Our galaxy is filled with strange
planets. Now, this should not surprise
anyone. You just need to look at our own
solar system to get a sense of the
variety that exists. Rocky planets, gas
giants, boiling surfaces, icy worlds.
Some planets with rings, others with
moons, some with storms the size of
entire smaller planets. But those pale
in comparison to the marvels that exist
once you start to look further a field.
To date, NASA has confirmed the
existence of more than 6,000 exoplanets
in the Milky Way. And while many of them
seem to follow the mold of what we would
see in our solar system, others, to say
the least, seem truly bizarre.
Worlds with multiple suns in their
skies. Worlds with rain made of molten
iron.
Worlds with rings the size of Venus's
orbit, worlds larger or hotter than
stars.
But out of all this, what are the
strangest planets we've discovered? What
are the marvels our galaxy has in store
for us? And what further wonders may
still be out there?
I'm Alex Mccoan and you're watching
Astramm. Join me today in this super cut
as we take a look at the Milky Way's
strangest exoplanets.
When it comes to planets, many things
can make them strange. For instance,
sometimes stranges is not found in the
planet itself, but how it moves around
its star. While we are used to some
large or small orbits in our solar
system, how long or short can a year
actually get? Let's start off close to
home. Mercury has a pretty short year,
only 88 Earth days, which is the
shortest orbit that we know of in our
solar system. And very interestingly,
due to the way Mercury rotates, one
Mercurian day is twice as long as its
year. Yes, there's exactly 2 years in
its solar day.
So, should anyone ever live on Mercury,
you would have to switch around your way
of thinking when it comes to describing
a shorter reference of time. But it's
kind of cheating to say a year on
Mercury takes half a day when it still
takes 88 Earth days. So, let's keep to
Earth time scales as the point of
reference. Outside of the solar system,
most exoplanets that we know of have
really short years, and that's because
of the way we detect the majority of
exoplanets. There are observatories that
look closely at thousands of stars at
the same time, looking to see if an
orbiting exoplanet passes in front of
its parent star, causing the star to dim
ever so slightly from our perspective.
In order for us to confirm that the dip
in brightness is caused by an exoplanet,
we need to see a pattern of dips in
regular intervals. So if an exoplanet
does transit a star, but it takes two
Earth years before it transits again,
then we would need to be constantly
monitoring the star for many years
before we can confirm that the dips in
the stars brightness are caused by an
exoplanet. On the other hand, exoplanets
with really short years, like a few days
or weeks long, can be confirmed as
exoplanets very quickly. This means that
most exoplanets that we know of only
take a few days to orbit. The famous
Trappist system exoplanets all orbit
their parent star within 1 month. The
closest planet to the star only takes
1.5 days.
So, out of all these thousands of
exoplanets that we have discovered so
far, which one has the shortest year?
Incredibly, there's a pulsar out there
which has a planet, although it could
also potentially be the core of another
collapse star which orbits around the
pulsar in only 48 minutes and 58.5
seconds. How can this happen? Well, not
only is the orbiting object extremely
close to the pulsar, but it is moving
very fast. A neutron star is also very
small, at most only around 30 km across.
Yet, it is incredibly massive. So much
so that its gravity even bends light
around it. A fastmoving object can stay
in orbit because the neutron stars
gravity is strong enough to keep it
there. Although you may say this is
cheating again, a pulsar isn't really a
star, more like a star remnant. So what
about a main sequence star? In this
case, the shortest year that we know of
goes to K2137b,
a planet that year lasts only 4.31
hours.
Because it is orbiting a main sequence
star, the Earthsiz exoplanet is surely
orbiting within the stars atmosphere and
will eventually either be ripped apart
by tidal forces or fall into the star
completely as atmospheric drag slows it
down. On the other end of the scale, the
longest year that we know of belonging
to a planetary mass object is this
beautifully named planet. This Jupiter
type object's year lasts roughly 1
million years.
Still, for some planets, talking about
how long it takes for them to orbit
their star doesn't make sense at all.
After all, some orbit more than one.
In our solar system, we like to keep
things simple. Just the one star found
in the center with everything else
orbiting around it. as is the case for
most planetary systems found in the
universe. However, there are some
planetary systems out there where things
get a bit more complicated.
Specifically, multi-star systems where
there are two or more stars that orbit
each other
in such configurations, what happens to
any planets orbiting them? In fact, can
planets orbit them at all with the
gravitational tugs from different
directions?
