Astronomers provide 'field guide' to exoplanets known as hot Jupiters


Date:
October 21, 2021
Source:
University of Arizona
Summary:
By combining Hubble Space Telescope observations with theoretical
models, a team of astronomers has gained insights into the
chemical and physical makeup of a variety of exoplanets known as
hot Jupiters. The findings provide a new and improved 'field guide'
for this group of planets and inform ideas about planet formation
in general.



FULL STORY ==========================================================================
Hot Jupiters -- giant gas planets that race around their host stars
in extremely tight orbits -- have become a little bit less mysterious
thanks to a new study combining theoretical modeling with observations
by the Hubble Space Telescope.


========================================================================== While previous studies mostly focused on individual worlds classified as
"hot Jupiters" due to their superficial similarity to the gas giant in
our own solar system, the new study is the first to look at a broader population of the strange worlds. Published in Nature Astronomy, the
study, led by a University of Arizona researcher, provides astronomers
with an unprecedented "field guide" to hot Jupiters and offers insight
into planet formation in general.

Although astronomers think that only about 1 in 10 stars host an
exoplanet in the hot Jupiter class, the peculiar planets make up a
sizeable portion of exoplanets discovered to date, due to the fact that
they are bigger and brighter than other types of exoplanets, such as
rocky, more Earthlike planets or smaller, cooler gas planets. Ranging
in size from about one-third the size of Jupiter to 10 Jupiter masses,
all hot Jupiters orbit their host star at an extremely close range,
usually much closer than Mercury, the innermost planet in our solar
system, is to the sun. A "year" on a typical hot Jupiter lasts hours,
or at most a few days. For comparison, Mercury takes almost three months
to complete a trip around the sun.

Because of their close orbits, most, if not all, hot Jupiters are
thought to be locked in a high-speed embrace with their host stars,
with one side eternally exposed to the star's radiation and the other
shrouded in perpetual darkness.

The surface of a typical hot Jupiter can get as hot as almost 5,000
degrees Fahrenheit, with "cooler" specimens reaching 1,400 degrees --
hot enough to melt aluminum.

The research, which was led by Megan Mansfield, a NASA Sagan Fellow at
the University of Arizona's Steward Observatory, used observations made
with the Hubble Space Telescope that allowed the team to directly measure emission spectra from hot Jupiters, despite the fact that Hubble can't
image any of these planets directly.

"These systems, these stars and their hot Jupiters, are too far away
to resolve the individual star and its planet," Mansfield said. "All we
can see is a point -- the combined light source of the two." Mansfield
and her team used a method known as secondary eclipsing to tease out information from the observations that allowed them to peer deep into the planets' atmospheres and gain insights into their structure and chemical makeup. The technique involves repeated observations of the same system, catching the planet at various places in its orbit, including when it
dips behind the star.



==========================================================================
"We basically measure the combined light coming from the star and its
planet and compare that measurement with what we see when the planet is
hidden behind its star," Mansfield said. "This allows us to subtract
the star's contribution and isolate the light emitted by the planet,
even though we can't see it directly." The eclipse data provided the researchers with insight into the thermal structure of the atmospheres
of hot Jupiters and allowed them to construct individual profiles
of temperatures and pressures for each one. The team then analyzed near-infrared light, which is a band of wavelengths just beyond the
range humans can see, coming from each hot Jupiter system for so-called absorption features. Because each molecule or atom has its own specific absorption profile, like a fingerprint, looking at different wavelengths
allows researchers to obtain information about the chemical makeup of
hot Jupiters.

For example, if water is present in the planet's atmosphere, it will
absorb light at 1.4 microns, which falls into the range of wavelengths
that Hubble can see very well.

"In a way, we use molecules to scan through the atmospheres on these
hot Jupiters," Mansfield said. "We can use the spectrum we observe
to get information on what the atmosphere is made of, and we can also
get information on what the structure of the atmosphere looks like."
The team went a step further by quantifying the observational data and comparing it to models of the physical processes believed to be at work
in the atmospheres of hot Jupiters. The two sets matched very well,
confirming that many predictions about the planets' nature based on
theoretical work appear to be correct, according to Mansfield, who said
the findings are "exciting because they were anything but guaranteed."
The results suggest that all hot Jupiters, not just the 19 included in
the study, are likely to contain similar sets of molecules, like water
and carbon monoxide, along with smaller amounts of other molecules. The differences among individual planets should mostly amount to varying
relative amounts of these molecules. The findings also revealed that the observed water absorption features varied slightly from one hot Jupiter
to the next.

"Taken together, our results tell us there is a good chance we have
the big picture items figured out that are happening in the chemistry
of these planets," Mansfield said. "At the same time, each planet has
its own chemical makeup, and that also influences what we see in our observations." According to the authors, the results can be used to guide expectations of what astronomers might be able to see when looking at a
hot Jupiter that hasn't been studied before. The launch of NASA's news
flagship telescope, the James Webb Space Telescope, slated for Dec. 18,
has exoplanet hunters excited because Webb can see in a much broader
range of infrared light, and will allow a much more detailed look at exoplanets, including hot Jupiters.

"There is a lot that we still don't know about how planets form in
general, and one of the ways we try to understand how that could
happen is by looking at the atmospheres of these hot Jupiters
and figuring out how they got to be where they are," Mansfield
said. "With the Hubble data, we can look at trends by studying the
water absorption, but when we are talking about the composition of
the atmosphere as a whole, there are many other important molecules
you want to look at, such as carbon monoxide and carbon dioxide,
and JWST will give us a chance to actually observe those as well." ========================================================================== Story Source: Materials provided by University_of_Arizona. Original
written by Daniel Stolte.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Megan Mansfield, Michael R. Line, Jacob L. Bean, Jonathan
J. Fortney,
Vivien Parmentier, Lindsey Wiser, Eliza M.-R. Kempton, Ehsan Gharib-
Nezhad, David K. Sing, Mercedes Lo'pez-Morales, Claire Baxter,
Jean- Michel De'sert, Mark R. Swain, Gael M. Roudier. A unique
hot Jupiter spectral sequence with evidence for compositional
diversity. Nature Astronomy, 2021; DOI: 10.1038/s41550-021-01455-4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211021121109.htm

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