by Nick Carne
Carne is a science journalist based in Adelaide.
Beyond the orbit of Neptune
the Kuiper Belt - and what else?
are those mystery
orbits in outermost reaches of solar system caused by an unknown
another group of astronomers
suggesting the answer is "no".
They believe it can all be explained by the combined gravitational
force of small objects orbiting the Sun beyond Neptune.
"The Planet Nine
hypothesis is a fascinating one, but if the hypothesized
ninth planet exists, it has so far avoided detection," says
Antranik Sefilian, from the Department of Applied Mathematics
and Theoretical Physics at the University of Cambridge in the
In a paper (Shepherding
in a Self-gravitating Disk of Trans-Neptunian Objects)
published in in The Astronomical Journal, Antranik
Sefilian and colleagues suggest there's a disc made up of
small icy bodies with a combined mass as much as 10 times that
When combined with a simplified model of the solar system, the
gravitational forces of the disc can account for the unusual orbital
architecture exhibited by some objects at the outer reaches of the
Theirs is not, they concede, the first theory involving such a disc,
but it is the first, they say, which explains the significant
features of the observed orbits while accounting for the mass and
gravity of the other eight planets in our solar system.
Beyond the orbit of Neptune lies the
Kuiper Belt, which is made up of
small bodies left over from the formation of the solar system.
Neptune and the other
giant planets gravitationally influence the objects in the Kuiper
Belt and beyond, collectively known as Trans-Neptunian Objects
which encircle the Sun on nearly-circular paths from almost all
However, astronomers have discovered some mysterious outliers...
Since 2003, around 30
TNOs on highly elliptical orbits have been spotted:
they stand out from
the rest by sharing, on average, the same spatial orientation.
The Planet Nine
10 or X aka Nibiru) hypothesis suggests that to
account for the unusual orbits of these TNOs, there would have to be
another planet, about 10 times bigger than Earth, lurking in the
distant reaches of the solar system and shepherding the TNOs in the
same direction through the combined effect of its gravity and that
of the rest of the solar system.
"We wanted to see
whether there could be another, less dramatic and perhaps more
natural, cause for the unusual orbits we see in some TNOs,"
"We thought, rather than allowing for a ninth planet, and then
worry about its formation and unusual orbit, why not simply
account for the gravity of small objects constituting a disc
beyond the orbit of Neptune and see what it does for us?"
He and Jihad Touma,
from the American University of Beirut, modeled the full spatial
dynamics of TNOs with the combined action of the giant outer planets
and a massive, extended disc beyond Neptune.
Their calculations revealed that such a model can explain the
perplexing spatially clustered orbits of some TNOs.
In the process, they were
able to identify ranges in the disc's mass, its "roundness" (or
eccentricity) and forced gradual shifts in its orientations (or
precession rate), which reproduced the outlier TNO orbits.
"If you remove planet
nine from the model and instead allow for lots of small objects
scattered across a wide area, collective attractions between
those objects could just as easily account for the eccentric
orbits we see in some TNOs," Sefilian says.
Of course, he adds, it's
possible that both things are true:
there could be a
massive disc and a ninth planet.
discovery of each new TNO, we gather more evidence that
might help explain their behavior."