To answer the latter question, the
simple answer is yes. Planets can orbit
in such situations. Although to answer
the former question, there is no one
answer rule about how a planetary system
in a multi-star system might look. What
we can do, however, is explore some of
the possibilities out there. But before
we look at planets, it would be good to
understand how multistar systems work.
For the most part, stable star systems
have organized themselves into
hierarchical systems. This is due to the
proximity in which they formed with each
other, which we will touch on a little
later. Binary star systems are generally
simple enough. Binary stars orbit around
a baris center or in other words, their
center of mass. If the masses of these
two stars are similar, then nearly
symmetrical elliptical orbits are often
seen. Although there can be occasions
where they orbit in circles in a similar
fashion to Pluto and Cheron.
In the case that one object is more
massive than the other, then the more
massive objects orbit doesn't take it as
far out compared to the less massive
object.
Beyond binary systems, you can have
three 4 5 6 7 or more stars in the same
system. And as you will see, there is a
structure within these systems to keep
them stable. In the case of three stars,
you'll have two stars orbiting each
other in a binary configuration with the
third orbiting around a bar center with
the other two. This keeps the system
stable because if three stars had their
orbits cross, one would certainly get
ejected from the system at some point.
In a three star system, two of the stars
are contained in their own enclosed
little system, acting as one star in the
grand scheme of the whole system itself.
We group this binary configuration into
a tier with that tier acting together in
its association with the single star. In
a way, once you have grouped the binary
configuration in the system, this upper
tier now acts like a two star system
again with the two stars and the one
star orbiting each other.
In the case of four stars, you'll either
have two binary configurations orbiting
around a barrier center or one binary
system orbiting a bar center with a
third star and all three of those stars
orbiting around a barrier center with a
fourth star. From here on is where the
hierarchical system really comes in
handy. With a chart like this, you can
easily see how the system works. In the
two binary configuration case, you have
the two binaries orbiting the system
center of mass. In the single binary and
two single stars configuration, you'll
add another lower tier in the
configuration. You have the binary here
which orbits this third star. And all
three of these stars are orbiting
together with the fourth star.
If there are more stars in the system,
say five, you can have an array of
configurations with various binaries or
single stars on a variety of tiers. Yet
with this chart, you can easily see
where the mass lies.
You can do the same for systems with
six, seven, or more stars, but anything
above seven is exceptionally rare and
probably won't remain stable. Although
maybe there are examples somewhere in
the universe where it exists.
So, let's add planets into the mix.
Could there be a planet out there with
seven suns in its sky? Well, yes. Let's
have a look at this hypothetical seven
star system again. By here, there are
three stars in a configuration. Two in a
binary configuration and a single star.
We see that the single star has planets
that specifically only orbit that star.
They have one sun. Beyond that, there
are planets that orbit both that star
and its binary companions.
These planets could be said to have
three suns as they are orbiting three
stars. Going to the top tier, there is a
planet here that orbits the entire
system. This planet could be said to
have seven suns. Let's see what that
looks like outside this graph.
Interestingly, in a sevenstar system,
from this distance, it only looks like
the orbit of a binary. That's because
each set of stars is found in one of
these two points.
Here is the planet with seven suns. Its
orbit takes it so far out that these
stars aren't very bright in the sky. So,
even though it has seven suns, they
don't really provide much in the way of
warmth or light.
Zooming in on the set containing three
stars, we see the planets which orbit
all three of these stars. These stars
would appear much larger in this planet
sky compared to the planet sky that was
orbiting all seven. In this example, the
two in the binary configuration appear
almost too close together to distinguish
with the single star clearly separate.
The other four stars that are part of
the other set in the system are dim in
the sky. They are very far away and
again don't provide much in the way of
light or warmth. Meaning there is a
clear day and night on these planets.
Interestingly though, this other set of
stars would get marginally brighter and
dimmer during their orbits and the
distance between their systems orbits
would get more noticeable as they get
closer.
Lastly, let's look at a planet orbiting
just the one star in this system. It
orbits closely to the star and is tidily
locked. Although the night side would be
disrupted by the binary configuration
this star orbits with as the planet's
orbit can take it between the single
star and the binary stars. The binaries
orbit together closely and so wouldn't
easily be distinguished in the sky
without a filter. Again, the other set
of stars in the system would be dim and
appear far away.
