There are eight known planets in our solar system. Beyond the orbit of Neptune, there are small icy worlds like Pluto and Eris, but none that rival the planets in size. But could there be a planet lurking in the depths of our solar system? 

It’s possible that there is a planet in the outer solar system, but it is so far away and so cold that it would be difficult to see. As we continue to make high resolution sky surveys, and as new telescopes such as the James Webb Space Telescope are launched, we will be able to find such planets in time. If any exist. But there is another way to look for these planets, and that’s by the gravitational tug they exert on other bodies.

Neptune was discovered in this way. After the discovery of Uranus, it was noticed that its motion didn’t quite agree with the predictions of Newtonian gravity. But the deflection of its orbit could be solved if it was caused by the pull of an undiscovered planet. In the mid 1800s John Couch Adams and Urbain Le Verrier calculated the position of such a planet independently. Soon afterward Johanne Galle discovered Neptune using Le Verrier’s predictions. This method has also failed, such as when strange motions of Mercury predicted a planet that doesn’t exist. It was later found that Mercury’s oddness is due to the effects of general relativity.

Seeing the gravitational tug from a much more distant world is much more difficult. The distances are greater and the gravitational influences much weaker. However there are hundreds of known objects beyond Neptune (or Trans-Neptunian Objects) and together they give us a statistical tool to look for other planets. In recent years there has been some evidence to support the existence of a super-Earth planet in the outer solar system. This was supported by the fact that the orbits of the most distant Trans-Neptunian Objects (TNOs) seemed to be clustered together. But the most recent survey of outer worlds clearly demonstrates that this clustering was likely due to detection bias, and the distribution of TNOs doesn’t support the existence of a super-Earth planet 9. Ethan Siegel has recently about this, and you should really check it out.

No if there’s no planet 9, is that it for solar system bodies? Maybe not. While the latest survey of TNOs rules out a super-Earth, it leaves open the possibility of a smaller planet, sometimes called Planet 10. The planets of our solar system have orbits that are all roughly along the same plane, known as the invariable plane. Since the solar system as a whole is gravitationally isolated, we would expect the orbits of TNOs to have a similar orientation. Orbits would vary a bit, but the average orientation should be near the invariable plane. But this latest survey found that the orbital orientations of distant TNOs are tilted about 8 degrees from the invariable plane. For a single orbit that isn’t unusual, but it is odd for lots of orbits to have this orientation. This kind of shift can occur due to a gravitational tug from a larger body, through what is known as the Kozai mechanism. Based on the data, this planet would be at least the size of Mars.

As it stands the evidence for planet 10 is relatively weak. There’s a small chance that the odd orbits of TNOs is merely random chance. But this work demonstrates that we are closing in on a final answer to the number of planets in our solar system. If they are out there, we will find them in the next several years, and if not we’ll confirm they don’t exist.

Paper: Kathryn Volk and Renu Malhotra. The curiously warped mean plane of the Kuiper belt.  arXiv:1704.02444 [astro-ph.EP] (2017)

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In the 1800s observations of the planet Uranus had some interesting irregularities. When compared against the predicted motion of Uranus due to the Sun and other planets, the actual motion didn’t quite match. This led some astronomers to suspect that there was another planet beyond Uranus. In 1846 Neptune was discovered close to its predicted position. Now a new paper in the Astronomical Journal, points to similar evidence that a large planet may lurk in our outer solar system. 

The evidence comes from the behavior of the outermost known solar system bodies (the six with an average distance greater than 250 AU). These bodies are so distant from the Sun that we can’t measure their gravitational deviation directly. We just haven’t observed their orbits well enough. So instead the authors of this new work looked at the statistical characteristics of the bodies.

The orbital shapes of distant bodies (red) compared with closer bodies. Credit: Batygin and Brown.

If one assumes the orbits of outer solar system bodies are Keplerian (which is a reasonable assumption) then you can plot them in terms of their orientation. Most of the outer solar system bodies are distributed fairly randomly, but the most distant ones are clustered. Their orbits seemed to be clumped together in a way that you wouldn’t otherwise expect. The authors calculate the likelihood of this happening purely by chance is about 0.007%. In scientific terms that’s not quite unusual enough to be conclusive, but it does strongly hint at either a bias in the way these outer bodies are discovered or a mechanism that has caused them to cluster.

The authors contend that a good explanation for the clustering is a gravitational perturbation by a Neptune-mass planet further away and on the other side of the solar system. While it’s certainly a good explanation for the clustering of these outer bodies, that doesn’t guarantee there’s a large world out there. There are other possible explanations that could account for the effect. But it is worth looking into, and that’s exactly what’s planned. If there is a planet out there, then infrared sky surveys such as NEOWISE have a chance of finding it.

