Hot Jupiter Planets: Any Way The Wind Blows

Hot Jupiter Planets: Any Way The Wind Blows

By Judith E Braffman-Miller

A generation ago, planet-hunting astronomers discovered the first exoplanets in orbit around distant stars beyond our own Sun--and these new discoveries have since provided a treasure chest filled with remarkable brave new worlds for scientists to ponder. Some of these remote planets show an almost eerie resemblance to the eight major planets in our own Solar System, while others are so exotic that they are unlike anything astronomers believed could exist--until they actually observed them. Hot Jupiter exoplanets do not resemble any of the major planets that circle our own Star, and these strange, distant, enormous, and gaseous worlds orbit their own parent-stars fast and close in roasting orbits. Although hot Jupiters bear a close resemblance to our own Solar System's banded behemoth gas-giant Jupiter, they are much closer to their parent-stars than Jupiter is to our Sun. Indeed, before their discovery, astronomers assumed that such gas-giant planets could only form far from their stars, in the outer regions of their planetary systems, where our own Jupiter is situated. In January 2018, a team of astronomers discovered yet another intriguing mystery about these bizarre gigantic worlds--the hottest region on a hot Jupiter exoplanet, closely clinging to its parent-star, isn't where astrophysicists expected it to be. This discovery challenges our scientific understanding of the numerous planets, of this exotic type, that are found in planetary systems orbiting stars beyond our own.

Unlike our Jupiter, hot Jupiters are so amazingly close to their parent-stars that it usually takes them less than three days to finish an orbit. In addition, one hemisphere of these distant worlds always faces its stellar parent, while the other face is always turned away from it, perpetually locked in darkness.

Of course, the "day" side of a hot Jupiter gets considerably hotter than the "night" side, and obviously the hottest point of all should be the region closest to the roiling, broiling star. Astrophysicists have proposed that these exoplanet "roasters" also experience powerful winds roaring eastward near their equators, which can at times displace the hot spot toward the east.

However, a mysterious hot Jupiter dubbed CoRoT-2b seems to travel to the beat of a different drummer. The hot spot on CoRoT-2b turns out to be located in the opposite direction--that is, it lies west of center. A team of astronomers at McGill University's McGill Space Institute (MSI) and the Institute for research on exoplanets (iREx) in Montreal, Canada, made this surprising discovery using NASA's Spitzer Space Telescope. Their findings are published in the January 22, 2018 issue of the journal Nature Astronomy.

"We've previously studied nine other hot Jupiter giant planets orbiting super close to their star. In every case, they have had winds blowing to the east, as theory would predict. But now, nature has thrown us a curveball. On this planet, the wind blows the wrong way. Since it's often the exceptions that prove the rule, we are hoping that studying this planet will help us understand what makes hot Jupiters tick," noted Dr. Nicolas Cowan in a January 22, 2018 McGill University Press Release. Dr. Cowan is a McGill astronomer, and a study co-author. He is also a researcher at MSI and iREx.

Distant "Roaster" Planets

The eight major planets of our Sun's family display a wide range of attributes. They are distinguished by two primary properties: their size and their orbit. The size of one of our Solar System's planetary denizens determines if it can have an atmosphere capable of sustaining life. The orbit affects the surface temperature and whether there could be life-loving liquid water pooling on the planet's surface. A planet that is considered to be potentially habitable not only should have liquid water on its surface, it must also be between about 80% to 200% the diameter of Earth.

The habitable zone surrounding a star is that "Goldilocks" region where the temperature is not too hot, not too cold, but just right for water to exist in its liquid phase. Liquid water is necessary to sustain life as we know it. Planets that circle their star in this "Goldilocks" region have the potential--though by no means the promise--of being inhabited by life as we know it.

Planets that are smaller than 8/10ths of an Earth-diameter, and also possess less than 50% an Earth-mass, do not have a sufficiently strong gravitational grip to hold onto a life-sustaining atmosphere. Conversely, planets that are more than twice the diameter of Earth sport about ten Earth-masses and sufficient gravity to hold onto hydrogen. Hydrogen is both the most abundant, as well as the lightest, atomic element. Such behemoth planets evolve into gas-giants like Jupiter and Saturn. Jupiter is more than ten times the diameter of our planet and over 300 times more massive.

The first exoplanet to be discovered circling a main-sequence (hydrogen burning) star, similar to our Sun, was 51 Pegasi b (51 Peg b, for short). Even though 51 Peg b has about the same mass as Jupiter, it is twenty times closer to its star than Earth is to the Sun. Jupiter is five times as far from our Sun as Earth, and it takes 12 years to orbit our Star. In sharp contrast, 51 Peg b orbits its star every 4 days.

51 Peg b was discovered in 1995 by astronomers using the radial velocity method, which was the first method used to successfully detect faraway exoplanets. The radial velocity technique spots the oscillations (wobbles) that these planets induce in the movements of their stellar parents, and the relatively large and rapid motions caused by giant, close-in planets in star-hugging orbits, are the easiest planets for astronomers to spot using this method. That is because this type of planet causes the greatest "wobble" in its parent-star. When 51 Peg b was discovered, it surprised planet-hunting astronomers because they did not think such giant, close-in, gas-laden worlds could exist.

As of January 1, 2018, there are 3,726 confirmed exoplanets in 2,792 systems, with 622 systems hosting more than one planet. There is also a population of free-floating (orphan) planets that do not belong to the family of any star at all, but instead wander as solitary worlds through interstellar space, without the compansionship of a stellar parent or planetary siblings. It is generally thought that originally these "orphan" worlds were members of a planetary system, but they were evicted from their original home as a result of the gravitational jostling of sibling planets. These sibling worlds hurled these tragic "orphans" out into the cold darkness of the space between stars, to wander alone and lost through the interstellar wilderness.

