Two new studies have been published providing fresh evidence for the existence of a massive planet at the edge of the Solar System.

The object, dubbed Planet Nine, was first proposed early this year, but it has yet to be directly observed. It is believed to be on average 70 times more distant from the Sun than Earth, so observations are difficult.

So far, astronomers have looked at other potential effects this object might have on the Solar System. Now, researchers from the California Institute of Technology (Caltech), including the proposers of Planet Nine – Mike Brown and Konstantin Batygin – suggest that the six-degree tilt observed in the Sun compared to the plane of the Solar System is actually caused by the pull of Planet Nine.

“Because Planet Nine is so massive and has an orbit tilted compared to the other planets, the Solar System has no choice but to slowly twist out of alignment,” said lead author Elizabeth Bailey in a statement.

The first paper, available online and submitted to the Astrophysical Journal, focuses on the angular momentum contribution of the potential planet on the spin of our star. Planet Nine is believed to arrive as close as 200 AU and as distant as 1,200 AU, with 1 AU (astronomical unit) being the Earth-Sun distance.

Having an object at least 10 times the mass of Earth, on such a wide orbit, and with an expected inclination of 30 degrees will generate a wobble in the Sun. And according to the calculations, it can explain the observed tilt.

The other study, published in the Astrophysical Journal Letters, focuses on a different piece of evidence. It appears that the orbit of four minor objects in the Kuiper belt, including the minor planet Sedna, are not random. The cause of this cosmic ballet seems to be a massive object with properties very similar to what is expected for Planet Nine.

“We analyzed the data of these most distant Kuiper Belt objects, and noticed something peculiar, suggesting they were in some kind of resonances with an unseen planet,” lead author Renu Malhotra, from the University of Arizona, said in a statement. “Our paper provides more specific estimates for the mass and orbit that this planet would have, and, more importantly, constraints on its current position within its orbit.”

Neither pieces of research are conclusive evidence for the existence of Planet Nine, but the scale is tilting in its favor. Brown and his colleagues are now searching the outer Solar System for signs of Planet Nine, and think it may take three or more years to finally observe it.




How close are we to finding Planet Nine? A new paper seems to suggest that the solution to the mystery is actually much closer than we previously thought.

Matthew Holman and Matthew Payne from the Harvard-Smithsonian Center for Astrophysics have used observations by the Cassini spacecraft to reduce the potential area in the sky where Planet Nine might be hiding. The region is found in the southern sky, roughly in the direction of the constellation of Cetus. The likely area extends over 20 degrees in all directions; by comparison, the full moon measures only half a degree across.

Planet Nine is a hypothetical planet proposed in January by Mike Brown and Konstantin Batygin to explain why objects beyond the orbit of Neptune have their closest point to the Sun in the same location.

In February, French researchers announced that by using data from the NASA/ESA Cassini spacecraft orbiting Saturn, they were able to narrow down the area where Planet Nine might be hiding. They used the perturbations, or lack thereof, in Saturn’s orbit to establish where the planet might be. Saturn would only be perturbed if Planet Nine was at its closest approach, so not seeing any perturbations tells us that Planet Nine is not very close to the Sun right now.

In the latest study, available online, the two researchers used a sophisticated statistical technique called Markov Chain Monte Carlo to reduce the potential hiding place of Planet Nine even further. While the data from Cassini doesn’t show any perturbations that cannot be explained with current models, those studies dismiss the perturbations as noise. Holman and Payne, therefore, decided to look for potential Planet Nine models that would not only fit the values, but also bolster support for the perturbations as a real effect.

“We put Planet Nine at a whole different slew of locations – all different possibilities on the sky, different distances, different masses – and tried to find out whether that constrains things even more,” said Payne to New Scientist.

According to the model, Planet Nine could be located in two narrow strips of the sky. The team then overlapped these regions with Batygin and Brown’s suggested orbit and got an even smaller area.

