Plan(et) 9 from outer space: What the still-hypothetical planet might look like

Earlier this year, we covered the cutting-edge work of astronomers and researchers working to demonstrate the existence of a ninth planet far beyond Neptune or the dwarf planet Pluto. Thus far, there’s no direct visual evidence for such a world, but the mathematical models that Konstantin Batygin and Mike Brown used to demonstrate its likely existence are being taken seriously.

Fresh research published by Christoph Mordasini and Ph.D. student Esther Linder at the University of Bern, suggests that Planet Nine is probably a smaller version of an ice giant like Uranus or Neptune.

High school science classes in the United States tend to lump the planets of the solar system into two groups: Rocky worlds (Mercury, Venus, Earth, Mars) and the gas giants (Jupiter, Saturn, Uranus, Neptune). While this is accurate, it obscures the significant differences between Jupiter and Saturn, which are composed almost entirely of hydrogen and helium, while both Uranus and Neptune contain significant amounts of methane, ammonia, and water ice. The last two planets in the solar system also contain significant amounts of rocky material, though the distribution and state of this material isn’t entirely known. Uranus, for example, might be missing part of its core after a collision with an Earth-sized protoplanet knocked it over on to its side, where it remains to this day.

The scientists at the University of Bern calculated that the planet would be approximately 10 Earth masses, smaller than either Uranus or Neptune, with an estimated temperature of -226C or 47K. Planet Nine’s hypothetical orbit is far enough from the sun that the vast majority of its energy would be provided by its own planetary core. Planets generate heat through three mechanisms — residual energy from the planet’s formation, heat generated as heavy metals continue to sink to the center of the planet, and heat generated by the decay of radiological elements. Because the hypothetical Planet 9 only receives a fraction of its energy from the sun, it might be easier to pick up in the infrared band as an emissive object rather than looking for the light it reflects via a traditional telescope.

The Bern team didn’t just predict the size and composition of the hypothetical Planet 9 — they also examined the capabilities of past local searches for planets and asteroids to determine whether these efforts had a reasonable chance of finding the theorized ice giant. In their opinion, existing surveys and telescopes haven’t been powerful enough to detect Planet 9, particularly if its currently at aphelion (the point of its orbit furthest from the sun). The upcoming Large Synoptic Survey Telescope (LSST) under construction in Chile should be capable of detecting whether Planet 9 actually exists or not. First light for the LSST is currently expected in 2019.

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