A Ninth Planet Discovered?

Artist's conception of the hypothetical "Planet 9" in the distant regions of the solar system. Credit: Caltech/R. Hurt (IPAC)

Artist’s conception of the hypothetical “Planet 9” in the distant regions of the solar system. Credit: Caltech/R. Hurt (IPAC)

It’s been 170 years since the eighth (and so far last) major planet, Neptune, was discovered in our solar system. Pluto, of course, was discovered in 1930, heralded as the ninth planet, but then demoted by consensus of the astronomical community, largely at the behest of the Caltech astronomer Mike Brown who reasoned that Pluto was not large enough to gravitationally clear its path of other bodies, one of the three criteria for a major planet. In the ten years since Pluto’s demotion, Brown has been asked if there are any other planets in our solar system. His answer: “Nope, that’s it.”

So it was a big surprise this week when the same Michael Brown and his Caltech colleague Konstantin Batygin announced research that suggests there is indeed a ninth major planet orbiting the Sun. They have not detected the planet directly, but they have inferred its presence by its gravitational effect on a handful of much smaller objects that revolve around the Sun in orbits far past Neptune. The motion of these small bodies has been a puzzle for some time because they tend to bunch up in their own orbital plane, a plane distinct from that of the inner planets in the solar system. The probability of such an arrangement just by chance is about 1 part in 14,000. So Brown and Batygin looked for another solution. When they ran a computer model of the motion of a handful of these small distant objects with a hypothetical large planet on the other side of the solar system, they were able to quite precisely explain the observed motions of these smaller objects.

Other solar-system researchers agree this is not the discovery of a new planet– not yet– but it is a first-rate analysis that suggests there is a very good chance a big planet is out there.

How far out there? Far. It’s predicted to be in a highly elliptical orbit in which its distance to the Sun ranges from an estimated 30 billion to 160 billion kilometers. That works out to 215 astronomical units (AU) to 1075 AU, which compares to a distance to the Sun of 30 AU for Neptune and 40 AU for Pluto. To explain the gathering of smaller objects in the outer solar system, the hypothetical planet must be an ice-giant, somewhat like small Neptune, with an orbital period of 10,000 to 20,000 years and a mass of about ten Earths. That’s more than massive enough to qualify as a major planet.

How did such a large planet get so far away from the Sun? After all, it’s more than ten times the distance from the Sun to Neptune on average. The working theory is that it formed in the inner solar system at the same time as Jupiter, Saturn, Uranus, and Neptune, but it approached one of these planets too closely and got whipsawed to the outer solar system.

So the hunt is now on to find this ninth planet. Now provisionally called Planet 9, the distant ice giant is too dim to see in all but the worlds’ largest telescopes such as the Subaru or Keck telescopes on Mauna Kea. Planet 9 is likely to be located on the opposite side of the solar system from where the orbits of the small bodies bunch up, but its precise location is unknown. Brown and Batygin expect a telescopic confirmation of the existence of Planet 9 some time in the next five to seven years.

Here’s a link to the (highly technical) paper by Brown and Batygin in the Astronomical Journal:



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