This may make you wonder how many
systems out there have more than one
star. Are we unusual? Or is it the trend
that there is only one star per system?
Well, if you look up into a clear night
sky, it may surprise you to know that
most of these stars are binaries. The
brightest star in the sky, Sirius, is a
binary. Alpha Centauri, the closest star
to us, is a binary or maybe even a
trinary if you include Proxima. Polaris,
the North Star, is also a binary. So,
for the longest time, astronomers
thought that we were the unusual ones.
However, it's coming to light with the
improvements in technology that this is
not the case and that most star systems
only contain one star. Perhaps the
reason why the brightest stars in our
sky are binaries is because they are
giving off twice the light. Single stars
are just generally dimmer and harder to
see in comparison.
It doesn't help either that 85% of all
stars in existence are red dwarfs. And
it seems that only about 25% of them
have companions. In fact, it's
interesting that binaries occur much
more frequently with the really massive
stars, the blue and white giants. This
could be because of how these stars
formed. Red dwarfs weren't pumped with
as much material as they were forming,
hence why they never attained the mass
of a blue giant. In an environment where
forming a blue giant is possible,
however, models suggest that several
stars could form at the same time. At
first, these systems would be chaotic.
The stars would be ejected until order
could be found. The ones that remained
and survived were the ones that ended up
absorbing the most interstellar medium,
thus becoming the most massive of stars.
The interstellar medium that ended up
caught in protolanetary discs around
these stars later form planets.
The first planet we ever discovered with
more than one sun in its sky was Kepler
16b spotted by NASA's Kepler Space
Telescope in 2011. At the time, it
became known as the Tatooine planet.
However, Kepler 16b is no desert world,
but is actually a gas giant around 0.3
times the mass of Jupiter. tracing its
orbit around its star has fascinated
scientists who are trying to unravel the
threebody problem. The complicated
mathematics that traces how three
orbiting bodies interact with each
other. Since then, a handful of other
Tatooine planets with their multiple
stars in their sky have been discovered,
indicating that they are not uncommon in
our universe. After all, more than half
of all stars in our galaxy have at least
one partner. But what would it be like
to live on a planet with two suns in the
sky? Or to help us visualize the point,
how much would having a second star
impact us here on Earth? How hot would
Earth get? What would the day and night
cycle be like? Would it be possible for
life as we know it to survive in a
multi-star system?
Well, it depends. Let's keep our sun as
it is and only make the second star the
variable in this thought experiment. As
we know, stars come in all sizes from
small, cool, and dim red dwarfs all the
way up to large, hot, and bright blue
super giants. In a binary star system,
the stars orbit around the systems
barrier center or the center of mass.
Depending on the mass of the stars and
the distances between them, you'll have
differences in how these orbits look.
For similarly sized stars, the orbit
could look circular in nature or in an
ellipse. On the other hand, the bigger
discrepancy there is between the stars
masses, the closer the baracenter will
be to the more massive star. Let's say
we plunk a red dwarf, which is the
smallest star type, in a close orbit
around our sun. Even though a red dwarf
can be as little as 7.5% the mass of our
sun, it's already going to have a big
impact on us. From our perspective, the
star would look like it orbits the sun,
meaning there would be times when it
transits in front of the sun and other
times where it is eclipsed by the sun.
Our year would be shorter if we stay one
astronomical unit from the stars bar
center. Because of the increase in mass
and gravity of the system from the extra
star, Earth's velocity would have to be
faster in order to not be pulled into
the stars. Either that or his orbit
would have to be slightly further out.
If we want to maintain our current
velocity, the increase in temperature
from the star would be noticeable too,
definitely making it unbearable for
humans. But exactly how hot would depend
on a variety of factors like a runaway
greenhouse effect, the heat of the star
and more. But let's say we place Earth's
orbit in such a location that we can
survive. Seasons would still be
massively impacted as the tilt of the
planet would be secondary to the
distance to the second star. When the
second star is as far away as possible
and a hemisphere on Earth was also
experiencing winter, it would get
extremely cold. On the other hand,
combine a summer with the second star
passing as closely as possible and it
will be incredibly hot. Additionally, a
curious phenomenon with red dwarfs is
that they also produce huge flares, much
larger than the ones our sun produces.
