14 mins read 17 Apr 2023

Undiscovered World - Why Planet Nine Might Not Be There

For a number of years now, scientists have bounced back and forth on the existence of an undiscovered massive planet in the far reaches of the Solar System. We take a look at why we think it might be there, what the evidence says so far, and how we might be able to find it, if at all, in the future.

Artist's concept of a hypothetical planet orbiting far from the Sun. Credit: Caltech/R. Hurt (IPAC).

In 2006, the familiar concept of our little corner of the Universe - the Solar System - drastically changed. The small, tiny planet Pluto (and its relatively large moon, Charon), which have humbly been known to be orbiting in the cold outer regions of our system since their discovery in the 1930s, were downgraded from planetary status to dwarf planet. 

Whilst the Pluto wars have been raging since, a new and more interesting science question has arisen, from the outer depths of Sol’s family. One that has captivated scientists, students, and the imaginations of millions over the years. What if there is an undiscovered planet out there? What if there is a Planet Nine (sometimes referred to as Planet X, pending on where you stand on Pluto’s planetary status).

Our knowledge of the ‘nearby’ cosmos is rather interesting. We know a lot (and are always learning) about the inner Solar System (between the Sun and asteroids, where the terrestrial planets like Earth are), and even the region where the giant planets are (from Jupiter to Neptune). Beyond this, there is the Kuiper Belt - a region of small, cold, icy bodies of which Pluto and Eris (another Kuiper Belt Object) are. So far, we have discovered a range of different bodies in this region. We sometimes call these Trans-Neptunian Objects (TNOs).

Then we jump out to the closest star system, (the Centauri system, which contains Proxima (the closest star to us)), as well as Alpha and Beta Centauri. They even have their own exoplanetary systems there too. Extensive study of the Centauri system means we have a fair amount of knowledge about it as well and given these systems contain bright stars that are relatively close by, we continue to probe their environments to learn more about our neighbours. 

However, there is much we need to learn about the in-between region - the space beyond Neptune and the TNOs, and before the Centauri star system. When speaking in astronomy terms, this space is small - only just over four light years in radius. But in our everyday terms, this is colossal - roughly 40,000,000,000,000 kilometres of space. Even travelling at the speed of the fastest human object ever created, the Parker Solar Probe - which used the Sun’s gravity to gain a velocity of around 535,000 km/h - it would still take over 8,500 years to cross it. 

We know that comets come from here, from an enormous sphere of icy bodies orbiting the Sun at this great distance (this sphere is known as the Oort Cloud). We also know about a handful of very distant bodies in the outer TNO region, known as the scattered disc. These objects are so far away, we consider them ETNOs, or Extreme Trans-Neptunian Objects

And here is where things get interesting for Planet Nine.

Clustering of the orbits of several ETNOs, and the expected orbit of Planet Nine that is causing this clustering. Credit: CalTech/R. Hurt(IPAC).

A few years back, astronomers noticed some weird coincidence in the orbits of several of these ETNOs, in particular, that about a dozen of the highly scattered objects were orbiting (and more specifically, clustering) in a configuration that could be (potentially) explained by adding a new, more massive body, out at these great distances (estimated to be about 300 times the Earth-Sun distance). According to the ETNO orbits, this new body could be used as an explanation for this clustering, if we could insert a predicted mass of about 5 - 10 times that of Earth. This isn’t the first time a planet has been postulated as a result of its indirect effects (gravitational influence) on other bodies in the Solar System. For example, Neptune was first considered due to the perturbations exerted on Uranus. This led to the Ice Giant’s discovery in 1846. 

And so, the hypothesis of an undiscovered Planet Nine was born. An idea, that this fairly large planet - with a radius approx. 2 - 4 that of Earth’s was slowly orbiting (with a period of approx. 10,000 - 20,000 years) in that cold, dark, and unknown ETNO region beyond the Kuiper Belt. Astronomers started crunching some numbers and soon worked out that (with a few assumptions) that it should have a magnitude of about +22 (this is very faint, which is why we haven’t seen it), a temperature that is very low (around -230℃, which would mean it would radiate in the far infrared spectrum), and at such great distances from the Sun, that it would reflect very little light. Given its great distance, it would also be moving fairly slowly relative to the background stars. All of these factors contribute to the ‘yet-to-be-discovered’ aspect of Planet Nine. 

