This artistic rendering shows the distant view from Planet Nine back towards the sun. The planet is thought to be gaseous, similar to Uranus and Neptune. Hypothetical lightning lights up the night side. Caltech/R. Hurt (IPAC)

Evidence is piling up for the solar system's Planet Nine, with a diameter four times that of the Earth and ten times as massive and circling the sun at a distant 90 billion kms. First proposed this January by Caltech astronomers based on theoretical calculations of orbits of six Kuiper Belt objects, a French team has now found that Planet Nine fits the bill for a massive object causing the tug on Cassini orbiting Saturn.

Konstantin Batygin and Mike Brown from the California Institute of Technology first made the case for the hidden planet's existence based on its gravitational effect on the icy bodies circling the sun beyond Neptune in the Kuiper Belt. This was extended to check for its gravitational effect on other objects in the neighbourhood.

Agnès Fienga at the Côte d'Azur Observatory in France and her colleagues checked whether a theoretical model including Planet Nine could better explain slight perturbations seen in spacecraft Cassini's orbit and not explained by considering the eight planets in the solar system, 200 asteroids and five massive Kuiper Belt objects. A placing of Planet Nine at 600 astronomical units (about 15 times the average distance of Pluto from the sun) away toward the constellation Cetus in the model gives a perfect fit for Cassini's tug.

Not surprisingly, this place has not been scanned by previous observations. Now Batygin and Brown have asked for observation time on the Subaru Telescope on Mauna Kea in Hawaii, reports Scientific American.

Luckily the region is part of the Dark Energy Survey looking for evidence of the repulsing force causing the universe to accelerate. Clinching visual evidence for the planet can come in as early as year or within five years.

The other method would be to look for the millimeter-wavelength light the planet radiates from its own internal heat, which could be significant if the planet is as massive as believed to be and detected from surveys of the cosmic microwave background (CMB), the pervasive afterglow of the big bang.

Planet Nine would be the fifth largest planet in size after Uranus and Neptune with Jupiter and Saturn in the lead. At the distance it is, the planet would take between 10,000 and 20,000 years to make just one full orbit around the sun.

First calculations at Caltech
Brown and Batygin were looking for a massive object that kept the orbits of six Kuiper belt objects pointed in same direction. The best match was obtained when they ran their simulations with a massive planet in an anti-aligned orbit or an orbit in which the planet's closest approach to the sun, or perihelion, is 180 degrees across from the perihelion of all the other objects and known planets.

Planet Nine also provided an explanation for the mysterious orbits of two Kuiper belt objects and also the detection of objects predicted to be in orbits inclined perpendicularly to the plane of the planets. In the last three years, observers have identified four objects tracing orbits roughly along one perpendicular line from Neptune and one object along another.

How Planet Nine was formed has many theories ranging from a hijack from a nearby solar system to one formed in our own solar system but kicked away into the distance by Jupiter or Saturn. Where scientists have long believed that the early solar system began with four planetary cores that went on to grab all of the gas around them, forming the four gas planets—Jupiter, Saturn, Uranus, and Neptune, Brown believes this could well have been five. Collisions and ejections resulted in the present solar system.

Once confirmed, Planet Nine would make our planetary system as common as the others detected so far. It would be like other sun-like systems where most of the planets have no single orbital range and range from very close to very far. Also, the most common planets around other stars range between 1 and 10 Earth-masses. This too will be confirmed with Planet Nine's existence, says Batygin.