Where is Planet X? Where is Nemesis?

Before Pluto was discovered, the world’s astronomers were captivated by the possibility of finding another massive planet beyond the orbit of Neptune. In 1930, Pluto was discovered lurking in what was considered to be the edge of the Solar System. However, it quickly became apparent that Pluto was tiny; it wasn’t the Planet X we were looking for. For the last 80 years, astronomers have been looking for a large planet that might go to some way of explaining interplanetary features such as the “Kuiper Cliff”, but Planet X has not been found. Unfortunately, the word “Planet X” has now become synonymous with conspiracy theories and doomsday, almost as notorious as the word “Nemesis”.

Nemesis is another unanswered question hanging over Solar System evolution: does the Sun have a binary twin? Is there a second, dim, hidden “sun” stalking it’s brighter counterpart from over a light year away? Some scientists have come forward to suggest that the existence of a hypothetical second sun — embodied as a brown dwarf or red dwarf — could explain some cyclical effects here on Earth (i.e. mass extinctions occurring with a strange regularity). Naturally, the discussion about Nemesis (like the discussion about the possibility of a massive Planet X) is purely academic, and only based on indirect observations and anecdotal evidence. Just because they might exist, doesn’t mean they do.

In a publication recently published to the arXiv database, one Italian researcher has dusted off this topic and asked a very basic question: Can we constrain the possible locations of Nemesis and/or Planet X if they did exist? His results are fascinating…

It’s nice to find a scientific publication about the possible existence of an unaccounted-for planet in the Solar System. The majority of articles I’ve written in the past 12 months have been examining the pseudo-science, fear, lies and nonsense surrounding the year 2012, of which “Planet X” seems to have a huge role to play. For some strange reason, certain unscrupulous authors have pinned every conceivable global doomsday event on a mythical planet that will be arriving at the inner Solar System on December 21st, 2012. Of course, this is total bunkum and the fear surrounding the name “Planet X” is completely unfounded. In fact, Planet X was originally the search for a massive planet beyond the orbit of Neptune, in the pre-Pluto era (some might say that we are now living in a “post-Pluto era” after the dwarf planet’s demotion… just a thought). Planet X is in fact the exciting astronomical journey the world took in the early 20th Century, culminating in the discovery of Pluto.

Searching for Planet X

Artist impression of the cold surface of Pluto (NASA)

Since Percival Lowell’s suggestion that there might be another planet out there perturbing the orbit of Neptune, the hunt for another planet was intense. The discovery of Pluto by Clyde Tombaugh in 1930 appeared to validate Lowell’s theory. However, by the 1970′s, it was found that Pluto was too small to account for any perturbations in any planet’s orbit, let alone the gas giant Neptune. However, as time went on and techniques became more advanced, the possible perturbations in Neptune’s orbit were put down to observational error. There was no longer any need for a Planet X, a hypothetical planetary body was no longer required to account for orbital perturbations. However, observations of the Kuiper Belt have reinvigorated the hunt for a Planet X (the “X” literally means “unknown”).

The Kuiper Belt is a region of space (in Pluto’s neighbourhood) where lots of icy, rocky bodies have been observed. As we have become rather good at observing small objects on our own doorstep (we’ve actually become rather good at observing objects in other star systems too), we have been able to plot the distribution of Kuiper Belt Objects (KBOs). It is in this distribution that a feature has been observed. At approximately 50 AU there is a sudden drop in KBO population. This has become known as the Kuiper Cliff and it possibly reveals that there is some significantly-sized planetary body (bigger than Pluto, but smaller than Earth) orbiting at a distance of 100 AU from the Sun. We have yet to discover anything that big shepherding the Kuiper Belt, but the Cliff is real, beyond 55 AU.

Other researchers have indicated that there may be a small planet orbiting at 60 AU (possibly explaining the behaviour of trans-Neptunian Objects, TNOs), or a massive planet (50% larger than Jupiter) patrolling a region of space over 1000 AU distant. However, there is still no strong evidence to support these theories, and there are certainly no observations of these possibilities.

The Sun’s Evil Twin?

Artist depiction of a dark star, or a brown dwarf (NASA)

So, we have some possible indirect observations of a Planet X out there, but what about the hypothetical Nemesis, the much feared “evil sun” that stalks our Solar System from afar?

In fact, it seems surprising that not more attention has been paid to Nemesis by conspiracy theorists and doomsayers. Planet X (a.k.a. Nibiru from the misunderstood Sumerian text), in comparison, seems like a petty concern when we are talking about a “second sun” that could be responsible for extinguishing life on Earth with alarming frequency. Although there is no direct evidence for the existence of Nemesis, some scientists have investigated this possibility. For a start, most stars observed in the galaxy are not single stars, they have a binary partner (often more). The Sun, as far as we know, is alone, there has never been any observation that our star has a binary partner. However, there are some indications that might point to the possibility of a faint, lightweight stellar companion that has remained secret till now. Key to this argument is the statistical regularity of mass extinctions on Earth, and its relationship with Oort Cloud objects.