Astronomers are already looking at the suggested region and will hopefully soon find out whether there are nine planets in the Solar System.

[H/T: New Scientist]



The possibility of a ninth planet has aroused plenty of excitement lately among astronomers. Evidence for this planet comes from its herding of more modest-sized worlds beyond Neptune, suggesting a large gravitational force is at play. However, attempts to explain how such a large object could be orbiting at these distances suggest it had to buck the odds to get there.

Until we have visual confirmation, we won’t know for sure that Planet Nine exists, but as evidence starts to build, the race is on to see if we can spot it. This in turn has led to another race to explain the planet’s history.

Dr. Gongjie Li of the Harvard-Smithsonian Center for Astrophysics has had a paper accepted by Astrophysical Journal Letters (preprint on arXiv) discussing some of the possibilities for how it formed. But she found that none could feasibly explain how it got to be in a wide and eccentric orbit.

“Evidence for the new planet is provided by the orbital alignment of Kuiper Belt objects, and other Solar System properties, which suggest a Neptune-mass object on an eccentric orbit with semi-major axis 400 − 1,500 AU [Astronomical Units, 1 AI is the distance from the Earth to the Sun],” Li and her co-author Fred Adams write. “With such a wide orbit, Planet Nine… is susceptible to disruption by passing stars.”

Evidence for Planet Nine rests on the way the orbits of Kuiper Belt objects come together as if something was herding them. Caltech/Robert Hurt.

Today, the nearest stars are so far away they would not disturb Planet Nine. Things would be different, however, if the Sun was part of a cluster, like the one in which it is believed to have formed. Within clusters, stars are so near each other that it would be very likely that a close encounter would disrupt Planet Nine, hurling it out of its orbit.

Stars begin their lives in clusters, which tend to be 1,000 times as densely packed as the Sun’s current neighborhood. Eventually, they break free, but how long this takes varies. If the Sun spent more than 100 million years within its birth cluster, Li and Adams think Planet Nine would have been unlikely to last the distance.

If the Sun spent less than 100 million years in such an environment, Li and Adams think Planet Nine would have had a reasonable prospect of maintaining its current orbit, but are still puzzled by how it got there in the first place.

An interaction with a nearby star (either before leaving the cluster or a chance encounter thereafter) could have pulled Planet Nine into its current theorized orbit, but only if it started out in a location that fit poorly with our current models of the Solar System.

Alternatively, Planet Nine could once have dwelt closer in and been thrown out by Jupiter or Saturn. However, the paper reports, “the window for scattering a planet into an orbit with a ∼1000 AU [1 AU is the Earth-Sun distance] is narrow.” More often, planets get thrown out of the Solar System entirely.

Another theory is that Planet Nine formed outside the Solar System before being captured by the Sun’s gravity, but Li and Adams conclude that the chance of this occurring is less than 1 percent. Not Leicester City level of unbelievability, but unlikely nevertheless.

Not everyone thinks Planet Nine is such a candidate for the finite improbability drive, however. Dr. Scott Kenyon, also of the Harvard-Smithsonian, has submitted two papers to the Astrophysical Journal with models for how a series of repeated encounters between Planet Nine and a larger planet could have placed it in its current orbit. Kenyon’s work has yet to pass peer review, but preprints of both are also available on arXiv.

Kenyon argues his theories can be observationally tested. If either is confirmed, it will provide unparalleled information about the Solar System’s early development.




A long-standing mystery in the Solar System might have been solved thanks to the help of an unlikely (and unconfirmed) object: Planet Nine.

Two different teams of astronomers, one from the US and one from Brazil and France, have independently discovered that the presence of a mysterious ninth planet beyond Neptune might have caused the orbit of the other eight planets to tilt with respect to the Sun.

The orbits of the eight planets we all know and love are not on the same plane. They have a small inclination compared to each other and they are tilted with respect to the Sun. But based on the interaction between the planets and the Sun, there shouldn’t be such an effect.

