. Image credit: Esther Linder / Christoph Mordasini.
“How big and how bright is Planet Nine if it really exists? What is its temperature and which telescope could find it?” These were the questions that Prof. Christoph Mordasini and Esther Linder, a PhD student, wanted to answer when they heard about Planet Nine.
“For me candidate Planet Nine is a close object, although it is about 700 times further away as the distance between the Earth and the Sun,” Linder said.
The scientists assume that the planet is a small ice giant with an envelope of hydrogen and helium.
With their evolution model they calculated how parameters like the planetary radius or the brightness evolved over time since the Solar System has formed 4.6 billion years ago.
They conclude that a planet with the projected mass equal to 10 Earth masses has a present-day radius of 3.66 Earth radii and a temperature of minus 375 degrees Fahrenheit.
“This means that the planet’s emission is dominated by the cooling of its core, otherwise the temperature would only be 10 Kelvin,” Linder said.
“Its intrinsic power is about 1,000 times bigger than its absorbed power.”
Therefore, the reflected sunlight contributes only a minor part to the total radiation that could be detected.
This also means that Planet Nine is much brighter in the infrared than in the visual.
“With our study candidate Planet Nine is now more than a simple point mass, it takes shape having physical properties,” Prof. Mordasini said.
The scientists also checked if their results explain why the planet hasn’t been detected by telescopes so far. They calculated the brightness of smaller and bigger planets on various orbits.
They conclude that the sky surveys performed in the past had only a small chance to detect an object with a mass of 20 Earth masses or less, especially if it is near the farthest point of its orbit around the Sun. But NASA’s WISE (Wide-field Infrared Survey Explorer) telescope may have spotted a planet with a mass equal to 50 Earth masses or more.
“This puts an interesting upper mass limit for the planet,” Linder said.
According to the scientists, future telescopes like the Large Synoptic Survey Telescope near Cerro Tololo in Chile or dedicated surveys should be able to find or rule out candidate Planet Nine.
“That is an exciting perspective,” Prof. Mordasini said.
The results were published online April 6, 2016 in the journal Astronomy & Astrophysics.
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E. F. Linder & C. Mordasini. Evolution and magnitudes of candidate Planet Nine. A&A, published online April 6, 2016; doi: 10.1051/0004-6361/201628350
