Searching for Planet Nine: What Is Lurking in the Outer Reaches of Our Solar System?

The Solar System has eight planets. In 2006, astronomers reclassified Pluto as a dwarf planet, the same class as contains Eris, Sedna, Quaoar, Ceres, and perhaps many more solar system small bodies. These are defined approximately as bodies that orbit the Sun but that are not massive enough (unlike regular planets) to gravitationally dominate their environments by clearing away material.

Astronomers wonder, though, whether there might not really be a ninth planet previously undiscovered but lurking in the outer reaches of the solar system, perhaps in the giant Oort cloud of objects that begins hundreds of astronomical units (au) from the Sun and extends outward. The notion that there may be a ninth massive planet in the outer solar system has taken on new appeal with recent data that show that the orbital parameters of some small bodies beyond Neptune (their inclinations, perihelion's, and retrograde motions) seem to behave as though they had been influenced by the gravity of a massive object in the outer solar system.

Although these data suffer from observational biases and statistical uncertainties, they have triggered renewed interest in the idea of the presence of another planet.

This speculative “Planet 9,” according to estimates, would be about 5-10 Earth-masses in size and orbit about 400-800 au from the Sun.

A planet at this distance would be extremely difficult to spot in normal optical sky searches because of its faintness, even to telescopes like Punsters and LSST. Most solar system objects were discovered at optical wavelengths via their reflected sunlight, but the sunlight they receive drops as one-over-their-distance-from-the-Sun squared; moreover, the reflected portion then travels back to telescopes on Earth and so declines again by a similar factor.

In the outer reaches of the solar system these objects, although cold, might emit more infrared radiation than the optical light they reflect, and astronomers in the past have used infrared surveys like the Wide-field Infrared Explorer (WISE) to search, but without success.

By Gajera International School, Utran

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