Myriad weird worlds circle stars beyond our own Sun, and some of these oddballs are so exotic that, until they were discovered, their possible existence did not even enter the wildest dreams of planet-hunting astronomers. Indeed, the first exoplanet observed in orbit around a star similar to our Sun was unlike any of the major planets in our Solar System. This exotic, giant sizzler, dubbed 51 Pegasi b--now known as Dimidium–was discovered a generation ago, and it circled its star fast and close in a roasting orbit. Like our own Solar System’s banded behemoth, Jupiter, Dimidium proved to be a gas-giant world but, until its discovery, astronomers thought that planets like Jupiter could only exist in orbits farther out from their parent-stars–approximately where Jupiter dwells in the outer region of our Solar System. Dimidium was the first hot Jupiter to be discovered, but it was far from the last–and it doesn’t even carry the distinction of being the weirdest. That title may possibly go to a newly discovered football-shaped hot Jupiter dubbed WASP-121 b, that is so extremely hot that it sends its atmospheric “heavy metals”–such as magnesium and iron gas–singing into the space between stars. This observation represents the first time that so-called “heavy metals”–meaning atomic elements heavier than hydrogen and helium–have been observed escaping from a hot Jupiter.
WASP-121 b‘s parent star is both hotter and brighter than our Sun. The oddball planet is so dangerously close to its star that the temperature of its upper atmosphere soars to a roiling, broiling 4, 600 degrees Fahrenheit. A blast of ultraviolet light from the parent-star is heating up the tormented planet’s upper atmosphere, which results in causing the magnesium and iron gas to go yowling into space. Observations conducted by astronomers using the Hubble Space Telescope’s (HST’s) Imaging Spectrograph revealed the spectral signatures of magnesium and iron far, far away from the sizzling giant planet. Worse, the planet is so close to its stellar parent that it is doomed to soon be torn apart by the star’s gravitational tidal forces. Indeed, the merciless and relentless gravitational forces are so powerful that they have changed the planet’s shape from a sphere into a football. The WASP-121 system is almost 900 light-years from Earth.
The observations of WASP-121 b represent the first time “heavy metals” have been detected fleeing from a hot Jupiter. Usually, hot Jupiters are still relatively cool enough inside to condense heavier atomic elements into clouds. But this is not the case with the blazing WASP 121 b. “Heavy metals have been seen in other hot Jupiters before, but only in the lower atmosphere. So you don’t know if they are escaping or not. With WASP 121 b, we see magnesium and iron gas so far away from the plane that they’re not gravitationally bound,” explained Dr. David Sing in an August 1, 2019 NASA Press Release. Dr. Sing, who is of the Johns Hopkins University in Baltimore, Maryland, is the lead researcher of the new study.
Ultraviolet light streaming out from the parent-star heats the upper atmosphere and helps heavy metals flee from their sizzing host planet into the space between stars. Also, the escaping magnesium and iron gas may add to the soaring temperature, Dr. Sing added. “These metals will make the atmosphere more opaque in the ultraviolet, which could be contributing to the heating of the upper atmosphere,” he continued to note.
Alas, the tormented roasting planet hugs it parent-star so closely that it is on the cusp of being torn apart.”We picked this planet because it is so extreme. We thought we had a chance of seeing heavy elements escaping. It’s so hot and so favorable to observe, it’s the best shot at finding the presence of heavy metals. We were mainly looking for magnesium, but there have been hints of iron in the atmospheres of other exoplanets. It was a surprise, though, to see it clearly in the data and at such great altitudes so far away from the planet. The heavy metals are escaping partly because the planet is so big and puffy that its gravity is relatively weak. This is a planet being actively stripped of its atmosphere,” Dr. Sing continued to explain in the August 1, 2019 NASA Press Release.
According to the terminology astronomers use, a metal refers to any atomic element that is heavier than helium. Thus, atomic elements such as oxygen, carbon, and neon are classified by astronomers as metals. The term metal has a different meaning for astronomers than it does for chemists.
Strange Sizzling Gas Giants
Hot Jupiter exoplanets are unlike any of the major planets in orbit around our Sun. Before their initial discovery back in 1995, astronomers thought that gas giant planets, like Jupiter and Saturn, could only be born far from their stellar parents, in the cooler outer regions of their planetary systems. Unlike Jupiter and Saturn, these enormous gaseous roasters hug their parent-stars so closely that it normally takes them less than three days to complete a single orbit. This means that one hemisphere of these distant exotic worlds always faces its stellar parent, while the other face is always turned away–swathed in an unchanging shroud of perpetual darkness.
