Astronomers Provide 'Field Guide' to Exoplanets Known as Hot Jupiters By Daniel Stolte, University Communications

This artist’s impression shows a hot Jupiter planet orbiting close to one of the stars in the rich old star cluster Messier 67, located between 2,500 and 3,000 light-years from Earth in the constellation of Cancer (The Crab).ESO/L. Calçada

The turbulent atmosphere of a hot, gaseous planet known as HD 80606b is shown in this simulation based on data from NASA's Spitzer Space Telescope. The planet spends most of its time far away from its star, but every 111 days, it swings extremely close to the star, experiencing a massive burst of heat.NASA/JPL-CalTech

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.

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 stars 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."

Resources for the Media 

Media contact:

Daniel Stolte
Science Writer, University Communications
stolte@arizona.edu
520-626-4402

Researcher contact:

Megan Mansfield
NASA Sagan Fellow, Steward Observatory
meganmansfield@arizona.edu

Source: The Arizona University/News

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Investigating the Mystery of Migrating 'Hot Jupiters'

The turbulent atmosphere of a hot, gaseous planet known as HD 80606b is shown in this simulation based on data from NASA's Spitzer Space Telescope

Image credit: NASA/JPL-Caltec. › Full image and caption

Astronomers watched an exoplanet called HD 80606b heat up and cool off during its sizzling-hot orbit around its star

Image credit: NASA/JPL-Caltech/MIT.  › Full image and caption

The Wild Temperature Swings of an Exoplanet
Youtube video

The last decade has seen a bonanza of exoplanet discoveries. Nearly 2,000 exoplanets -- planets outside our solar system -- have been confirmed so far, and more than 5,000 candidate exoplanets have been identified. Many of these exotic worlds belong to a class known as "hot Jupiters." These are gas giants like Jupiter but much hotter, with orbits that take them feverishly close to their stars. 

At first, hot Jupiters were considered oddballs, since we don't have anything like them in our own solar system. But as more were found, in addition to many other smaller planets that orbit very closely to their stars, our solar system started to seem like the real misfit.

"We thought our solar system was normal, but that's not so much the case," said astronomer Greg Laughlin of the University of California, Santa Cruz, co-author of a new study from NASA's Spitzer Space Telescope that investigates hot Jupiter formation. 

As common as hot Jupiters are now known to be, they are still shrouded in mystery. How did these massive orbs form, and how did they wind up so shockingly close to their stars? 

The Spitzer telescope found new clues by observing a hot Jupiter known as HD 80606b, situated 190 light-years from Earth. This planet is unusual in that it has a wildly eccentric orbit almost like that of a comet, swinging very close to its star and then back out to much greater distances over and over again every 111 days. One side of the planet is thought to become dramatically hotter than the other during its harrowing close approaches. In fact, when the planet is closest to its host star, the side facing the star quickly heats up to more than 2,000 degrees Fahrenheit (1,100 degrees Celsius). 

"As the planet gets closer to the star, it feels a burst of starlight, or radiation. The atmosphere becomes a cauldron of chemical reactions, and the winds ramp up far beyond hurricane force," said Laughlin, a co-author on the Spitzer study, which is accepted for publication in The Astrophysical Journal. 

HD 80606b is thought to be in the process of migrating from a more distant orbit to a much tighter one typical of hot Jupiters. One of the leading theories of hot-Jupiter formation holds that gas giants in distant orbits become hot Jupiters when the gravitational influences from nearby stars or planets drive them into closer orbits. The planets start out in eccentric orbits, then, over a period of hundreds of millions of years, are thought to gradually settle down into tight, circular orbits.

"This planet is thought to be caught in the act of migrating inward," said Julien de Wit of the Massachusetts Institute of Technology, Cambridge, lead author of the new study. "By studying it, we are able to test theories of hot Jupiter formation."

Spitzer previously studied HD 80606b in 2009. The latest observations are more detailed, thanks to a longer observing time -- 85 hours -- and improvements in Spitzer's sensitivity to exoplanets. 

"The Spitzer data are pristine," said de Wit. "And we were able to observe the planet for much longer this time, giving us more insight into its coldest temperature and how fast it heats up, cools down and rotates."

A key question addressed in the new study is: How long is HD 80606b taking to migrate from an eccentric to a circular orbit? One way to assess this is to look at how "squishy" the planet is. When HD 80606b whips closely by its star, the gravity of the star squeezes it. If the planet is squishier, or more pliable, it can better dissipate this gravitational energy as heat. And the more heat that is dissipated, the faster the planet will transition to a circular orbit, a process known as circularization.

"If you take a Nerf ball and squeeze it a bunch of times really fast, you'll see that it heats up," said Laughlin. "That's because the Nerf ball is good at transferring that mechanical energy into heat. It's squishy as a result."

The Spitzer results show that HD 80606b does not dissipate much heat when it is squeezed by gravity during its close encounters - and thus is not squishy, but rather stiffer as a whole. This suggests the planet is not circularizing its orbit as fast as expected, and may take another 10 billion years or more to complete.

"We are starting to learn how long it may take for hot Jupiter migration to occur," said de Wit. "Our theories said it shouldn't take that long because we don't see migrating hot Jupiters very often."

"The long time scales we are observing here suggest that a leading migration mechanism may not be as efficient for hot Jupiter formation as once believed," said Laughlin. 

The Spitzer study suggests that competing theories for hot Jupiter formation -- in which gas giants form "in situ," or close to their stars, or smoothly spiral inward with the help of planet-forming disks -- may be preferred.

The new study is also the first to measure the rotation rate of an exoplanet orbiting a sun-like star. Spitzer observed changes in the planet's brightness as the planet spun on its axis, finding a rotation period of 90 hours. 

"Fifty years ago, we were measuring the rotation rates of planets in our own solar system for the first time. Now we are doing the same thing for planets orbiting other stars. That's pretty amazing," said Laughlin.

A rotation rate of 90 hours is much slower than what is predicted for HD 80606b, puzzling astronomers, and adding to the enduring mystique of hot Jupiters.

Additional study authors are: Nikole Lewis of the Space Telescope Science Institute in Baltimore; Jonathan Langton of Principia College, Elsah, Illinois; Drake Deming of University of Maryland, College Park; Konstantin Batygin of the California Institute of Technology, Pasadena; and Jonathan Fortney of the University of California, Santa Cruz.

NASA's Jet Propulsion Laboratory, Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at Caltech. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA. 

For more information about Spitzer, visit:  http://spitzer.caltech.edu / http://www.nasa.gov/spitzer

Technical journal article:  http://iopscience.iop.org/article/10.3847/2041-8205/820/2/L33


Media Contact

Whitney Clavin
Jet Propulsion Laboratory, Pasadena, California
818-354-4673
whitney.clavin@jpl.nasa.gov

Source:  JPL-Caltech/news