They would easily knock satellites
offline on a regular basis and Earth
would have spectacular aurora. Our power
grid as it stands would also be under
serious threat from these stellar flares
as they would act like hemispherewide
EMP bombs. The interactions with the
second star could well make our sun more
active too, meaning it too may produce
more flares. So even with the smallest
type of star, our habitability on Earth
would be under serious threat. Increase
the mass of the second star and you'll
start to get additional problems like
increased UV radiation, making going
outside more and more dangerous. You'll
also have two shadows a lot of the time.
Once you turn the second star into
something like a blue super giant, there
really won't be a place in the solar
system where there is even a hope of
habitability. Blue super giants can be
many times the mass of our sun. The
theoretical limit being 150 solar
masses, although some argue this should
be higher. Their volume is also big.
They make the sun look absolutely puny
in comparison. They can be millions of
times more luminous than our sun too
with devastating stellar winds. Enough
to rip our atmosphere off over a
relatively short time frame. Larger
again are yellow super giants and then
red super giants. While not as massive
or as luminous as blue super giants, red
super giants are the largest stars in
existence. If you plop the largest known
of these stars directly into the center
of our solar system, not only would they
easily encompass Earth, but everything
up until Saturn. That's 10 billion times
the volume of our sun. However, there is
a scenario where we could be in a binary
star system with a super giant and still
be on a habitable planet. You see,
binary stars can orbit very far apart,
taking thousands of years to complete
one orbit. The most extreme cases can
see a binary star system with a
separation of over a light year. And
what can happen is that planetary
systems will form around each of these
stars separately. Meaning that if Earth
was in one of these planetary systems,
it would only have the one parent star.
Even if that star was part of a binary,
the second star would be easily visible
in the night sky, but may not make much
of an impact during the day depending on
its luminosity. Stella winds from the
other star would have very little impact
on Earth, as our sun's powerful magnetic
field would redirect most of it away.
The big problem with a super giant on
your doorstep is that it is a ticking
time bomb. Super giants tend to be on
the verge of erupting in a supernova. A
supernova going off only one lightyear
away would be catastrophic,
probably sterilizing the entire planet
as radiation from the shock wave passes
over. And that's not to mention the
gammaray burst from the resulting
neutron star. In fact, a recent study
has suggested that being within 50
lighty years of a supernova going off
would be close enough to be catastrophic
in nature. However, the good news is
that we don't know of a star capable of
erupting in a supernova within 100 light
years. Going back to the single parent
star binary star configuration on Earth
with a single sun, we have a set day and
night cycle. However, should we have a
second sun outside of our orbit, but
still pretty close to us, it's going to
mess with our day and night cycle pretty
badly. There would only be tiny parts of
the year where you would get a proper
day and night cycle. And as the year
progresses, you'd get less and less of a
night until at one point you'd have no
night at all. So living on a planet with
more than one sun is no walk in the
park. No wonder Luke wanted to leave
Tatooine. Still, they are likely more
hospitable than conditions on our next
category of strange planets, rogue
planets.
Rogue planets are one of the great
mysteries left in the universe. They are
planetary-sized objects that are not
gravitationally bound to a star. We
don't fully understand how they formed.
Perhaps they were born in a planetary
system but got ejected during the
systems turbulent beginnings. Following
that thought, perhaps our own solar
system also had additional planets at
the beginning before they were cast away
to forever roam the galaxy alone.
Rogue planets could also be protoars
that simply failed to absorb the mass
needed to become a star or even a brown
dwarf.
We expect there to be billions to
trillions of them out there in just our
galaxy alone. Although because they are
hard to detect, this really is an
educated guess at best. However, since
the turn of the century, we have started
to detect a few of these mysterious
objects.
How can that be when there are no stars
lighting them up? And since they don't
emit their own light, if they are pitch
black, how do we have any hope of
detecting them at all? Well, this is
where a very interesting detection
technique comes in called gravitational
microlensing.
On this channel, we have explored
various exoplanet hunting techniques in
the past, mainly the radial velocity
method and the earlier mentioned transit
method. The radial velocity method
measures the wobble of a star caused by
the gravity of orbiting exoplanets. As
we talked about earlier, the transit
method measures the brightness of a star
over a long period of time and looks for
the dips in the stars brightness when an
exoplanet passes in front of it.
However, these methods are only useful
for detecting exoplanets around stars.
And generally, these exoplanets tend to
be large and have very close orbits. The
gravitational microl lensing method is
an exciting method because not only can
it find exoplanets much further away
from their host stars, but these planets
can also be as small as Mars or possibly
even smaller than that and still be
detected.
