'Pluto killing' astronomer Mike Brown announced that
he and his colleagues had found evidence that a
massive, icy planet could be lurking on the edge of
the Solar System, just past Neptune, about 149
billion km from the Sun.
No oneís ever seen it (being 75 times more distant
than Pluto doesnít help), but Brown's estimated that
the hypothetical 'Planet Nine' orbits our Sun every
10,000 to 20,000 years, and is about 10 times more
massive than Earth and four times the size.
Now, Swiss astronomers have used the available data
surrounding Planet Nine to figure out what the upper
and lower limits on how big, bright, and warm it
could actually be. The evidence suggests itís
like a 'mini-Uranus',
with a solid iron core surrounded in ice and a dense
layer of gas.
"With our study, candidate Planet 9 is now more than
a simple point mass, it takes shape having physical
one of the team, Christoph
Mordasini from the University of Bern.
Mordasini and PhD student Esther Linder decided to
try and nail down some specifics for Planet Nine by
applying the evidence surrounding Planet Nine to
existing planetary evolution models. Not only does
this allow them to make estimates on various
characteristics of the mystery planet, but it also
could explain about why after hundreds of years of
getting to know the Solar System, weíve only
recently seen hints of its existence.
Based on the fact that our strongest evidence for
the existence of Planet Nine is the weird behaviour
of some Kuiper Belt Objects out on the edge of the
Solar System - which suggests the gravitational pull
of something massive is
messing with them -
Mordasini and Linder agree with previous estimates
that itís likely no smaller than around 10 times
more massive than Earth.
But the fact that multiple infrared surveys, such as
Infrared Survey Explorer (WISE)
mission, have seen no signs of it means it canít be
much bigger than 10 times more massive than Earth.
And knowing its approximate distance from the Sun
(based on where the weird Kuiper Belt Objects are)
too, they could extrapolate a whole lot more.
"By their reckoning, Planet Nine should have a
radius 3.7 times that of Earth and an upper
atmosphere temperature of -226 degrees Celsius (or
47 Kelvin)," Ian
OíNeill reports for Discovery News. "They
arrived at these numbers by considering Planet
Nineís predicted orbit around the Sun and the age of
our Solar System; the hypothetical world would have
formed from our Sunís protoplanetary disk that began
to condense into planets some 4.6 billion years
Interestingly, the models showed that at that
distance from the Sun, Planet Nine should be a lot
colder if our star is its only source of heat.
Having an estimate for its temperature means they
were then able to figure out how bright it could be. As
when planets form, the incredible amount of
gravitational energy generated in their cores can
keep them molten hot for billions of years, and this
heat could contribute to the overall temperature of
So to maintain a temperature of -226 degrees Celsius
some 149 billion km from the Sun, the researchers
suggest that Planet Nineís 'intrinsic' - or internal
- power is about 1,000
times bigger than
its absorbed power.
"This means that the reflected sunlight would be
minuscule compared to the internal heat the world is
currently generating, making its infrared signal
vastly more powerful than looking for reflected
sunlight in optical wavelengths," says
Of all the radiation that could be detected around
Planet Nine, sunlight makes up only a tiny part of
that, and - if it exists - it should be much
brighter in the infrared wavelengths than in the
The team has also come up with estimates of what the
planet could be made of, which you can see in the
So... why havenít we found it yet? Mordasini and
Linder suggest that the sky surveys that have been
performed in the past were very unlikely to have
detected anything in the Solar System with a mass
equivalent to 20 Earth masses or less, especially if
they were looking when the planet was near its
farthest point from the Sun.
But there is hope - future telescopes such as the Large
Synoptic Survey Telescope, which
is currently under construction in Chile, should be
powerful enough to spot something smaller, or more
focused surveys might be able to turn up more
evidence that something really is out there.
We have to be patient for either of those
possibilities, but one thingís for sure - answers
are coming, and we canít wait to find out what they