The results may explain why telescopes haven’t spotted it yet.
Although scientists have not yet been able to spot the elusive Planet Nine — a potentially hidden planet hypothesized to exist near the outer edges of our solar system — that hasn’t stopped them from imagining how it looks or what it is made of.
Christoph Mordasini, a professor at the University of Bern, and his PhD student Esther Linder, wanted to find out the upper and lower limits of the size, temperature, and brightness of Planet Nine — if it indeed exists. Not only that, they wanted to determine if telescopes will ever be able to spot it.
The two scientists are experts in modeling the evolution of planets. However, they usually study the formation of young exoplanets in disks around stars light years away, and how to directly image these objects.
To model the mysterious planet, the astrophysicists assumed that Planet Nine is a smaller version of Uranus and Neptune — a small ice giant with an atmosphere of hydrogen and helium. Using their planet evolution model, they calculated how the planet’s radius and brightness evolved over time since the solar system formed 4.6 billion years ago.
Their results, published in the journal Astronomy and Astrophysics and financed by the research project of the Swiss National Science Foundation PlanetsInTime and the National Center for Competence in Research (NCCR) PlanetS, agreed with previous estimates that Planet Nine likely weighs no less than 10 Earth masses.
“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),” Ian O’Neill reported for Discovery News.
They also stated that although the planet is bigger than Earth, it is still too small for our telescopes to see. Sky surveys performed in the past only had a tiny chance of detecting an object with a mass of 20 Earths or less, especially if it is located near the furthest point of its orbit around the sun.
According to Mordasini and Linder, scientists may be able to increase the odds of sighting Planet Nine by switching to infrared telescopes. The 47 Kelvin temperature “means that the planet's emission is dominated by the cooling of its core, otherwise the temperature would only be 10 Kelvin,” explained Esther Linder in the press release.
In other words, if the planet was relying primarily on the sun for warmth, it would be far colder. Since it has another source of heat — its core — it would be more visible in the infrared since it will be warmer than the cooler objects around it. “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,” said O’Neill to Discovery News.
The team also came up with estimates of what the planet could be made of, which you can see in the graphic above.
Future telescopes, like the Large Synoptic Survey Telescope under construction near Cerro Tololo in Chile, or surveys dedicated to finding Planet Nine, should be able to, once and for all, determine if the planet really does exist.