Additionally, rogue planets with no star
at all can be detected with this method.
Sounds almost too good to be true, but
there is one catch. So, how does it
work? Gravitational lensing is a
well-known phenomenon in astronomy and
has been used for years to detect some
of the most distant galaxies that we
know of. Einstein predicted this
phenomenon back in 1936. And with the
development of telescope technology, it
has been confirmed by observation.
Basically, the further away a light
source is from us, the dimmer it
becomes, thanks to the inverse square
law. The reason objects get dimmer with
distance is that photons spread out as
they travel. Meaning, the further away
you are from the object, the fewer
photons that reach your eyes. However,
when there is a body with a large amount
of mass, like a galaxy or galaxy
cluster, the mass of the object warps
the curvature of spaceime depending on
how massive it is. Light follows the
curvature of spaceime. Meaning that if
light emitted by a distant object
travels past a massive object, the light
photons that would have otherwise gone
off in these directions bend back around
thanks to the object's gravity, making
the background object appear brighter to
an observer here than it otherwise would
have done as more light photons are
reaching them. Now, gravitational
lensing is very obvious when we look at
the biggest types of objects, galaxy
clusters, because these objects warp
spaceime the most. This means that if we
are aligned just right with a galaxy
cluster and a distant galaxy, the
distant galaxy would appear much
brighter. The light from the distant
galaxy bending around the nearer galaxy
would also actually make the galaxy
appear stretched, sometimes into the
form of a ring. You may have seen these
Hubble images before where distant
galaxies appear totally distorted thanks
to this gravitational lensing effect.
And here is a CGI example of what you
are seeing which may help you understand
why this happens.
Now, as I mentioned, more recently, the
use of gravitational lensing has also
been used to help astronomers detect
exoplanets.
Stars and planets are much less massive
than galaxy clusters, but gravitational
lensing still happens to a small degree.
When a star transits in front of a
distant star, the distant star will
become brighter as its light follows the
curvature in spaceime around the
foreground star. Additionally, should
the foreground star have an exoplanet or
exoplanets,
the distant star will also get slightly
brighter when these transit in front of
the distant star too. This is the
process of gravitational microl lensing.
This process is so accurate that small
earth or Mars-ized exoplanets can be
detected and potentially even large
exomoons.
Astronomers simply need to measure the
little peaks in brightness to
distinguish between the individual
bodies orbiting the foreground star.
Now, as I mentioned, there is a catch
with this method and that is that star
transits are actually quite rare because
they really need to be aligned just
right from our perspective and the
galaxy is simply huge. There's so much
space in between stars. This is why up
until now, the transit method has had a
far better success rate at detecting
exoplanets because exoplanets orbit and
transit their stars a lot quicker than
stars transit other stars.
So, in order to spot these events, what
we need is to observe a huge swath of
sky in one go. Enter the Nancy Grace
Roman Space Telescope, previously known
as the W First Observatory.
This telescope, due to be launched no
later than 2027, is comparable in size
and will provide images with a sharpness
similar to the Hubble Space Telescope,
except the Roman Space Telescope will be
equipped with a 228
megapixel camera and a 2.8 squared
degree field of view, which is 100 times
larger than Hubble. With this field of
view, it will be able to keep track of a
much larger section of the sky and
monitor for microl lensing events.
How does this all fit in with rogue
planets? Well, much like stars, rogue
planets that transit in front of a
distant star will also make that star
slightly brighter. And if the Roman
Space Telescope is looking at that
section of the sky, it should be able to
detect it. In fact, throughout its
5-year initial mission, scientists hope
that the Roman Space Telescope can give
us a much better indication of how many
rogue planets and Earth-sized exoplanets
there are out there.
Now, while it is possible that rogue
planets are in any section of the sky,
the Roman Space Telescope will focus its
time looking towards the center of our
galaxy. There are a lot more stars here,
so transit should be more frequent. And
this will increase the chances of
spotting microl lensing events. It is
amazing to me that there are ways to
detect these otherwise invisible objects
that emit no light. And all you need to
do is look at the brightness of a
background star. And as a rogue planet
passes in front of it, the gravitational
microl lensing event will reveal key
details about the rogue planet like its
mass.
