Planet 9: The Hunt Intensifies

Exciting news, science fans!

Last week there was a flurry of papers all on the hunt for the elusive Planet 9 in the Astrophysical Journal Letters. The articles, all open source, deal with the likely characteristics and potential position of the yet to be discovered body.

One paper, written by Mike Brown (the guy responsible for Pluto’s demotion) and Konstantin Batygin, the guys who first published the possible existence of Planet 9 back in the new year, tries to narrow down the location by looking at the unusual orbits of certain Kuiper Belt Objects (KBOs). If you look at the orbits of Sedna and half a dozen other KBOs then the most likely explanation for how they got where they are is that their orbits were perturbed by a large mass 5-20 times that of the earth. They also predict that, if it exists, the plane of the orbit of Planet 9 is 30 degrees out from that of the rest of the solar system and it can be found somewhere in the region of the constellation Orion.

The paper by Malhotra et al conducts similar analyses, looking to fine tune the orbit and position of Planet 9 but also sounds a strong sceptical note. They highlight that the entire argument for Planet 9 is based on the odd orbits of a very small number of objects and that far more data is needed before we could be even vaguely confident that the rogue planet exists.

Finally, the paper by Fortney et al looks at the potential atmosphere of Planet 9. Given its extreme distance from the sun and subsequent low temperature, estimated at only 35-50 Kelvin, they hypothesise that the methane in the atmosphere would have condensed out leaving behind an atmosphere comprised mostly of hydrogen and helium. These are both highly reflective and could result in 75% of the light that hits it being reflected back. That’ll certainly make it easier to find if true.

Assuming that these theories are all correct, and that is still a big assumption at this point, it sounds like we’re making progress in tracking Planet 9 down: we know roughly where it is, it’s way bigger than earth and shiny to boot. What’s the hold up in finding it? That would be the distance. The average distance between the earth and the sun is known as 1 astronomical unit. Pluto is about 40 au from the sun. Planet 9 is currently predicted to be more than 1500 au from the sun; that’s about 250 billion km.

Don’t be daunted, though, fortune favours the bold. Teams of scientists have weeks of time booked on the world’s best telescopes later this year and that’s when the hunt will really begin in earnest. In the meantime scientists will be trying to use more eccentric KBO orbits to further refine their predictions. More power to their elbow.


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