This video is sponsored by Magellan TV.
Welcome back to Launch Pad, I'm Christian Ready, your friendly neighborhood astronomer.
And, astronomers using the Hubble Space Telescope found an exoplanet that seems to behave like
the long-sought "Planet Nine" that we've been searching for here in our solar system. So
this would be the first discovery of an "exoplanet nine."
Now, to quickly recap, Planet 9 is a planet that's thought to exist in the distant outer
solar system. It was proposed as a way of explaining the orbits of distant solar system
objects like Sedna, the Goblin, and many others. All of these objects are on "detached" orbits
from the rest of the solar system. And that means they don't feel any gravitational disturbance
from the rest of the planets. They're all aligned in roughly the same direction, and
seem to be in the same inclined plane with respect to the rest of the planets.
Astronomers realized that a planet with a mass between 5 to 10 times that of Earth orbiting
in the opposite direction would have shepherded these objects into their present-day orbits.
This new object was dubbed "Planet 9" and astronomers have been searching for it ever
since.
Obviously, they haven't found it yet. But then again, that's not entirely surprising
because Planet 9 would have to have an average distance of 300 to 500 astronomical units
from the Sun. And Pluto orbits the Sun from about 40 AU away, so we're talking about 10
times the distance from the Sun to Pluto!
So that would make for a fairly faint target in a very big sky. So, astronomers use models
to predict Planet 9's expected orbit and therefore determine where in the sky to search.
While the idea of a "mini Neptune" lurking in the outer reaches of the solar system has
captured everyone's imagination, it's also been met with a healthy amount of skepticism.
That's partly because even though we've found more than 4000 exoplanets to date, we've yet
to see anything that behaves the way Planet 9 is thought to.
So this new finding of an "exoplanet 9" is interesting because it shows that such planets
are possible, and, in a way, it may indirectly help astronomers to find Planet Nine here
in the outer solar system. So, we'll talk about this new exoplanet and its implications
in a moment, but first, I'd like to thank Magellan TV who are very kindly sponsoring
today's video.
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share it with not one, but two of your favorite curious people.
And while you're there, check out Planet Hunters: the Search for Earth's Twin. It's a great
documentary about the search for a planet just like ours: with the right size, the right
orbit around its sun, with the right temperatures that are not too hot and not too cold to support
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the link in the description of this video.
HD 106906 is a binary star system about 103 parsecs or 336 light-years from the Sun. It's
part of a larger association of stars between the constellations Centaurus and Crux.
The system consists of two stars of spectral type F, which makes each of them a little
bit more massive and hotter than the Sun. They're very close together, and orbit each
other once every 49 days. It's also very young at just 15 million years.
In fact, it's so young it still has a large protoplanetary disk surrounding it.
It's about 800 AU across so it's much larger than our Kuiper belt. The disk appears to
have a small clearing near the center that extends out to about 50 AU from the star.
Now for reference, Pluto orbits the Sun at about 40 AU, so it's likely there's a nascent
planetary system clearing out the inner disk.
But the only way to reveal the outer disk is by blocking out the light from the star
as shown here in an earlier Hubble Space Telescope image. This technique also reveals a planet
about 737 AU from the central binary star.
The planet goes by the license plate of HD 106906 b. The planet doesn't appear to be
in the same plane as the disk, although it isn't clear if this was because the planet
is on an inclined orbit around the disk. or if it was perhaps an optical illusion caused
by the planet being in the foreground or something.
But the disk is asymmetric. The eastern side is relatively thick and extends out to about
370 AU, while the western side is thinner, but longer, extending out to about 550 AU.
If the planet is on a highly inclined orbit, it's likely responsible for the disk's appearance,
because it would have disrupted the orbits of objects in the disk from circular and dragged
them out into more elongated orbits.
But to confirm this hypothesis, the planet's orbit would need to be mapped out. However,
the planet is more than 700 AU from its host star, it would have something like a 10,000-year
orbit, and that's way too long to wait because we're gonna miss out on The Expanse.
Fortunately, we don't have to wait quite that long. A team of astronomers used archival
data taken by Hubble to make precise measurements of the planet's motion over a 14-year period.
From those measurements, they were able to come up with range of possible orbits that
fit the observations.
They found that the allowed orbits would have to be fairly elongated - or eccentric - and
those orbits would have to be inclined 35 to 68 degrees above the plane of the disk.
The model predicts the planet's periastron - or its closest approach to the star - to
be no closer than 190 AU. The small clearing at the center of the disk extends out to about
50 AU, so even at 190 AU close approach, the planet's orbit would be detached from any
planets circling the star.
Overall, that's pretty much like the kind of orbit predicted for Planet 9! But Planet
9 is thought to be between 5 and 10 times the mass of Earth. HD 106906 b on the other
hand, is thought to be somewhere around 11 times the mass of Jupiter! That's nearly 3,500
Earth masses! So how does something that heavy end up so far away from its parent star?
Well, we think Planet 9 got to it's present-day orbit because it formed in the same part of
the solar system as the other giant planets, and was later kicked out by Jupiter.
HD 106906 b might have formed even closer to its star, at about 3 AU or so. At the time,
the inner disk would have been churned up by the shifting gravitational pull of the
two stars as they orbited each other. And this would have created a kind of chaos zone
of turbulent gas and dust.
As the planet orbited in the disk, drag caused it to spiral inward to the chaos zone. With
a combined mass of nearly 3 times the Sun, the binary stars could have easily kicked
the planet into an unstable, eccentric orbit out of the plane of the disk. With each close
approach, the planet picked up more and more orbital energy, and might have gotten thrown
out of the system entirely, had it not been for a passing star.
If the star had gotten close enough, it would have raised the planet's periastron, and detached
it from the inner disk. That would stabilized its orbit and keep the planet far enough away
to (not) feel any significant pertubations from the binary star.
In order for this to work, the passing star would have had to have gotten relatively close,
on the order of a tenth of a parsec, but probably even closer, perhaps as close as 0.05 parsec.
Now that's about 10,300 astronomical units, and that would just be close enough to fully
stabilize the planet's orbit.
As luck would have it, the team identified two possible interlopers. HIP 59715 and HIP
59721 are about 11 parsecs from HD 106906. Their relative velocities suggest an encounter
with the binary system would have occurred about 2-3 million years ago. The stars certainly
came to within a parsec of each other, though it's not clear if they came to within the
0.05 parsec needed to stabilize the planet's orbit.
Alternatively, it's possible the planet didn't form with HD 106906 at all, but was captured
by the binary after getting ejected from another system. The way to tell will be to take the
planet's spectrum and measure its radial velocity. That would tell us whether or not it's moving
away from us rotating in the same direction as the disk. Or, if it was moving toward us
and moving opposite the disk's rotation. That would put it in a retrograde orbit, and that
would certainly imply it was captured from another system. On the other hand, if it's
rotating with the disk, then it certainly formed within the system and was kicked into
its present-day orbit.
Now, those radial velocity measurements haven't been done yet, but so far, all of the evidence
thus far favors the prograde, "formed with the disk the got kicked into its present orbit"
hypothesis.
So, will this help us find Planet 9 in our solar system? Of course...not. At least, not
directly. But this does show that a planet can get onto a Planet 9-like orbit very early
in its system's evolution. It's also possible that a more precise measurements of HD 106906
b's orbit will help to validate and refine the models being used to estimate Planet 9's
location in the sky. It would be pretty funny if an Exoplanet Nine played a role, however
indirectly, in helping us to find our Planet Nine here in our solar system.
Now, I made some videos about the Planet Nine hypothesis and how astronomers are searching
for it so I'll see you over there when we're done here.
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