Astronomers Find Planet Hotter Than Most Stars

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Astronomers Find Planet Hotter Than Most Stars
Jet Propulsion Laboratory
June 5, 2017

A newly discovered Jupiter-like world is so hot, it's being vaporized
by its own star.

With a dayside temperature of more than 7,800 degrees Fahrenheit (4,600
Kelvin), KELT-9b is a planet that is hotter than most stars. But its blue
A-type star, called KELT-9, is even hotter -- in fact, it is probably
unraveling the planet through evaporation.

"This is the hottest gas giant planet that has ever been discovered,"
said Scott Gaudi, astronomy professor at The Ohio State University in
Columbus, who led a study on the topic. He worked on this study while
on sabbatical at NASA's Jet Propulsion Laboratory, Pasadena, California.
The unusual planet is described in the journal Nature and at a presentation
at the American Astronomical Society summer meeting this week in Austin,
Texas.

KELT-9b is 2.8 times more massive than Jupiter, but only half as dense.
Scientists would expect the planet to have a smaller radius, but the extreme
radiation from its host star has caused the planet's atmosphere to puff
up like a balloon.

Because the planet is tidally locked to its star -- as the moon is to
Earth -- one side of the planet is always facing toward the star, and
one side is in perpetual darkness. Molecules such as water, carbon dioxide
and methane can't form on the dayside because it is bombarded by too much
ultraviolet radiation. The properties of the nightside are still mysterious
-- molecules may be able to form there, but probably only temporarily.

"It's a planet by any of the typical definitions of mass, but its atmosphere
is almost certainly unlike any other planet we've ever seen just because
of the temperature of its dayside," Gaudi said.

The KELT-9 star is only 300 million years old, which is young in star
time. It is more than twice as large, and nearly twice as hot, as our
sun. Given that the planet's atmosphere is constantly blasted with high
levels of ultraviolet radiation, the planet may even be shedding a tail
of evaporated planetary material like a comet.

"KELT-9 radiates so much ultraviolet radiation that it may completely
evaporate the planet," said Keivan Stassun, a professor of physics and
astronomy at Vanderbilt University, Nashville, Tennessee, who directed
the study with Gaudi.

But this scenario assumes the star doesn't grow to engulf the planet first.

"KELT-9 will swell to become a red giant star in a few hundred million
years," said Stassun. "The long-term prospects for life, or real estate
for that matter, on KELT-9b are not looking good."

The planet is also unusual in that it orbits perpendicular to the spin
axis of the star. That would be analogous to the planet orbiting perpendicular
to the plane of our solar system. One "year" on this planet is less than
two days.

KELT-9b is nowhere close to habitable, but Gaudi said there's a good reason
to study worlds that are unlivable in the extreme.

"As has been highlighted by the recent discoveries from the MEarth collaboration,
the planet around Proxima Centauri, and the astonishing system discovered
around TRAPPIST-1, the astronomical community is clearly focused on finding
Earthlike planets around small, cooler stars like our sun. They are easy
targets and there's a lot that can be learned about potentially habitable
planets orbiting very low-mass stars in general. On the other hand, because
KELT-9b's host star is bigger and hotter than the sun, it complements
those efforts and provides a kind of touchstone for understanding how
planetary systems form around hot, massive stars," Gaudi said.

The KELT-9b planet was found using one of the two telescopes called KELT,
or Kilodegree Extremely Little Telescope. In late May and early June 2016,
astronomers using the KELT-North telescope at Winer Observatory in Arizona
noticed a tiny drop in the star's brightness -- only about half of one
percent -- which indicated that a planet may have passed in front of the
star. The brightness dipped once every 1.5 days, which means the planet
completes a "yearly" circuit around its star every 1.5 days.

Subsequent observations confirmed the signal to be due to a planet, and
revealed it to be what astronomers call a "hot Jupiter" -- the kind of
planet the KELT telescopes are designed to spot.

Astronomers at Ohio State, Lehigh University in Bethlehem, Pennsylvania,
and Vanderbilt jointly operate two KELTs (one each in the northern and
southern hemispheres) to fill a large gap in the available technologies
for finding exoplanets. Other telescopes are designed to look at very
faint stars in much smaller sections of the sky, and at very high resolution.
The KELTs, in contrast, look at millions of very bright stars at once,
over broad sections of sky, and at low resolution.

"This discovery is a testament to the discovery power of small telescopes,
and the ability of citizen scientists to directly contribute to cutting-edge
scientific research," said Joshua Pepper, astronomer and assistant professor
of physics at Lehigh University in Bethlehem, Pennsylvania, who built
the two KELT telescopes.

The astronomers hope to take a closer look at KELT-9b with other telescopes
-- including NASA's Spitzer and Hubble space telescopes, and eventually
the James Webb Space Telescope, which is scheduled to launch in 2018.
Observations with Hubble would enable them to see if the planet really
does have a cometary tail, and allow them to determine how much longer
that planet will survive its current hellish condition.

"Thanks to this planet's star-like heat, it is an exceptional target to
observe at all wavelengths, from ultraviolet to infrared, in both transit
and eclipse. Such observations will allow us to get as complete a view
of its atmosphere as is possible for a planet outside our solar system,"
said Knicole Colon, paper co-author who was based at NASA Ames Research
Center in California's Silicon Valley during the time of this study.

The study was largely funded by the National Science Foundation (NSF)
through an NSF CAREER Grant, NSF PAARE Grant and an NSF Graduate Research
Fellowship. Additional support came from NASA via the Jet Propulsion Laboratory
and the Exoplanet Exploration Program; the Harvard Future Faculty Leaders
Postdoctoral Fellowship; Theodore Dunham, Jr., Grant from the Fund for
Astronomical Research; and the Japan Society for the Promotion of Science.

For more information about exoplanets, visit:

https://exoplanets.nasa.gov

News Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabet...@jpl.nasa.gov

Pam Frost Gorder
The Ohio State University, Columbus, Ohio
614-292-9475
Gord...@osu.edu

Written with Pam Frost Gorder

2017-160