Planet Nine: will we ever find it?
How many planets are there in the Solar System? Well, it seems not so easy to say. Our current
classifications say there are 8 planets. The four smaller inner planets, Mercury,
Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal.
The four outer planets are giant planets, being substantially more massive than the terrestrials.
The two largest outer planets, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen
and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed
mostly of substances with relatively high melting points compared with hydrogen and helium, called
volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie
within a nearly flat disc called the ecliptic. But now astronomers are asking themselves
if there could be a 9th planet. Why is that? Well, observational data
show an unusual clustering of orbits for a group of Trans Neptunian objects. (TNOs). These tend to
make their closest approaches to the Sun in one sector, and their orbits are similarly tilted!
Follow me on this video to get to know more about the famous Planet Nine!
Planet Nine, sometimes incorrectly referred to as Planet X,
is a hypothetical planet in the outer region of the Solar System.
Does it exist? Well, we don't know yet.
As we said, its gravitational effects could explain the unusual clustering of orbits
for a group of extreme trans-Neptunian objects (eTNOs), bodies beyond Neptune that orbit the Sun
at distances averaging more than 250 times that of the Earth.
These eTNOs tend to make their closest approaches to the Sun in one sector, and their orbits are
similarly tilted. These alignments suggest that an undiscovered planet may be shepherding the orbits
of the most distant known Solar System objects. But how Planet Nine would be, if it existed?
Based on earlier considerations, it would be a super-Earth sized planet, with a mass
of five to ten times that of the Earth, and a huge elongated orbit, 400 to 800 times as far
from the Sun as the Earth. If the Sun was in New York and the Earth was in Washington, one
would have to cover about ten times the distance from New York to Beijing to reach planet Nine!
Konstantin Batygin and Michael E. Brown suggested that Planet Nine could be the core of a giant
planet that was ejected from its original orbit by Jupiter during the genesis of the Solar System.
Others proposed that the planet was captured from another star, was once a rogue planet,
or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.
Anyway, if it existed, we would not have any doubt about its planet nature.
In fact, according to IAU (international astronomical union),
a planet is a celestial body which: 1)is in orbit around the Sun;
2)has sufficient mass to assume hydrostatic equilibrium (a nearly round shape);
3)has "cleared the neighbourhood" around its orbit.
(Among other things, this definition caused Pluto to no longer be classified as a planet,
a change from how it had been widely considered until that point. We want to spend some words here
reporting a fun fact: when Pluto was finally classified as a dwarf planet, some people
joined the “Pluto is a planet” protest. Here are some photos taken during those protests.)
If Planet Nine exists, its mass is sufficient to clear its orbit of large bodies in 4.6
billion years, the age of the Solar System, and its gravity dominates the outer edge of
the Solar System, which is sufficient to make it a planet by the above definition.
As of February 2021, no observation of Planet Nine had been announced.
But if planet nine exists, why can't we find it?
The answer is that while we can see the effects of this hypothetical planet,
finding it is a different matter. In fact, in order to find planets,
astronomers make use of various methods.
The most famous, and perhaps the one you’ve heard of, is the transit method.
From our position, if a planet crosses across its star, we can see the dip in light that causes.
Measuring three of these dips, we can work out the mass and orbit of the planet.
Not all planets transit their star with respect to us, however, so we can’t use this for every star.
Another method is called the radial velocity. This involves noting the tiny, tiny gravitational tug
a planet exerts on its star. For smaller planets in wide orbits, this is incredibly difficult,
but for larger planets in tighter orbits, such as hot Jupiters, this can be quite useful.
Then there’s gravitational microlensing, where we can see a more distant star being
magnified by the gravity of a closer star. If there’s a planet in orbit,
we can sometimes see a small deviation in the light from the distant star.
This method is pretty much the only way we’re able to find rogue planets, those wandering
without a star, by noting their gravitational effects as they pass in front of other stars.
But in our own Solar System, things are a bit more tricky.
We can’t really use these methods to see Planet Nine or Planet Ten, except maybe microlensing,
because they don’t pass near the Sun relative to us. We can, however, guess where they should be.
We can’t use the transit method, because planet nine doesn't cross the Sun,
and other stars are too distant. We can’t use the radial velocity method,
because it doesn't have much effect on the Sun. So our best bets are either microlensing, hoping
it passes in front of a distant star and warp its light, or more promisingly direct imaging. The
latter is, as its name suggests, seeing light reflected by the planets in actual images.
Doing so takes time, however. We’ve essentially got to look across the whole sky
(although the area is a bit smaller, based on where we think the planets are). As we speak,
people are trawling through images of the sky to find glints of light from these planets.
If Planet Nine is out there, there’s a good chance we’ll find it given enough
time. And some estimates suggest there could be many more planetary bodies
hiding in the outer Solar System. Finding them will be just as tricky, though.
However, another answer could be that we can't find it simply because...it doesn't exist!
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What if there was no planet Nine?
We’re taught to believe that science is pure and objective. But science is a human
endeavour, and the scientists conducting it necessarily bring with them their own biases.
Some astronomers question the idea that the hypothetical planet exists
and instead assert that the clustering of the eTNOs orbits is due to observing biases,
resulting from the difficulty of discovering and tracking these objects during much of the year.
According to the authors of a new study being published in the Planetary Science Journal,
“Planet Nine” could only be the apparent result of a statistical accident . Brown and Batygin's
original study looked at the orbits of only six extreme transneptunian objects, which then
increased to fourteen in the following years. Since these objects, in addition to being few,
are small and weak, very dark and difficult to find except on rare, fortuitous occasions,
the authors of the new study warn of possible problems introduced by observational techniques.
According to the authors, the greater sensitivity of an observatory in the direction of pointing
could lead to selecting objects in that only direction , ignoring the presence of other objects
falling outside it and thus leaving one mistakenly thinking of an accumulation of trans-Neptunians
in that portion of the sky. It cannot be excluded that their distribution is
actually uniform on the celestial vault and the others have not simply been observed.
The researchers, led by Kevin Napier of the University of Michigan,
then tried to test their hypothesis by analyzing the orbits of 14 different trans-Neptunian objects
than those in the original study by Brown and Batygin and discovered through
three different observatories scattered around. different points of the globe.
Their result is that using different observers, there is no accumulation of trans-Neptunians
of any kind - a conclusion that does not rule out the possibility that Planet Nine exists,
but that makes it a little less likely. "The densification is a consequence of where and
when we look," Napier tells Science. "You don't need a different model to understand the data."
However, Batygin does not agree with the conclusions:
according to the Russian-American researcher, trans-Neptunian objects have already been searched
in a large area of ​​the sky around where the bodies of the original study are located.
"The correct question to ask," says Batygin, " is whether their analysis is able to
distinguish between an accumulation and a uniform distribution of extreme transneptunian bodies,
and the answer appears to be no." To unravel this problem,
further observations will be necessary that lead to the knowledge of an ever-increasing
number of trans-Neptunian objects, so as to determine once and for
all whether the accumulation that led to hypothesize Planet Nine is there or not.
As you can see, we have some huge unsolved problems to answer.
Science is an ongoing process, an infinite staircase.
And every time we make a step, there is a wall to climb in order to reach the next one.
We know how to detect exoplanets, yet we still debate whether Planet Nine exists or not.
But this is ok. We just have to keep on trying. Never give up!
"This video ends here! Thanks for watching everyone! Did you
know of the hypothetical existence of planet nine?
What's your favourite planet in the Solar system? Is there anything more you want to hear? Let us
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