Artist's
impression of the hot star KELT-9 and its planet KELT-9b, a hot
Jupiter. Researchers have now detected the extended hydrogen atmosphere
of the planet, which is "boiling off" due to the central star's great
heat. Image: MPIA . JPG_2303x1358 - TIF_2303x1358
Astronomers have found that the atmosphere of the hottest known
exoplanet, the hot Jupiter-like planet KELT-9b, is "boiling off," with
the escaping gas being captured by the host star. Using the CARMENES
instrument at Calar Alto Observatory, Fei Yan and Thomas Henning of the
Max Planck Institute for Astronomy in Heidelberg were able to detect the
escaping hydrogen atmosphere of the planet. Their observations indicate
a spread-out hydrogen envelope that is being pulled towards the host
star.
By all definitions, KELT-9b is a hellish kind of exoplanet: Due to
its proximity to an extremely hot host star, the planet itself is the
hottest exoplanet yet discovered. Now Fei Yan and Thomas Henning of the
Max Planck Institute for Astronomy have detected that planet's extended
atmosphere, showing that the star is not only heating up the planet's
hydrogen atmosphere – it is then using its gravity to pull the hydrogen
onto itself.
Specifically, the planet's host star KELT-9 is an extremely hot star
with a temperature of up to 10,000 K (compare this with the Sun's much
more modest 5800 K, or 5500 degrees Celsius). The planet's orbit is
extremely small – ten times smaller than the orbit of Mercury in our
Solar system (corresponding to only about 3% of the diameter of Earth's
orbit around the Sun). When the planet was discovered in 2017 by a team
of astronomers led by B. Scott Gaudi (Ohio State University), the
astronomers measured its day-side temperature to be at 4600 K (4300
degrees Celsius), which is hotter than many stars!
The planet itself is a significantly larger version of our Solar
System's Jupiter, at almost 3 times Jupiter's mass and almost twice
Jupiter's diameter. These properties combined place KELT-9b firmly in
the class of what astronomers call "hot Jupiter".
The planet's orbit regularly takes it between the host star and an
observer on Earth – during each such transit, the planet blocks some of
the starlight, causing the star to dim a little bit as measured by
telescope on Earth. The planet was initially discovered by astronomers
looking for that kind of regular little dip in the star's apparent
brightness (the so-called transit method).
When Yan and Henning observed KELT-9b using the CARMENES spectrograph
installed at the 3.5 meter telescope at Calar Alto Observatory, they
found traces of the planet's atmosphere: Whenever the planet was in
front of its star, there would be a clear absorption line for hydrogen
(Ha), a narrow wavelength region where the planet's hydrogen-rich
atmosphere absorbs some of it's host star's bright light. CARMENES gives
a particularly detailed, high-resolution view of stellar spectral
making it an excellent tool for this kind of observation.
The extended hydrogen atmosphere surrounding KELT-9b is surprisingly
large – more than half as large again as the planet's radius. Models of
how the star's gravity will pull on the planet's gas show that this is
close to the maximal size of such an atmosphere. The large size suggests
that the planet is losing hydrogen gas at a significant rate of more
than 100,000 tons of hydrogen per second. The star is "boiling off" the
planet's atmosphere, and pulling the gas onto itself, in a blatant case
of interplanetary theft.
The way the wavelength of the absorption line changes during the
transit amounts to a rare direct detection of the planet's motion: the
wavelength shift is due to the Doppler shift, which tells us how fast
the planet is moving towards us or away from us. Fey Yan, lead author of
the article, says: "This is a very special kind of measurement – this
kind of direct measurement of planetary motion has only been possible
for about half a dozen exoplanets so far."
Thomas Henning, director at the MPIA and co-author of the study,
says: "This planet reminds me of the mythical Icarus, who came to close
to the Sun and crashed. Our planet will not crash, but it will certainly
lose an essential part of itself, namely its atmosphere."
Background information
The results described here are published as F. Yan and Th. Henning,
"An extended hydrogen envelope of the extremely hot giant exoplanet
KELT-9b" in the journal Nature Astronomy. Both authors are at the Max
Planck Institute for Astronomy.
For advance access to the paper before the end of the embargo, journalists should please contact
CARMENES [kár-men-es](short for the "Calar Alto
high-Resolution search for M dwarfs with Exoearths with Near-infrared
and optical Échelle Spectrographs") is a next-generation instrument
built for the 3.5m telescope at the Calar Alto Observatory by a
consortium of German and Spanish institutions. The principal
investigators of CARMENES are Andreas Quirrenbach of the Zentrum für
Astronomie of Heidelberg University and Pedro J. Amado of IAA Granada.
The Max Planck Institute for Astronomy is part of the CARMENES
consortium, and the instrument has received funding from the Max Planck
Society.
Science Contact:
Fei Yan
Phone: (+49/0) 6221528-358
Email: fyan@mpia.de
Room: 308/3
Links: Personal homepage
Thomas K. Henning
Director
Phone: (+49|0) 6221 528-200
Email: henning@mpia.de
Room: 216 G
Links: Personal homepage
Public Information Officer:
Markus Pössel
Managing scientist, HdA, and senior MPIA outreach scientist
Phone: (+49|0) 6221 528-261
Email: info@hda-hd.de
Room: H-517
Links: Personal homepage
