A ninth planet once again?

MYSTERIOUS

It is good news for skywatchers. In an article published in The Astronomical Journal on January 20, 2016, researchers Konstantin Batygin and Michael Brown of the California Institute of Technology have provided convincing evidence for what will now be the ninth planet in our solar system, generating interesting debate among astronomers. For most of us, the idea of solar system ends with Pluto, which was the ninth planet until 2006. However, that year, Pluto was demoted as a dwarf planet, and our solar system was left with only eight planets, with Neptune being the farthest from the Sun.

Neptune orbits the Sun in an elliptical path, with its farthest position from the Sun (aphelion) being about 30 Astronomical Units (distances in the solar system is measured in Astronomical Unit, which is the mean distance between the centre of the Sun and the centre of Earth. It is equivalent to about 150 million km). Beyond Neptune lies a region known as Kuiper Belt, consisting mainly of dwarf planets and lots of small icy objects scattered over a region up to 50 astronomical units (AU). In fact, Pluto itself is one of the large Kuiper Belt Objects (KBOs).

In 2003, Michael Brown discovered another object in the Kuiper Belt, which was named Sedna. The peculiarity of Sedna is that it has a highly eccentric orbit with its closest distance from the Sun (perihelion) at 76 AU and far outside the influence of the gravity of Neptune. He wondered if something massive and well beyond Neptune was pulling Sedna into such a distant orbit. Eleven years later, astronomers C A Trujillo and S S Sheppard discovered another KBO, VP113, having a similar stretched out orbit with a perihelion of 80 AU. They also observed that Sedna, VP113 and a few other KBOs, all exhibited another peculiarity property — their perihelion lay in the plane of the solar system and all moved from north to south while crossing that plane. Since then, a handful of other icy objects from the Kuiper Belt have turned up with similar orbits.

Intrigued by these strange orbital properties, Konstantin and Michael studied them further and found that in addition to being clustered at the perihelion, the long axis of their orbits fell in the same quadrant in the sky. This meant that they were clustered in space also. Factoring these parameters into a computer simulation, they found “that such clustering has only a probability of 0.007 per cent, thus requiring a dynamical origin”.

The dynamical origin they proposed was that of a hitherto undiscovered huge planet in the solar system, with a diameter two to four times, and mass ten times that of Earth. It is an ice giant similar in composition to Uranus and Neptune — a mixture of rock and ice covered with an atmosphere of hydrogen and helium.

They also deduced that its proposed orbit lies in approximately the same plane as those of the distant KBOs, but the perihelion being about 180 degree away from those of the KBOs. This means that the orientation of its orbit is approximately opposite to the orientation of the orbits of the KBOs. Thus the planet is predicted to have a highly eccentric orbit around the Sun. Even its perihelion is five times as far from the Sun as Pluto and its looping orbit extends up to 600 to 1200 AU from the Sun, far beyond the Kuiper Belt, with an orbital period of 15,000 Earth years.

One intriguing question is how Planet 9 ended up so far from the Sun? At such distances, the primordial gas and dust from which the solar system formed would be too thin to support the growth of a planet of this size. The authors propose that the core of Planet 9 might have been formed much closer to the Sun, alongside those of Jupiter, Saturn, Uranus and Neptune (other giant planets), but “was ejected during the nebular epoch of the solar system evolution”.

If so, why did not Planet 9 loop back to where it started or leave the solar system entirely? Konstantin contends that the “interaction with the Sun’s birth clusters and possibly the gaseous components of the nebula would have facilitated the retention of the scattered planet on a bound orbit”.

Seeing is believing
Notwithstanding these predictions, Planet 9 has not yet been sighted. Many astronomers remain skeptical until it is directly observed. But observation is not easy because of the planet’s inferred size (smaller than Uranus and Neptune) and its exceedingly large distance from the Sun, which would make it extremely faint — 600 times fainter than Pluto.

Telescopes like Hubble Space Telescope and the 10-metre Keck Telescope which can see dim objects at such distances have a very narrow field of view. “It would be like looking for a needle in a haystack by peering through a drinking straw,” says Eric Hand, a Science commentator.

Nevertheless, Konstantin and Michael are confident that Planet 9 will be cited within the next five years. They pin their hopes on an eight-metre telescope called Saburu in Hawaii. In addition to having a high light gathering area to detect such faint objects, it has huge field of view — 75 times larger than that of Keck Telescope, permitting astronomers to view large swaths of the sky each night.

Whether Planet 9 is cited or not, this will not be the last major study of our solar system. Sun’s gravitational field extends up to more than 1,00,000 AU and an overwhelming majority of the solar system is still to be explored. There may lurk in that cold darkness many exotic objects, waiting to be discovered.