January 20, 2016
Today, two scientists announced evidence that a body nearly the size of Neptune - but as yet unseen - orbits the sun every 15,000 years. During the solar system's infancy 4.5 billion years ago, they say, the giant planet was knocked out of the planet-forming region near the sun.
Slowed down by gas, the planet settled
into a distant elliptical orbit, where it still lurks today.
But the new evidence comes from a pair of respected planetary scientists, Konstantin Batygin and Mike Brown of the California Institute of Technology (Caltech) in Pasadena, who prepared for the inevitable skepticism with detailed analyses of the orbits of other distant objects and months of computer simulations.
Batygin and Brown inferred its presence from the peculiar clustering of six previously known objects that orbit beyond Neptune.
They say there's only a 0.007% chance,
or about one in 15,000, that the clustering could be a coincidence.
Instead, they say, a planet with the mass of 10 Earths has
shepherded the six objects into their strange elliptical orbits,
tilted out of the plane of the solar system.
Its closest approach to the sun is seven times farther than Neptune, or 200 astronomical units - AUs - (an AU is the distance between Earth and the sun, about 150 million kilometers.)
And Planet X could roam as far as 600 to 1200 AU, well beyond the Kuiper belt, the region of small icy worlds that begins at Neptune's edge about 30 AU. If Planet X is out there, Brown and Batygin say, astronomers ought to find more objects in telltale orbits, shaped by the pull of the hidden giant.
But Brown knows that no one will really believe in the discovery until Planet X itself appears within a telescope viewfinder.
The team has time on the one large
telescope in Hawaii that is suited for the search, and they hope
other astronomers will join in the hunt.
but this is head and shoulders
above everything else.
Alessandro Morbidelli, a
planetary dynamicist at the Nice Observatory in France, performed
the peer review for the paper. In a statement, he says Batygin and
Brown made a "very solid argument" and that he is "quite convinced
by the existence of a distant planet."
as a dwarf planet - a
saga Brown recounted in his book 'How
I Killed Pluto.'
In 1846, for example, the French
mathematician Urbain Le Verrier predicted the existence of a
giant planet from irregularities in the orbit of Uranus. Astronomers
at the Berlin Observatory found the new planet, Neptune, where it
was supposed to be, sparking a media sensation.
In 1930, Pluto turned up - but it was
far too small to tug meaningfully on Uranus. More than half a
century later, new calculations based on measurements by the Voyager
spacecraft revealed that the orbits of Uranus and Neptune were just
fine on their own: No Planet X was needed.
In the 1980s, for example, researchers proposed that an unseen brown dwarf star could cause periodic extinctions on Earth by triggering fusillades of comets. In the 1990s, scientists invoked a Jupiter-sized planet at the solar system's edge to explain the origin of certain oddball comets.
Just last month, researchers claimed to
have detected the faint microwave glow of an outsized rocky planet
some 300 AU away, using an array of telescope dishes in Chile called
the Atacama Large Millimeter Array (ALMA).
(Brown was one of many skeptics, noting that ALMA's narrow field of
view made the chances of finding such an object vanishingly slim.)
Sedna's odd, far-flung orbit made it the most distant known object in the solar system at the time. Its perihelion, or closest point to the sun, lay at 76 AU, beyond the Kuiper belt and far outside the influence of Neptune's gravity.
The implication was clear:
(DIAGRAM) A. CUADRA/SCIENCE
Sedna's gravitational nudge could have
come from a passing star, or from one of the many other stellar
nurseries that surrounded the nascent sun at the time of the solar
By combining Sedna with five other weirdos, Brown says he has ruled out stars as the unseen influence:
Of his three major discoveries - Eris, Sedna, and now, potentially, Planet X - Brown says the last is the most sensational.
Brown and Batygin were nearly beaten to the punch.
For years, Sedna was a lone clue to a perturbation from beyond Neptune. Then, in 2014, Scott Sheppard and Chad Trujillo (a former graduate student of Brown's) published a paper describing the discovery of VP113, another object that never comes close to the sun.
Sheppard, of the Carnegie Institution for Science in Washington, D.C., and Trujillo, of the Gemini Observatory in Hawaii, were well aware of the implications.
They began to examine the orbits of the two objects along with 10 other oddballs. They noticed that, at perihelion, all came very near the plane of solar system in which Earth orbits, called the ecliptic.
In a paper, Sheppard and Trujillo
pointed out the peculiar clumping and raised the possibility that a
distant large planet had herded the objects near the ecliptic. But
they didn't press the result any further.
