ESA’s new exoplanet mission, Cheops, has
found a nearby planetary system to contain one of the hottest and most extreme
extra-solar planets known to date: WASP-189 b. The finding, the very first from
the mission, demonstrates Cheops’ unique ability to shed light on the Universe
around us by revealing the secrets of these alien worlds.
Launched in December 2019, Cheops (the Characterising Exoplanet Satellite) is designed to observe nearby stars known to host planets. By
ultra-precisely measuring changes in the levels of light coming from these
systems as the planets orbit their stars, Cheops can initially characterise
these planets — and, in turn, increase our understanding of how they form and
evolve.
The
new finding concerns a so-called ‘ultra-hot Jupiter’ named WASP-189 b. Hot
Jupiters, as the name suggests, are giant gas planets a bit like Jupiter in our
own Solar System; however, they orbit far, far closer to their host star, and
so are heated to extreme temperatures.
WASP-189
b sits around 20 times closer to its star than Earth does to the Sun, and
completes a full orbit in just 2.7 days. Its host star is larger and more than
2000 degrees hotter than the Sun, and so appears to glow blue. “Only a handful
of planets are known to exist around stars this hot, and this system is by far
the brightest,” says Monika Lendl of the University of Geneva, Switzerland,
lead author of the new study. “WASP-189b is also the brightest hot Jupiter that
we can observe as it passes in front of or behind its star, making the whole
system really intriguing.”
First,
Monika and colleagues used Cheops to observe WASP-189 b as it passed behind its
host star – an occultation. “As the planet is so bright, there is actually a
noticeable dip in the light we see coming from the system as it briefly slips
out of view,” explains Monika. “We used this to measure the planet’s brightness
and constrain its temperature to a scorching 3200 degrees C.”
This
makes WASP-189 b one of the hottest and most extreme planets, and entirely unlike
any of the planets of the Solar System. At such temperatures, even metals such
as iron melt and turn to gas, making the planet a clearly uninhabitable one.
Next,
Cheops watched as WASP-189 b passed in front of its star – a transit. Transits
can reveal much about the size, shape, and orbital characteristics of a planet.
This was true for WASP-189 b, which was found to be larger than thought at
almost 1.6 times the radius of Jupiter.
“We
also saw that the star itself is interesting – it’s not perfectly round, but
larger and cooler at its equator than at the poles, making the poles of the
star appear brighter,” says Monika. “It’s spinning around so fast that it’s
being pulled outwards at its equator! Adding to this asymmetry is the fact that
WASP-189 b’s orbit is inclined; it doesn’t travel around the equator, but
passes close to the star’s poles.”
Seeing
such a tilted orbit adds to the existing mystery of how hot Jupiters form. For
a planet to have such an inclined orbit, it must have formed further out and
then been pushed inwards. This is thought to happen as multiple planets
within a system jostle for position, or as an external influence – another
star, for instance – disturbs the system, pushing gas giants towards their star
and onto very short orbits that are highly tilted. “As we measured such a tilt
with Cheops, this suggests that WASP-189 b has undergone such interactions in
the past,” adds Monika.
Monika
and colleagues used Cheops’ highly precise observations and optical
capabilities to reveal the secrets of WASP-189 b. Cheops opened its ‘eye’ in January of this year and began routine science operations in April, and has been working to expand our understanding of exoplanets and the
nearby cosmos in the months since.
“This
first result from Cheops is hugely exciting: it is early definitive evidence
that the mission is living up to its promise in terms of precision and
performance,” says Kate Isaak, Cheops project scientist at ESA.
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