So, have we detected any rogue planets
this way at all? As it happens, yes, we
have. There are a few groundbased
programs on the hunt for rogue planets
operating right now, like Auggle, MOA,
and Super Macho. At present, they have
found 22 rogue planet candidates in all
with the most exciting one being Ouggle
2012 BLG1323.
If this rogue planet gets confirmed, it
will be the smallest rogue planet
discovered by some ways, being roughly
the mass of Earth. How a body like this
comes to be free floating in space, we
don't really know yet.
What might conditions be like on a rogue
planet? The first fact to consider is
that they would be incredibly dark
places. No nearby star means that
conditions there would be pitch black.
You might think that this might mean
that they would be extremely cold places
too, but this might not actually be the
case. No nearby star also means that
there is no nearby source of atmosphere
stripping solar winds, which might
actually help a rogue planet retain any
atmosphere it started out with. A 1999
research paper suggested that a
hydrogen-rich atmosphere around a rogue
planet could actually allow it to trap
in enough of its heat that it could
sustain Earthlike oceans. Still, any
life that wanted to live on such a
planet would have to be more akin to our
chemosynthetic species on Earth that
survive next to deep sea vents, relying
on the planet's molten core to provide
heat and nutrients. Photosynthesis would
obviously be a nogo on a rogue planet.
Other than that, rogue planets could
potentially come in all shapes and
sizes. Any planet that was flung out of
its home system would, by definition, be
a rogue planet. They could be gas giants
or rocky planets. So, the context of
where a planet is can make it pretty
strange. But what about the planet
themselves? What are some of the
weirdest, strangest phenomena we might
encounter on an exoplanet?
Well, let's step away from dark, chilly
rogue planets to something at the other
extreme.
WASP 76b is an exoplanet discovered in
2013 as part of the wide angle search
for planets program. Since its first
discovery, it has fascinated scientists
due to its unusual properties. ESO's
very large telescope recently found one
special characteristic in particular. On
this planet, temperatures are so hot
that instead of water rain, it rains
molten iron. How can this be? Exoplanets
that are very close to their stars with
very small orbits are the easiest to
discover because we can see a very clear
pattern on the stars light curve over a
short period of time. WASP 76b is one
such planet. Plus, it is huge, way
bigger than Jupiter. And combine this
with the fact that it only takes 1.8
Earth days to make one orbit, it made it
comparatively easy to detect.
But detecting the presence of an
exoplanet is one thing. How do
astronomers know anything about its
physical characteristics?
Interestingly, the first thing
astronomers do is find out the physical
characteristics of the parent star, WASP
76. The distance to the star is
determined and then the star is
classified based on its brightness and
color.
Knowing the distance helps us determine
how bright it is and we measure its
color simply by observing it which helps
us determine how hot it is. If the star
is on the main sequence then this chart
also helps us know the radius and mass
of the star as they all tend to follow a
pattern. Once we have that information
we can determine the characteristics of
the orbiting planet itself. Knowing the
mass and radius of the star means we can
measure the mass of the orbiting planet
using some clever equations based on the
law of universal gravitation. As it
happens, WASP 76b is a super Jupiter,
way bigger than our Jupiter. That means
that although it is massive, this mass
is spread out across a large volume,
likely making it a gas giant. Orbiting
this close to the star means the planet
is probably tidily locked. Only one side
faces the star at any given time. Also,
due to the proximity of the planet, it
orbits within the stars atmosphere, the
physics of which we really don't
understand yet. However, the star-facing
side will be extremely hot, estimated to
be around 2,400°
C, easily hot enough to vaporize metals.
Models go on to suggest the night side
is about 1,500°
C. Still blisteringly hot, but much
cooler. To find out what WASP 76b is
made of, though, we need to go back to
the light curve of the transit.
Scientists look for differences in the
light when the planet passes in front of
the star. As light from the star will
shine through the planet's atmosphere.
Certain atoms block certain wavelengths
of light. So any reduced wavelengths
help us know what is in the atmosphere.
This is known as spectroscopy.
For WASP 76b, the biggest surprise that
scientists detected was an abundance of
iron in the atmosphere. Based on what we
know about the planet so far, it seems
like iron exposed to the day side of the
planet is vaporized where it is
transported through strong wind
processes to the terminator line between
the day and the night side of the
planet. Here the temperature is low
enough for the iron to start to cool and
condense, producing iron raindrops which
fall deep into the atmosphere. By the
time the wind has reached the morning
side of the planet, iron can no longer
be detected. This remarkable measurement
taken by the Espresso instrument on
ISO's Very Large Telescope is the first
time variations have been spotted like
this on an ultra hot gas giant. Although
it probably won't be the last time.
