Artist's impression of
Planet Nine as an ice giant eclipsing the central Milky Way, with a
star-like Sun in the distance. (Tomruen, nagualdesign; background taken
from File:ESO , CC BY-SA)
Ever since a study suggested that a “Planet Nine” could be lurking in the outskirts of our solar system, astronomers have been busy trying to pin it down.
As nobody has actually observed the planet yet, this research has
been largely computational. The existence of the planet was only
suggested after scientists noticed that objects in its vicinity were
moving strangely.
Since it was proposed in January, astronomers have modelled Planet
Nine’s structure, orbit, estimated threat to Earth and possible origin.
But with all this data at hand, are we any closer to actually finding
it? Let’s take a look at some recent results and what they really mean.
It May Be an Exoplanet
The latest such study
has come up with two different possible scenarios for Planet Nine’s
origin. One is that it may have started as a forming outer planet core
from our own early solar system which was expelled to the edge of the
solar system by some process, perhaps a collision.
However, another possibility is that our sun may have stolen Planet
Nine from a nearby star in the Milky Way 4.5 billion years ago, which
would make it our nearest extrasolar planet. As star formation regions
are relatively dense with stars – the sun was born in a cluster with
perhaps 1,000 other stars – these can indeed interact.
While this is entirely possible, the research assumes that
Neptune-sized objects were relatively common in this region – something
we simply don’t know. The study also suggest that further observations
and modelling of the positions and orbits of minor objects in the solar
system beyond Neptune now may provide further clues as to the origin of
the proposed Planet Nine – whether this is core expulsion as originally
proposed or exoplanet capture.
At the moment, the lack of direct observations of Planet Nine and the
whole range of objects which may be affected by it mean that the
explanations are poorly constrained. In the meantime, this kind of work
provides interesting ideas – but ultimately we need proof. Excitingly,
if it does exist and turns out to be a captured exoplanet, it is likely
to be our best bet for visiting an exoplanet in the near future.
It Could Be Made Up of Iron and Ice
Planet Nine could be similar to Uranus. (NASA, ESA, and M. Showalter)
Other computer simulations
assume that Planet Nine was a distant ice giant similar to Neptune and
Uranus. They calculate the evolution of the size, temperature,
luminosity and colour of such a body, having moved from its possible
formation point nearer the sun to its distant position at about 700 AU.
This research suggests Planet Nine is like a “mini-Uranus”, with an
iron core, silicate mantle, water ice shell and hydrogen/helium outer
layers. Its temperature would be about -226°C (or 47 Kelvin) – and most
of this would be internal heat rather than absorbed sunlight, which
means it would be difficult to see with optical telescopes. In fact, the
planet would reflect very little sunlight, which means it would be
visible in infrared wavelengths (heat) instead of visible light.
The research is useful for scientists modelling and searching for the
planet, as it helps them to know what they should look for.
It May Be Harder to Detect Than We Thought
The proposers of Planet Nine, and other astronomers, are busy using
an array of telescopes to search for their target. These includes
further work with data from the WISE survey, as well as Catalina Sky
Survey and Pan-STARRS. So far, these searches have been unsuccessful.
Current and planned searches are underway and proposed using telescopes
including Subaru at Mauna Kea observatory, ALMA, the Dark Energy Survey
and the James Webb Space Telescope.
Mauna Kea’s Subaru. (Denys/wikimedia, CC BY-SA)
But there may be other ways to spot it. For example, there was a prediction that the orbit of the Cassini spacecraft
at Saturn may be affected by Planet Nine’s gravitational pull, based on
its possible location in Cetus at about 630 AU (1 AU = the distance
between the Earth and the sun). However, many scientists are sceptical
of this.
So why haven’t we seen it? The study that modelled its interior also
postulated how easy it would be to detect the object using surveys such
as WISE, and estimated that Planet Nine’s current size is just 3.7 times
Earth. This is considerably less than the 10 times our planet which was
initially suggested. They therefore argue that it would be very hard to
spot the planet with current instruments, but suggest future telescopes
may be able to.
What’s more, some suggest that the planet may currently be at
aphelion (its farthest point from the sun), which would also make it
even more difficult to see. However, one study has managed to make the
search area smaller by modelling the orbit and its inclination. The search is narrowing, but slowly.
It Probably Won’t Wipe Us Out
With its suggested orbit between 200 and 1,200-2,000 AU – much
further away from us than the sun– it seems we should be safe from
Planet Nine. But conspiracy theorists were quick to suggest that,
as this may be the first of several such objects, at least one may have
our name on it. However, there is no evidence that any known or
postulated object poses any threat to Earth. But observers are keeping a
careful eye on near-Earth objects for potential problems.
Planet Nine, if it exists, would certainly be a difficult object to
detect. We know that the effects on six nearby objects look consistent
with its existence, although even this is not universally accepted by
astronomers. However, it is certainly strong enough to prompt a detailed
search. Thanks to the computational modelling, the search is narrowing.
At the same time, technology is developing and observations in this
region are improving. If Planet Nine exists, it should be found in the
next few years.
Andrew Coates, Professor of Physics, Deputy Director (Solar System) at the Mullard Space Science Laboratory, UCL. This article was originally published on The Conversation. Read the original article.
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