An interesting thought to consider emerges from the scenario that if Planet Nine does exist, then how did even get there in the first place? Several competing ideas have been put forward to try and explain this important aspect of this argument. One theory states that as Jupiter moved around in the early Solar System days, it caused Neptune to also push outwards into the far reaches of our system. This triggered the scattering of the ETNOs, and Planet Nine might have been ejected to these far-flung regions during these dynamic processes. Given its postulated mass, it most certainly would have had to have formed within the Solar System region where the gas and ice giants reside, as there would never have been enough material in the outer scattered Kuiper Belt regions, to accumulate the predicted mass model. Another theory, states that Planet Nine might have even been captured by our Sun, through an ancient interaction with another star. In other words, maybe we stole another system’s planet. In one paper, the tilt of Uranus is even blamed on an outward migrating Planet Nine. 

Could We Ever Find Planet Nine?

Artist illustration of Planet Nine with several moons. Credit: Science Photo Library/Alamy.

With all the number crunching, several published papers and even more theories (some, rather wild), would we ever be able to observe Planet Nine, and what has the evidence told us so far? Let’s approach this firstly from two angles. Direct observations of Planet Nine, or indirect observations of it. 

Given its great distance, its low (assumed) albedo, and its expected small angular diameter, Planet Nine might likely be impossible to detect in the visible light regime from the ground, even when incorporating adaptive optics techniques with current telescope technology. Even using Hubble’s cameras might prove difficult to resolve the angular diameter of such a small body at these great distances, especially since it is not producing any light and reflecting very small amounts of light from the Sun. 

That Planet Nine is expected to be very cold might allow us to look for it in the far infrared bands. This rules out most Earth-based telescopes (due to atmospheric opaqueness at these frequencies) but several space-based telescopes have data in these observed bands - such as the Herschel Space Observatory, Spitzer Space Telescope, Infrared Astronomical Satellite and Infrared Space Observatory. Could Planet Nine be lurking in the archival data of these telescopes? Astronomers could potentially search this data (even using Machine Learning technology) to try and find an object with a high degree of parallax shift relative to the background stars. However, an issue with this approach is that we don’t know where Planet Nine actually is, and so the search region to look for this tiny back-and-forth parallax jump is vast. 

We might consider using sub-mm band observations, as per the range that ALMA surveys use to look for the electromagnetic signature of Planet Nine. Though, determining which is the planet vs. the thousands of other minor bodies in this temperature band would be challenging for this kind of analysis. A recent pre-print paper has suggested that Planet Nine might even have moons, which experience tidal heating effects, raising their temperatures higher than some of the many background bodies, and so ALMA could potentially detect these - if they existed. 

Indirectly, Planet Nine could also reveal itself to us. Due to its mass, and its passage across space, it might suddenly amplify the brightness of a background star, through gravitational lensing effects. This of course would require a very high coincidence (read: an extremely lucky) opportunity for our telescopes to be looking in the right part of the sky at just the right time, just as Planet Nine and a background star align from our perspective view. 

Another interesting method to indirectly observe it (and my favourite, though I am completely biased here) is through pulsar timing methods, which measure any perturbations on the Solar System Barycentre (to a very precise value) created by any (and all) of the masses in our system. This process has determined the masses of the known gas and ice giants to several decimal places already, and so if Planet Nine exists, it should also cause an effect on the barycentre as measured through pulsar timing arrays. 

So, What Does the Evidence Say?

The expected interior structure of Planet Nine, based on simulations. Credit: E. Linder/C. Morsasini/Uni. Of Bern.

A number of studies have now searched for Planet Nine, and sadly, come up with nothing. Whilst it might be easy to assume that the planet does not exist, there is still much more for us to search and learn about before we completely rule it out. Though, the evidence so far does not point to its existence. 

For example, astronomers used the Wide Field Infrared Survey Explorer (WISE) telescope to search for Jupiter or Saturn-sized objects between 10,000 - 20,000 AU and did not find anything. The GAIA spacecraft has also now catalogued an enormous number of stars and objects in our Galaxy, and it too still has not found any evidence of Planet Nine. Other space-based telescopes such as TESS have also looked for the elusive planet and not been able to make any detections. The public archive of the Zwicky Transient Facility survey was also searched, with no promising results

Using seven years of data across two hemispheres, the Catalina Sky Survey as well as the Siding Springs Survey looked for new objects in the optical bands, but also found no evidence of Planet Nine. Similar results for the Dark Energy Survey (which boasts one of the most powerful cameras on Earth) - where they indeed found several new TNOs, but no evidence for Planet Nine, though it has placed limits on detectability. 