Plotting the orbits of the distant
objects, Batygin says, they realized that the pattern that Sheppard
and Trujillo had noticed "was only half of the story." Not only were
the objects near the ecliptic at perihelia, but their perihelia were
physically clustered in space (see diagram, above).
But he made his mark there by modeling the fate of the solar system over billions of years, showing that, in rare cases, it was unstable: Mercury may plunge into the sun or collide with Venus.
Brown, 50, is the observational astronomer, with a flair for dramatic discoveries and the confidence to match.
He wears shorts and sandals to work,
puts his feet up on his desk, and has a breeziness that masks
intensity and ambition. He has a program all set to sift for Planet
X in data from a major telescope the moment they become publicly
available later this year.
They even became exercise buddies, and
discussed their ideas while waiting to get in the water at a Los
Angeles, California, triathlon in the spring of 2015.
A favored size for Planet X emerged - between five and 15 Earth masses - as well as a preferred orbit:
The orbits of the six cross that of Planet X, but not when the big bully is nearby and could disrupt them.
The final epiphany came 2 months ago, when Batygin's simulations showed that Planet X should also sculpt the orbits of objects that swoop into the solar system from above and below, nearly orthogonal to the ecliptic.
It turns out that, since 2002, five of these highly inclined Kuiper belt objects have been discovered, and their origins are largely unexplained.
Sheppard, who with Trujillo had also suspected an unseen planet, says Batygin and Brown,
Others, like planetary scientist Dave Jewitt, who discovered the Kuiper belt, are more cautious.
The 0.007% chance that the clustering of the six objects is coincidental gives the planet claim a statistical significance of 3.8 sigma - beyond the 3-sigma threshold typically required to be taken seriously, but short of the 5 sigma that is sometimes used in fields like particle physics.
That worries Jewitt, who has seen plenty of 3-sigma results disappear before.
By reducing the dozen objects examined by Sheppard and Trujillo to six for their analysis, Batygin and Brown weakened their claim, he says.
Astronomers say a
Neptune-sized planet lurks unseen in the solar system
A. CUADRA/SCIENCE; NASA/JHUAPL/SWRI;
(DIAGRAM) A. CUADRA/SCIENCE
It ruled out the existence of a Saturn-or-larger planet as far out as 10,000 AU, according to a 2013 study by Kevin Luhman, an astronomer at Pennsylvania State University, University Park.
But Luhman notes that if Planet X is Neptune-sized or smaller, as Batygin and Brown say, WISE would have missed it. He says there is a slim chance of detection in another WISE data set at longer wavelengths - sensitive to cooler radiation - which was collected for 20% of the sky.
Luhman is now analyzing those data.
And even if Planet X did get a foothold
as a planetesimal, it would have moved too slowly in its vast, lazy
orbit to hoover up enough material to become a giant.
Another embryonic giant planet could easily have formed there, only to be booted outward by a gravitational kick from another gas giant. It's harder to explain why Planet X didn't either loop back around to where it started or leave the solar system entirely.
But Batygin says that residual gas in the protoplanetary disk might have exerted enough drag to slow the planet just enough for it to settle into a distant orbit and remain in the solar system.
That could have happened if the ejection
took place when the solar system was between 3 million and 10
million years old, he says, before all the gas in the disk was lost
Other researchers are more positive.
The proposed scenario is plausible, Laughlin says.
All this means that Planet X will remain
in limbo until it is actually found.
Most telescopes capable of seeing a dim
object at such distances, such as the Hubble Space Telescope or the
10-meter Keck telescopes in Hawaii, have extremely tiny fields of
view. It would be like looking for a needle in a haystack by peering
through a drinking straw.
It has enough light-gathering area to detect such a faint object, coupled with a huge field of view - 75 times larger than that of a Keck telescope. That allows astronomers to scan large swaths of the sky each night.
Batygin and Brown are using Subaru to look for Planet X - and they are coordinating their efforts with their erstwhile competitors, Sheppard and Trujillo, who have also joined the hunt with Subaru.
Brown says it will take about 5 years
for the two teams to search most of the area where Planet X could be
has a large field of view - enabling it
to search efficiently
for Planet X.
Brown says it's too early to worry about that and scrupulously avoids offering up suggestions. For now, he and Batygin are calling it 'Planet Nine' (and, for the past year, informally, Planet Phattie - 1990s slang for "cool").
Brown notes that neither Uranus nor Neptune - the two planets discovered in modern times - ended up being named by their discoverers, and he thinks that that's probably a good thing.
It's bigger than any one person, he says:
He is sure, however, that Planet X - unlike Pluto - deserves to be called a planet.
Something the size of Neptune in the solar system? Don't even ask...
A New 9th Planet for the Solar System?