The 7 most extreme planets ever discovered

Scientists recently discovered the hottest
planet
ever found – with a surface temperature greater than
some stars. As the hunt for planets outside our own solar system
continues, we have discovered many other worlds with extreme
features. And the ongoing exploration of our own solar system has
revealed some pretty weird contenders, too. Here are seven of the
most extreme.

The hottest

How hot a planet gets depends primarily on how close it is to its
host star – and on how hot that star burns. In our own solar
system, Mercury is the closest planet to the sun at a mean
distance of 57,910,000km. Temperatures on its dayside reach about
430°C, while the sun itself has a surface temperature of 5,500°C.

But stars more massive than the sun burn hotter. The star HD
195689 – also known as KELT-9 – is 2.5 times more massive than
the sun and has a surface temperature of almost 10,000°C. Its
planet, KELT-9b, is much closer to its host star than
Mercury is to the sun.

Though we cannot measure the exact distance from afar, it circles
its host star every 1.5 days (Mercury’s orbit takes 88 days).
This results in a whopping 4300°C – which is hotter than many of
the stars with a lower mass than our sun. The rocky planet
Mercury would be a molten droplet of lava at this temperature.
KELT-9b, however, is a Jupiter-type gas giant. It is shrivelling
away as the molecules in its atmosphere are breaking down to
their constituent atoms – and burning off.

The coldest

At a temperature of just 50 degrees above absolute zero – -223°C
OGLE-2005-BLG-390Lb snatches the title of the
coldest planet. At about 5.5 times the Earth’s mass it is likely
to be a rocky planet too. Though not too distant from its host
star at an orbit that would put it somewhere between Mars and
Jupiter in our solar system, its host star is a low mass, cool
star known as a red dwarf.


OGLE-2005-BLG-390Lb
Freezing
but Earth-like.


ESO


The planet is popularly referred to as Hoth
in reference to an icy planet in the Star Wars franchise.
Contrary to its fictional counterpart, however, it won’t be able
to sustain much of an atmosphere (nor life, for that matter).
This because most of its gases will be frozen solid – adding to
the snow on the surface.

The biggest

If a planet can be as hot as a star, what then makes the
difference between stars and planets? Stars are so much more
massive than planets that they are ignited by fusion processes as
a result of the huge gravitational forces in their cores. Common
stars like our sun burn by fusing hydrogen into helium. But there
is a form of star called a brown dwarf, which are big enough to start
some fusion processes but not large enough to sustain them.


pl2
Artist’s
impression of the biggest planet known.


NASA
and G. Bacon (STScI)



Planet DENIS-P J082303.1-491201 b with the equally
unpronounceable alias 2MASS J08230313-4912012 b has 28.5 times
the mass of Jupiter – making it the most massive planet listed in
NASA’s exoplanet archive. It is so massive
that it is debated whether it still is a planet (it would be a
Jupiter-class gas giant) or whether it should actually be
classified as a brown dwarf star. Ironically, its host star is a
confirmed brown dwarf itself.

The smallest

Just slightly larger than our moon and smaller than Mercury,
Kepler-37b is the smallest exoplanet yet
discovered. A rocky world, it is closer to its host star than
Mercury is to the sun. That means the planet is too hot to
support liquid water and hence life on its surface.

The oldest

PSR B1620-26 b, at 12.7 billion years, is the oldest known planet. A gas giant 2.5 times
the mass of Jupiter it has been seemingly around forever. Our
universe at 13.8 billion years is only a billion years older.

PSR B1620-26 b has two host stars rotating around each other –
and it has outseen the lives of both. These are a neutron star
and a white dwarf, which are what is left when a star has burned
all its fuel and exploded in a supernova. However, as it formed
so early in the universe’s history, it probably doesn’t have
enough of the heavy elements such as carbon and oxygen (which
formed later) needed for life to evolve.

The youngest

The planetary system V830 Tauri is only 2m years old. The host star
has the same mass as our sun but twice the radius, which means it
has not fully contracted into its final shape yet. The planet – a
gas giant with three quarters the mass of Jupiter – is likewise
probably still growing. That means it is acquiring more mass by
frequently colliding with other planetary bodies like asteroids
in its path – making it an unsafe place to be.

The worst weather

Because exoplanets are too far away for us to be able to observe
any weather patterns we have to turn our eyes back to our solar
system. If you have seen the giant swirling hurricanes photographed by the Juno
spacecraft
flying over Jupiter’s poles, the largest planet in
our solar system is certainly a good contender. However, the
title goes to Venus. A planet the same size of Earth, it is
shrouded in clouds of sulfuric acid.


venus
The greenhouse effect
creates hellish temperatures of at least 462°C on Venus’s
surface.

NSSDC/GSFC/NASA

The atmosphere moves around the planet much faster than the
planet rotates, with winds reaching hurricane speeds of 360km/h.
Double-eyed cyclones are sustained above each pole. Its
atmosphere is almost 100 times denser than Earth’s and made up of
over 95% carbon dioxide. The resulting greenhouse effect creates
hellish temperatures of at least 462°C on the surface, which is
actually hotter than Mercury. Though bone-dry and hostile to
life, the heat may explain why Venus has fewer volcanoes than Earth.

 

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