What is
important is that there is disturbance.
Had it been zero, then the possibility of another large planet would
have become rather implausible. It is
now a plausible prospect.
Of course, we
are unlikely to find it anytime soon and that remains that. However we now do have ample reason to engage
in sky scanning to discover micro lensing in general. It will be much like the decades of
photographic interpretation that most astronomy became during the twentieth
century. At least now a computer can do
it.
It will still
need a vast number of eyeballs, but that is coming also in time. After all we do know how to at least narrow
the band of observation to the solar system’s plane from which we can widen it.
New dwarf planet hints
at giant world far beyond Pluto
A surprise monster may be lurking in our solar
system. A newly discovered dwarf planet has grabbed the crown as the most
distant known object in our solar system – and its orbit hints at a giant,
unseen rocky world, 10 times the mass of Earth and orbiting far beyond Pluto.
The dwarf planet, for now dubbed 2012 VP113 because
it was spotted in images taken in November 2012 – is an interesting discovery
in itself. Scott Sheppard of
the Carnegie Institution for Science in Washington DC and his colleagues found
that it is a lump of rock and ice 450 kilometres wide and lies at 80
astronomical units from the sun at its closest approach (1 AU is Earth's
distance from the sun).
That's twice as far as the most famous dwarf
planet, Pluto,
which is 2340 kilometres wide and also beats the previous record holder, a
1000-kilometre-wide planetoid called Sedna, discovered in 2003, with a closest
approach of 76 AU out.
Objects orbiting this far from the sun, in the
"inner Oort cloud", are useful to probe the early solar system.
That's because they lie too far away to be perturbed by the gas planets, but
too close to the sun to be affected by the gravity of other stars in our galaxy
– so their orbits and behaviour are thought to be almost unchanged since they
first formed. "Once we find more objects in this region, we'll be able to
start to strongly constrain the possible formation scenarios," says Sheppard.
Distant
puppeteer
The discovery of 2012 VP113 instantly doubles the
number of objects known in this part of space, from 1 to 2 – and the team
estimates that many more exist in this region and are just waiting to be
discovered. "We thought Pluto was unique for over 70 years, but we now
know that it shares its orbit with thousands of other objects," says
Sheppard. "Sedna was unique for about 10 years but it's now clear that
Sedna and 2012 VP113 are just the tip of the iceberg."
Intriguingly, Sheppard's team also found a strange
alignment when they looked at the orbits of 2012 VP113, Sedna and 10 other
objects that lie closer to the sun. "It was a big surprise to us," he
says.
One explanation for the alignment could be the tug
of a rocky planet that is 10 times the mass of Earth that orbits the sun at 250
AU, the team calculate. That world would be cold and faint – and would push and
pull at the closer objects like a distant but powerful puppeteer.
NASA's
Wide-field Infrared Survey Explorer (WISE) scoured this region of space in 2010 and 2011
searching for a so-called Planet X and came up empty.
However, WISE was looking for the tell-tale warmth
of gas giants – a rocky "super-Earth", like the one Sheppard's team
suggest, would be too cold for the telescope to pick up. "This is too
faint for WISE," says Ned Wright, the space telescope's principal
investigator. Even if the planet has a small internal heat source – and absorbs
some sunlight, it would still not generate enough heat to register, he adds.
Rogue planets
Nonetheless, if it exists, how could such a monster
possibly have escaped notice until now? It may seem strange, particularly given
our recent ability to
spot more than 1000 far more distant planets, in alien solar systems. But instruments such
as NASA's Kepler space telescope, which has had particular success in finding such
exoplanets, would have no chance of spotting a planet like this one.
Kepler is designed to spot tell-tale dips in the
brightness of alien stars as planets pass in front of them. "No way would
Kepler find this planet," says William Welsh of
San Diego State University, a member of the Kepler team.
The only method currently available that might have
a chance is gravitational microlensing, suggests Welsh, where the gravity of a
massive object lying between us and the planet could be used as a cosmic
magnifying glass. Recently this method was used to spot an object that might be the first moon glimpsed
outside our solar system.
However, there are other ways to explain the orbital
alignment that Sheppard's team found, including the pull of stars or "rogue" planets kicked out of our solar system eons ago.
For that reason, Mike Brown of the California Institute of Technology, a
co-discoverer of Sedna, isn't holding out hope of a giant planet just yet.
"It is possible that some undiscovered large object out there is doing
this, but there are likely many other explanations, too, most of them sadly
more mundane."
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