[astro] constraints on planet x and nemesis from solar system's inner dynamics

[Eugen Leitl]

http://www.astroengine.com/?p=4695

Where is Planet X? Where is Nemesis?

By Ian O'Neill

Artists impression of the hypothetical star, Nemesis (Wikipedia)

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 250
AU, 750 AU, 13,500 AU and 500,000 AU respectively. To put these distances in
perspective, the minimum possible distance a Mars-mass Planet X could orbit
is over 6 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 a little under 2
light-years away (or 125,000 AU). A red dwarf (0.075-0.5 solar masses) would
have a minimum orbital distance of 2.1-5.6 light years away (132,000-354,000
AU). To put these numbers into perspective, the Oort Cloud is thought to be
roughly 50,000 AU (~1 light year) away and the nearest star to our Sun,
Proxima Centauri, is 4.2 light years away.

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
a lot more obvious, 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
2 light years away (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.

On the other hand, Planet X could exist a little closer to the Sun, but I
think we can safely say that it’s not going to make an appearance any time
soon. A Planet X the size of Mars could be 250 AU from the Sun, but as its
mass gets larger, the distance increases. A Planet X the mass of Jupiter for
example would be orbiting some 135,000 AU (0.2 light years) away. That’s not
to say that smaller KBOs aren’t out there. There may well be other dwarf
planets (possibly of Pluto-mass) shaping the Kuiper Cliff for example.

To cut a long story short, it looks like we have discovered all the large
planets (of Mars mass and above) within 250 AU of the Sun, and anything else
probably will have very little influence on inner Solar System dynamics for
millennia for millions of years to come.

Source: Constraints on Planet X and Nemesis from Solar System’s inner
dynamics, Lorenzo Iorio, 2009. arXiv:0904.1562v1 [gr-qc]