The discovery of other planetary systems has allowed us to understand our own Solar System better, and several explanations have been put forward to explain the tilt. A temporary interaction with a passing star might have given the planets the right tug, or maybe the slant was there right from the start as the magnetic field of the young Sun moved the protoplanetary disk that formed the planets.

Those hypotheses are appealing but they lack a smoking gun, an undeniable proof to be the likely cause of the tilt. For this reason, the two teams were looking at alternative culprits, and the existence of a massive potential ninth planet might fit the bill.

Planet Nine has not been discovered yet, but it was proposed by Caltech’s Michael Brown and Konstantin Batygin to explain the complex orbits of objects in the outer Solar System. Now, with Caltech’s Elizabeth Bailey, who led the study (available on arXiv), they have estimated the impact of Planet Nine on the orbit of objects in the inner Solar System.

“Because we think Planet Nine has a significant inclination, if it exists, then that means it would tilt things,” Bailey told New Scientist. “It’s one puzzle piece that seems to fit together, and it really seems to be in support of the Planet Nine hypothesis.”

Planet Nine is thought to be between five and 20 times the mass of Earth, and the large size of the icy planet could have created the tilt in the early Solar System. In their paper, the researchers show that the induced tilt from the simulation is consistent with the value that has been measured.

Independently, researchers from Brazil and France have come to the same conclusion, although the driving mechanism is not Planet Nine’s mass but the tilt of its orbit.

Planet Nine can help explain a lot of curious phenomena in the Solar System, but until somebody sees it with a telescope, it remains just a great hypothesis.

[H/T: New Scientist]


We don’t yet know for certain where the hypothesized ninth planet of the Solar System is, if it even exists. But one team of researchers has narrowed down the area that it might be lurking.

Planet Nine was first proposed to exist by astronomers Mike Brown and Konstantin Batygin last month. By modeling the motion of objects in the Kuiper Belt, an area of icy bodies outside Neptune’s orbit, they suggested there was a Neptune-sized planet 10 times the mass of Earth orbiting the Sun at a distance of more than 200 AU (astronomical unit, 1 AU is the Earth-Sun distance).

We don’t have any direct evidence for the planet yet, but in an effort to help astronomers locate it, a team of French scientists has refined where it could be. Using radio data from the Cassini spacecraft orbiting Saturn, they looked for the gravitational effects of Planet Nine on the gas giant. Not finding anything, they ruled out certain areas where it could reside.

Their results are published in the journal Astronomy and Astrophysics.

The gravitational effects of Planet Nine are only thought to be detectable for half of its highly elliptical orbit, which can take up to 20,000 years. So if the planet is at the point in its orbit furthest from us, we probably won’t be able to detect its gravitational effect on the outer planets – although we could spot it via other methods, perhaps direct images or its effect on other bodies.

But if the planet is in the nearer half of its orbit, the French scientists say they can exclude 50 percent of the predicted orbital path based on the lack of perturbations on Saturn. “We have cut the work in half,” co-author Jacques Laskar from the Paris Observatory told Agence France-Presse (AFP). In fact, they have noted a zone where the planet can most likely be found, seen below.

The red zone is excluded by the study. The pink zone could be studied if Cassini is extended to 2020, and the green zone is the most probable zone for Planet Nine. Fienga et al.

The team said that if the Cassini mission was extended to 2020 – currently it is due to end in 2017 with a dive into Saturn’s atmosphere – then they could further narrow the search field. That is unlikely to happen, but the team notes that the upcoming Juno mission to Jupiter – due to arrive on July 4 this year – could partially help, although the effects of Planet Nine on Jupiter are thought to be less than at Saturn.

Nonetheless, these results will hopefully help astronomers refine where Planet Nine could be. Its hypothesized existence made headlines around the world, so just imagine the excitement if (or perhaps when) a direct detection is made.