Paper: Konstantin Batygin and Michael E. Brown. Evidence for a Distant Giant Planet in the Solar System. The Astronomical Journal, Volume 151, Number 2 (2016)

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Astronomers using data from the Atacama Large Millimeter/submillimeter Array (ALMA) have found a distant object in the direction of Alpha Centauri. The object appears to be in the outer region of our solar system, and depending on its distance could be a hypothesized “super-Earth.”

ALMA is capable of precise observations at short microwave wavelengths, typically emitted by cold gas and dust. But objects on the edge of our solar system also emit light in this range, and would be too cool and distant to be observed by infrared telescopes. In 2014, ALMA found a faint object in the direction of Alpha Centauri A & B. The object was again observed in May of this year, this time more clearly. Given that the object is within a few arcseconds of the Alpha Centauri system, it would seem reasonable to presume that it could be part of that system, possibly gravitationally bound as Alpha Centauri D. The Centauri system is about 4 light years away, and at that distance (given the object’s brightness at submillimeter wavelengths) it would have to be a red dwarf star. But such a star would also be clearly visible in the infrared, so if this object is Alpha Centauri D we should have seen it long ago.

The new object (labelled U) as seen by ALMA. Credit: R. Liseau, et al.

Since it doesn’t seem to be part of the Alpha Centauri system, it must be closer and correspondingly smaller. With just two observations it isn’t possible to determine the object’s orbit, so we can only guess at its distance and size. One possibility (and the one I think most likely) is that it’s an extreme trans-Neptunian object about 100 astronomical units away from the Sun, which is further than Sedna at 86 AU. This would make it the most distant known object in the solar system, but likely smaller than Pluto.

Another possibility (which seems more likely to the object’s discoverers) is that it is about 300 AU away and about 1.5 times the size of Earth, making it the first “super-Earth” found in our solar system. Observations of trans-Neptunian objects have led to some speculation that one or two super-Earth’s could lurk in the outer solar system, so it’s not out of the question. There’s reason to be cautious of this idea, however, because of its location. Alpha Centauri is about 42 degrees away from the ecliptic. Most large solar system lay within a few degrees of the ecliptic, and even Sedna’s orbit is only inclined about 12 degrees from it. The chances of a super-Earth with such a highly inclined orbit seems very unlikely.

A third possibility is that the object is a cool brown dwarf about 20,000 AU away. Such an object should also be visible in the infrared, so there would still be the question as to why it wasn’t discovered by earlier infrared sky surveys. It’s proximity to Alpha Centauri would seem to make such an object easy to find.

The only way to know for sure is to gather more observations. Either by tracking its motion or by gathering observations at other wavelengths we can eventually get a handle on its size and distance. Whether dwarf planet, super-Earth or small star, it seems clear that something is lurking on the outer edge of our solar system.

Paper: R. Liseau, et al. A new submm source within a few arcseconds of α Centauri: ALMA discovers the most distant object of the solar system. arXiv:1512.02652 [astro-ph.SR]

This article first appeared on Forbes.

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At the moment there are 8 known planets in the solar system (alas, poor Pluto) and (officially) 5 dwarf planets. With the exception of Ceres, the dwarf planets are all trans-Neptunian objects. We know of several other such objects of similar size, but these haven’t yet been designated as dwarf planets. But what about larger bodies on the edge of our solar system?

At the moment, we know of nothing larger than Eris beyond the orbit of Neptune, but a recent paper in MNRAS hints at the possibility of larger trans-Neptunian planets. The paper looks at the orbits of 13 trans-Neptunian objects and notices an interesting pattern. According to current models, these small distant bodies should fairly close to the invariable plane of the solar system (where most planetary orbits are) and they should have an average orbital distance (semi-major axis) of about 150 AU (astronomical units). What the team found was that the bodies have a semi-major axis of 150 – 520 AU, and their orbits can tilt 20 degrees or more from the invariable plane.

So the team looked at gravitational simulations of these orbits, to see how they might be affected by larger gravitational bodies. In particular, they looked at in interaction known as the Kozai mechanism, where the gravity of a larger body can pull the orbit of a smaller one toward the orbital plane of the larger one at the cost of making orbits less circular. They found that the orbits of these trans-Neptunian objects could be explained by the presence of one or two larger bodies in the region. Their results suggest super-Earth massed bodies in the region of 250 AU.

This wouldn’t be the first time a new planet has been discovered by its gravitational influence, though it should be stressed that the evidence here is not particularly strong. Thirteen bodies is a small statistical sample, so don’t bet on a large planet beyond Neptune just yet. But the hint is definitely there, and we are bound to find more objects waiting in the dark as our detection methods continue to improve.

Paper: C. de la Fuente Marcos, R. de la Fuente Marcos. Extreme trans-Neptunian objects and the Kozai mechanism: signalling the presence of trans-Plutonian planets? Monthly Notices of the Royal Astronomical Society Letters 443(1): L59-L63  (2014)

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