Planets hot enough that they rain molten
iron are certainly hot places to be, but
they are not the hottest exoplanet we've
seen. So, how hot can a planet get?
Nestled in a tight orbit only 5 million
km from its parent star, roughly 10
times closer than Mercury orbits our
sun, sits a planet that is a raging
inferno.
Kelt 9 is a mercilessly hot star. At
9,700°
C is almost twice the temperature of our
own sun. The lone planet that orbits it
known as Kelt 9b is a gas giant almost
twice Jupiter's size and three times its
mass. And orbiting that close to such a
blaze, its daytime temperature surpasses
that of some stars. Kelt 9b was recorded
to have a temperature of 4,300°
C, making it hotter than your average
red dwarf star. Unsurprisingly, its
hydrogen atmosphere is boiling off into
space, but is then being dragged back
into its star at a predicted rate of
100,000
tons of hydrogen per second.
In time, all of Kelt 9b's atmosphere
will be drained away, leaving the
planet's core exposed to that blistering
heat. Kelt 9b's core will either be
eaten then or the planet as a whole will
be consumed when the star Kelt 9 expands
in 300 million years. In the meantime,
the surface of Kelt 9b is hot. Hot
enough that molecules on its surface
break down, ripped apart by all that
energy. All in all, it would not be a
nice place to live.
On the note of planets hotter than
stars, is it possible for planets to
exist that are larger than their parent
stars, too?
If you were to imagine a star system,
you'd probably think of a giant star
being orbited by smaller planets. And
generally speaking, this is the standard
for star systems across the universe.
However, solar systems come in many
shapes and sizes. Could it be that there
are planets out there that are bigger
than the star they are orbiting? And
leading on from that, how big can
planets get? What's the biggest one that
we know of?
First of all, let's define one point.
When we talk of big, what we are
actually referring to is the volume of
an object, not its mass. In order for a
star to be a star, it has to be over a
certain mass, namely 0.08 08 solar
masses or 8% of our sun's mass.
Below this threshold, the object is
classified as a brown dwarf because it
didn't become massive enough for nuclear
fusion to take place in its core.
Lower masses than brown dwarfs are
simply planets, asteroids, and dust.
So, the minimum mass of a star is 0.08
solar masses. Is there also an upper
limit? Theoretically, the most amount of
mass a star can attain is about 150
solar masses before models suggest it
would lose its stability.
Although, interestingly enough, there
are some stars in existence that
seemingly contradict that theory with
masses that are estimated to be around
300 solar masses.
Regardless, just because a star is
massive doesn't mean that it's big. Some
of the densest stars in existence,
neutron stars, are 1 to two solar masses
and can be only 30 km across. Imagine
fitting the vast mass of our star, an
object so huge that it dwarfs Jupiter
considerably, let alone us, and squeeze
up to two of these in a space the size
of a large city.
While these stars are tiny volumewise,
they easily have enough mass to host a
grand solar system that could stretch
far beyond our own. We have discovered
exoplanets around neutron stars.
However, due to the detection method
used to discover them, we can only
really define their mass, not their
radius.
It would make sense though that these
planets are many times bigger than the
tiny star as we have asteroids in our
solar system which can be hundreds of
kilome across, let alone planets
thousands of km across.
White dwarfs are another example of tiny
stars that likely have larger planets
orbiting them. White dwarfs are the
remnants of red giant stars that weren't
massive enough to become neutron stars
in a supernova explosion when they
reached the end of their lives.
Instead, they shed their outer layers in
a planetary nebula, leaving behind the
core of the star, or what is known as a
white dwarf. Nuclear fusion does not
take place in the white dwarf, meaning
their luminosity comes solely from
stored thermal radiation.
This makes them quite dim stars, and
they only get dimmer over time.
Planets that would have survived the
stars evolution from main sequence to
red giant to white dwarf could well be
larger than the small remnant white
dwarf which is typically the radius of
earth.
There came a confirmation of such a
planetary system in 2020 thanks to the
test observatory an exoplanet known as
WD1856b.
What's remarkable about this system is
that the planet orbits the white dwarf
relatively closely, meaning it survived
the star going through its red giant
phase. The red giant phase would have
easily enveloped where the planet now
resides. So, this means the planet must
have fallen or scattered into this orbit
after the event, perhaps thanks to the
gravitational influences of the stars
other undiscovered planets or even from
nearby stars that the white dwarf is
gravitationally bound to. In any case,
it's an incredible system indeed.