Every 25 million years or so (over the last 250 million years), there appears to be some kind of extinction event on Earth. Could it be that a stellar partner, called Nemesis, passes closer to the Sun during its orbit, disturbing objects in the Oort cloud? If this is the case, there may be a mechanism for the regularity of comet impacts on Earth, thus causing the statistical regularity of extinctions. Once again, this is a hypothetical argument, but it is based on good science and historical evidence. If these extinction events are related to comet impacts after the comets have been kicked out of the Oort cloud by a binary brown dwarf or red dwarf, this suggests a binary orbital period of approximately 25 million years.

Where Are They?

Could Nemesis be a red dwarf? Probably not, according to precession data of the inner Solar System planets (NASA)

For argument’s sake, let’s say Planet X and Nemesis could be out there. If so, how far away from the Sun could they orbit? Lorenzo Iorio from the National Institute of Nuclear Physics in Pisa, Italy, has investigated this question, using data derived from the dynamics of inner Solar System planets. In particular, Iorio has computed the Newtonian/Einsteinian perihelion precession of planets within 1.5 AU of the Sun that could be caused by a massive, unknown, distant body. From his computations, it is assumed that no matter where the inner planets are located in their orbits, the gravitational force felt by the planets will be constant. Therefore, if there is a massive body out there (either Planet X or Nemesis), what is the minimum possible orbital distance allowed by the computed precession of the inner Solar System planets?

Iorio concludes that the minimum possible distances at which a Mars-mass, Earth-mass, Jupiter-mass and Sun-mass object can orbit around the Sun are 62 AU, 430 AU, 886 AU and 8995 AU respectively. To put these distances in perspective, the minimum possible distance a Mars-mass Planet X could orbit is over two times further away from the Sun than Pluto’s 39 AU (average) distance from the Sun.

If we consider the minimum possible orbit for a brown dwarf-mass object (often cited as a possible “failed star” candidate for Nemesis), with a mass of 75-80 Jupiters, its minimum orbital distance would be approximately 0.06 light years away (or 3,736-3,817 AU). A red dwarf (0.075-0.5 solar masses) would have a minimum orbital distance of 0.06-0.11 light years away (3,793-7,139 AU).

In Conclusion

Iorio has basically set the constraints on the closest possible orbital radii for unknown planets and small stellar objects as yet to be discovered in our Solar System. If they were any closer, their gravitational presence would be felt, and we’d easily be able to detect perturbations in the dynamics of the inner planets.

If Nemesis (the Sun’s binary partner) is out there, it isn’t any closer than ~3,800 AU (if it’s a large brown dwarf, or a small red dwarf). Therefore, it seems unlikely that Nemesis will have a very stable orbit as it would be affected by the gravity of other stars in different systems. From this evidence alone, Nemesis will remain a myth. In light of the updated paper, the minimum distance for a Nemesis candidate has reduced, and could therefore have a stable binary orbit with the Sun. However, something this large will have been observed by now.

According to a paper by David Jewitt, at the Institute for Astronomy, University of Hawaii, a Jupiter-sized planet could be detected up to a distance of 2140 AU (the minimum distance that a Jupiter-mass planet could exist is ~886 AU according to Iorio). To put this into perspective, a Pluto-sized planetary body can be detected up to a distance of 320 AU according to Jewitt, so it would appear there is nothing of significant mass out there up to 320 AU away (if you can call Pluto’s mass “significant” that is!).

(A special thank you to Ricardo De Castro for providing this additional information.)

Therefore, there is little chance that Planet X does exist, Iorio’s data suggests that the minimum distance a Mars-mass object can orbit is 62 AU (twice the distance of Pluto’s orbit), but Jewitt’s data suggests that if something the size of Mars was orbiting the Sun at a distance of 62 AU, it would have been discovered by now. According to Jewitt, a Pluto-sized object is detectable up to a distance of 320 AU. Mars is far bigger than Pluto, meaning anything the size of Mars would have made its presence very obvious by now. A tiny Planet X within 320 AU is very hard to imagine, and anything bigger could be seen coming from a vast distance (a couple of thousand AU). If Planet X is improbable, the larger Nemesis seems even more so.

To cut a long story short, it looks like we have discovered all the large planets (of Mars mass and above) and anything else probably will have very little influence on inner Solar System dynamics for millennia for millions of years to come.

Sources:
“Constraints on Planet X and Nemesis from Solar System’s inner dynamics“, Lorenzo Iorio, 2009. arXiv:0904.1562v1 [gr-qc]
“Project Pan-STARRS and the Outer Solar System“, David Jewitt, 2004.