The first sign of an exoplanet beyond our Solar System was noted as early as 1917, but its true identity was not recognized at the time. The first scientific discovery of an exoplanet was in 1988. Soon afterwards, the first validated detection came in 1992.

Today, the discovery of thousands of exoplanets has become almost routine for astronomers who are on the hunt for distant worlds. The search for planets belonging to the families of remote stars, beyond our Sun, historically proved to be a difficult quest. At last, in 1992, the very first batch of genuinely bizarre exoplanets were discovered in orbit around a small, dense, and rapidly whirling stellar corpse termed a pulsar. Dr. Alexander Wolszczan of Pennsylvania State University, after carefully studying radio emissions streaming out from a compact millisecond pulsar with the drab name of PSR B1257+12, made the historic announcement that it was being circled by several extraordinarily weird small worlds. A pulsar is only about 12 miles in diameter--and it is actually the collapsed core of what was once a massive main-sequence star on the Hertzsprung-Russell Diagram of Stellar Evolution. This genuinely bizarre, dense, tiny stellar relic is all that is left of a heavy star that has finished burning its necessary supply of hydrogen fuel, and has perished in the fabulous final blaze of glory that characterizes a brilliant supernova blast.

In 1995, 51 Peg b was discovered in orbit around a normal Sun-like star. This discovery was first made by Dr. Michel Mayor and Dr. Didier Queloz of Switzerland's Geneva Observatory. It was soon confirmed by a team of American planet-hunting astronomers using the Lick Observatory's three-meter telescope atop Mount Hamilton in California.

Following the historic discovery of 51 Peg b, new theories were quickly devised to explain the surprising existence of hot Jupiters. Some astronomers proposed that these strange "roasters" were actually enormous molten rocks, while others suggested that they were gas-giant planets that had been born about 100 times farther away from their parent-stars. According to the latter theory, hot Jupiters were shot back towards their roiling, broiling parent-stars as a result of near-collisions with other sibling planets--or, possibly, by the gravitational jostling of a binary stellar companion of their own star.

One proposal suggests that hot Jupiters are born much farther from their parent-star, before they make their journey inward, at a distance that is comparable to that of our Solar System's Jupiter. This is because these "roasters" gradually lose energy as a result of interactions with what is termed the protoplanetary accretion disk. This disk, composed of gas and dust, swirls around young stars, and it is the birthplace of their retinue of orbiting planets. The newborn giant planet, as a result of this interaction, spirals inward towards the searing-hot and bright inner regions of the planetary system, closer to its youthful parent-star. As a result of its unfortunate journey towards its roiling, broiling stellar parent, the baby gas-giant travels very far from its more remote birthplace, far from its star, where it is both much colder and darker.

Hot Jupiter exoplanets may be doomed behemoths, destined to undergo a very violent demise within the seething-hot furious furnaces of their glaring parent-stars. However, until they make their final, fatal plunge into their stars' churning fires, these unfortunate "roasters" orbit their stellar-parents fast and close.

Roasting hot Jupiters actually are a diverse bunch. However, they do have certain properties in common:

--Many show low densities.

--Most have circular orbits.

--By definition they all possess large masses and short orbital periods around their parent-stars.

--They are apparently more common in orbit around F-and G-type stars, but less common around K-type stars. They are usually not found circling tiny red dwarf stars. Red dwarfs are both the most abundant, as well as the smallest, true stars to populate our Milky Way Galaxy.

--Many hot Jupiters are blanketed by extreme and exotic atmospheres, resulting from their short orbital periods, relatively long days, and tidal locking.

The Strange Case Of CoRoT-2b

The strange "roaster", dubbed CoRoT-2b, was discovered a decade ago by a French-led space observatory mission. This nonconformist alien world resides 930 light-years from our planet and, even though many other hot Jupiters have been discovered in recent decades, CoRoT-2b has continued to sing a sirens' song to astronomers because of its two mysterious attributes: its bloated size and the exotic spectrum of light emissions coming from its surface.

"Both of these factors suggest there is something unusual happening in the atmosphere of this hot Jupiter," commented Lisa Dang in the January 22, 2018 McGill University Press Release. Ms. Dang, who is the lead author of the new study, is a doctoral student at McGill. By using Spitzer's Infrared Array Camera to observe the planet while it finished an orbit around its parent-star, the astronomers were able to map the planet's surface brightness for the first time. This is how its unusual westward hot spot was discovered.

The team of astronomers offer three possible explanations for the puzzling attributes of CoRoT-2b:

--The planet could be spinning so slowly that a single rotation takes longer than one full orbit of its parent-star. This slow rotation could create winds that blow toward the west rather than the east. However, this possibility could weaken theories about planet-star interactions.

--CoRoT-2b's atmosphere could be interacting with its magnetic field to modify its wind pattern. This possibility could give astronomers the rare chance to study an exoplanet's magnetic field.

--Large clouds blanketing the eastern side of CoRoT-2b might make it appear darker than it would otherwise. This possibility could challenge current models of atmospheric circulation on such planets.

Lisa Dang commented in the January 22, 2018 McGill University Press Release that "We'll need better data to shed light on the questions raised by our finding. Fortunately, the James Webb Space Telescope, scheduled to launch next year, should be capable of tackling this problem. Armed with a mirror that has 100 times the collecting power of Spitzer's, it should provide us with exquisite data like never before."

The new research is published under the title: Detection of a westward hotspot offset in the atmosphere of hot gas giant CoRoT-2b, published in the January 22, 2018 issue of Nature Astronomy. Study co-authors are Lisa Dang, Nicholas B. Cowan, and Joel C. Schwartz.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, newspapers, and magazines. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.

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