When it comes to main sequence stars,
planets larger than them are tricky to
find, too. We believe that main sequence
stars make up the majority of stars in
the universe. These can be very big
objects, as internal pressure from the
reactions in the core keeps the radius
large. We'd have to look for stars right
around the dividing line between brown
dwarfs and red dwarfs to find a main
sequence star small enough to host a
larger planet.
Remember the difference between a brown
dwarf and a main sequence star is
whether nuclear fusion takes place in
the stars core which we believe happens
above 0.08 solar masses. And we also
believe the dividing line between a
planet and a brown dwarf is around 13
Jupiter masses.
So, one of our best bets to find a tiny
star with a large planet is looking
around the red dwarf VHS1256-1257.
Its exoplanet is one of the few
exoplanets to be discovered using direct
imaging. Its host star is dim enough.
The exoplanet is far enough away from
the star and big enough to be seen
simply by using a powerful telescope.
Unfortunately though, this means that
while the mass of the exoplanet has been
determined, we can't know of its radius
for sure. However, this is one of the
most likely candidates that we know of.
Also, because of the margins of error
involved, it could well be that either
or both objects are brown dwarfs,
meaning we can't say if this is truly a
star and planet system.
So, while we don't have definitive proof
of a planet being bigger than its host
star, we have found some promising
candidates, and there's almost certainly
cases out there that we haven't found
yet. Some of the smallest stars out
there have a radius of roughly 70,000
km. Some of the biggest planets out
there can be double or triple that.
Which leads on to the final question I
wanted to cover here. What is the
biggest exoplanet that we know of?
Unfortunately, it is not clear-cut. One
possible answer is GQ Loopy B. It is
another directly imaged exoplanet, which
again means we don't have a good grasp
on its physical characteristics. From
the margins of area involved, it could
be a brown dwarf, but it's probably the
largest exoplanet that we know of.
Scientists have estimated its radius to
be three times the size of Jupiter, but
again there are margins of error
involved. Other contenders to this
throne would be DH Tori B and rocks
42BB.
We've looked at a lot of exoplanets in
the course of this video today. some of
the hottest, the coldest, planets with
multiple stars, planets with none, the
fastest orbits, and the longest. But I
want to finish with an exoplanet that
would be visually stunning if its
existence were to be confirmed. J1407b
touted as the first exoplanet discovered
with a ring system like Saturn.
Sometimes it is surprisingly tricky to
spot ring systems around planets. It was
only in 1977 that Uranus's rings were
discovered. Jupiter's rings were only
spotted in 1979 when they were imaged by
the passing Voyager probe, and Neptune's
rings weren't seen until the early
1980s.
This is because in the absence of nearby
probes to photograph them, rings are
usually discovered via either the
transit method or stellar occultation
when they pass in front of a near or
distant star, causing the light reaching
us to momentarily dim. Usually, you have
to be looking carefully. However, not so
with J1407b.
When planets like Jupiter pass in front
of its neighboring star, they can
sometimes block as little as 1% of the
stars light. Their rings much less than
that. When J1407b
passed in front of its star, it blocked
an incredible 95%
of the stars light. Why was this? In
2012, researchers who noticed this
phenomenon in the super wasp data came
up with the answer. J1407b
had rings, massive ones.
By evaluating the dips in the light,
astronomers were able to detect a
complex ring structure around the planet
that covers more than 40,000 times the
area of Saturn's rings. For point of
reference, this represented rings as
large as the orbit of Venus around our
sun. Just like with Saturn, at least one
small moon had begun to form within the
rings, creating gaps and indicating that
the planet's rings will not last
forever. However, right now, if this
interpretation of the data is correct,
they would be a sight to behold.
Sadly, there is a reason to look at this
answer with suspicion. J1407b
supposedly travels around a star once
every 10 years, but since that initial
eclipse, no equivalent dimming events
have been cited. Scientists are
attempting to explain whether this means
J1407b
was actually just a one-off event, a
free floating brown dwarf with its own
protolanetary disc that just so happen
to pass by the star by coincidence, or
whether J1407b
could still be an orbiting planet with
massive rings, albeit ones that are much
thinner than first anticipated.
So, there we have it. some of the
strangest exoplanets out there. Are
there any you feel I've missed? What
were your favorites? Let me know in the
comments below.
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