For this reason, the day side of a hot Jupiter is considerably hotter than its night side–and, of course, the hottest area of all is the region closest to its glaring parent star. It is thought that hot Jupiters also are tormented by strong winds that rage eastward close to their equators. This can sometimes displace the hot region toward the east.
After the surprising and historic discovery of Dimidium, new theories were quickly proposed to explain the existence of these star-hugging gas giants. Some planet-hunting astronomers suggested that these exotic sizzling worlds were really gigantic moten rocks. However, other planetary scientists proposed that they really were gas-giant planets that had been born approximately 100 times farther from their stars. Unfortunately, these ill-fated worlds were sent screaming towards their fiery stellar parents because of near smash-ups with other sibling planets–or, alternatively, by the gravitational jiggling of a binary stellar companion of their own star.
Another theory, that has been devised, also suggests that hot Jupiters were originally born as denizens of the outer portions of their planetary systems–at a distance similar to that of Jupiter in our Sun’s own family. Alas, these giant planets gradually lost energy because of destructive interactions with their protoplanetary accretion disks. These swirling, whirling disks are made up of gas and dust, and they circle young stars–but they also serve as the birthplace of baby planets. The neonatal gas giants, as a result of such interactions, begin to spiral closer and closer to the warm and well-lit inner regions of their planetary systems–closer to the gravitational embrace of their stars. Unfortunately, this migration means that the traveling planet is doomed, and it is destined to ultimately experience a violent and horrible end when it plunges into its stellar parent’s roaring fires.
Sizzling hot Jupiters are a diverse lot that nevertheless display certain important attributes in common:
–By definition they all have short orbital periods around their stellar parents.
–They all possess hefty masses.
–Many of them are of low density.
–Most have circular orbits around their stars.
In addition, hot Jupiters are usually not found in orbit around small red dwarf stars–which are the most abundant, as well as the smallest, true stars inhabiting our Milky Way Galaxy. Also, many of these exotic roasting planets are enshrouded by extreme and bizarre atmospheres that result from their short orbital periods.
Hot Jupiters are generally more common circling F-and G-type stars, but are less frequently seen orbiting K-type stars.
The Case Of The Searing-Hot, Football-Shaped Hot Jupiter
Dr. Sing and his colleagues used HST’s Imaging Spectrograph to hunt, using ultraviolet light, for the spectral signature of magnesium and iron. This signature would be imprinted on starlight, filtering through WASP-121 b’s atmosphere, as the distant world passed in front of (transited) the glaring face of its parent-star.
This searing-hot and weirdly-shaped alien planet is also a perfect target for NASA’s upcoming James Webb Space Telescope (JWST). The JWST will have the capacity to search for infrared light, indicating the presence of water and carbon dioxide, both of which can be seen at longer, redder wavelengths. The combination of HST and JWST images would provide astronomers with a more complete inventory of the chemical elements that compose the football-shaped planet’s atmosphere.
The WASP-121 b study is part of the Panchromatic Comparative Exoplanet Treasury (PanCET) survey, which is HST’s program that aims to hunt for 20 exoplanets, ranging in size from super-Earths (several times our planet’s mass) to Jupiter’s (more than 100 times our planet’s mass). This will be the first large-scale ultraviolet, visible, and infrared comparative study of distant alien worlds.
The observations of WASP-121 b contribute to the developing story of how planets lose their primordial atmospheres. When planet’s are born, they snare an atmosphere that contains gas originating in the accretion disk from which the planet and its parent-star emerged. These early atmospheres are composed primarily of the primeval, lighter-weight gases hydrogen and helium–both of which were born in the Big Bang birth of the Universe almost 14 billion years ago, and are the lightest and most abundant of atomic elements. Hydrogen and helium are not classified as metals in the terminology astronomers use. These primordial planetary atmospheres eventually dissipate as the baby planet travels ever closer and closer to its fiery, glaring parent-star.
“The hot Jupiters are mostly made of hydrogen, and Hubble is very sensitive to hydrogen, so we know these planets can lose the gas relatively easily. But in the case of WASP-121 b, the hydrogen and helium gas is outflowing, almost like a river, and is dragging these metals with them. It’s a very efficient mechanism for mass loss,” Dr. Sing explained in the August 1, 2019 NASA Press Release.
The results of this study are published in the August 1, 2019 online edition of The Astronomical Journal.