Another dedicated TNO search survey was the Outer Solar System Origins Survey (OOSSAO), which to date, has found hundreds of outer system bodies, including ETNOs, and even probed the scattering/clustering effects of ETNOs. This study found no evidence for Planet Nine but more likely evidence for observational biases that could explain this clustering effect. 

Astronomers have even used the Solar System ephemerides with known bodies to place constraints on the existence of Planet Nine. For example, using the astrometry of Pluto and TNOs, the existing ephemeris data appeared to be correct without the need to include Planet Nine. Some hope was raised when an attempt to show that Planet Nine was perturbing Saturn’s orbit, as measured in situ by Cassini but these ideas were quickly dismissed by NASA’s JPL scientists (who are in charge of creating the ephemerides models). 

Pulsar timing array searches for any effects that Planet Nine would have on the Solar System barycentre have also not resulted in any evidence of its existence. Even citizen scientists have been getting in on the search, within the platform Zooniverse, in which a project named ‘Backyard Worlds: Planet Nine’ has been underway for several years now (where the citizen scientists help catalogue and classify astrophysical objects), and to date, there has been no Planet Nine in these data sets either. 

Alternate Theories for ETNO Clustering

One paper has suggested that Planet Nine could be a primordial black hole, lurking in the outer regions of our system. Credit: Science Magazine.

A number of theories have emerged of what could be causing the clustering of the ETNOs, instead of Planet Nine. For example, one study suggests that the massive and moderately eccentric disc of TNOs (which, we might have not discovered fully as yet), could gravitationally shepherd the nature of clustering that has been observed. Or maybe, an even bigger, Jupiter-sized object could exist out at 5000 - 9000 AU, causing ETNOs like Sedna to have the orbit they do, but the findings from the WISE study ruled this out. 

Another concept is that a primordial black hole was captured by our system and could be an alternate explanation of Planet Nine, though the lack of observed Gamma-rays that would be produced through localised dark matter annihilating (expected around these objects), has ruled this out. As has the lack of evidence for any gravitational lensing observations so far that a black hole of this nature might cause. 

A more likely solution (channelling Occam’s Razor) is that it is likely selection and/or observational biases. 

Future Searches for Planet Nine

Artist rendition of the dim Sun, out from the orbit of Trans-Neptunian Objects. Credit: NASA/ESA/G. Bacon (STScI).

Hope for Planet Nine is not completely lost, however, with several prospective options still being considered. For example, a suite of very large aperture telescopes (about 10 - 40 metres in diameter) is currently under development at several global locations, which will be able to survey the sky in greater detail and speed than ever before. Using advancements in adaptive optics, their resolution will also be unprecedented. Whilst they might not be able to detect anything in the far infrared bands, there could be an opportunity to observe (in particular, with wide-angle cameras) low-light objects that exhibit tiny amounts of motion relative to the background stars, which could be further investigated. AI and machine learning capabilities would assist to sift through the high volumes of data, looking for that tiny parallax movement amongst the background stars. 

The JWST is an infrared telescope, but its observational frequency bands do not extend into the far infrared range, so Planet Nine lies outside of its capabilities. Future far-infrared space-based telescopes could tackle this problem, in particular, if they have wide, yet detailed, fields of view. 

Pulsar timing also presents an opportunity to continue to observe the Solar System Barycentre for Planet Nine’s signature. These data sets are now coming up to several decades, which has allowed the determination of Jupiter’s orbital noise on the barycentre to be quantified and soon will allow for a full Saturnian orbit. Once both these have been fully accounted for in pulsar timing sets, their noise parameters (which are expected to be the largest of all bodies) can be de-coupled from the barycentric movement, and more refined searches of Planet Nine can take place in the pulsar timing data sets. 

There has even been an idea to use powerful lasers to propel hundreds of tiny spacecraft that contain accurate ticking clocks out to all directions in the Solar System and measure their ticks over the course of decades to determine if any are affected by the gravitational influence of Planet Nine. That is, as these tiny clocks interact with the gravitational fields of unknown bodies, their ticking (as measured back on Earth) and direction would slightly change, revealing the presence of any unknown bodies out there in the darkness. 

In summary, all of our observational evidence so far points to Planet Nine being an outcome of our bias when considering the orbital clustering of ETNOs, but I (like many scientists) hope that this is wrong - because finding Planet Nine would change our understanding of our little corner of the Universe radically and re-shape our thinking on the evolution of our special little, somewhat weird, Solar System.