Hot Jupiters are wacky Wonder Worlds that cling closely to their parent-stars in incredibly speedy, roasting orbits. These bewitching behemoths are gas giant exoplanets with orbital periods that are less than 10 days long, and their extremely hot orbits around their roiling, broiling, fiery stars usually carry them less than 0.1 Astronomical Units (AU) from their stellar hosts–which amounts to only one-tenth of the distance between Earth and Sun. One AU is the distance between our planet and our Star, which is 93,000,000 miles. Ever since astronomers first calculated the immense size of these weird exoplanets almost a generation ago, they have confronted a tantalizing mystery–how did these searing-hot, distant, and puffy alien “oddballs” manage to grow so large? In November 2017, thanks to a recent discovery of tattle-tale twin planets, a team of astronomers are getting closer and closer to solving this puffy planet puzzle.
The astronomers, who detected this dynamic duo of twin planets, are led by graduate student Samuel Grunblatt of the University of Hawaii’s Institute for Astronomy (IfA). Thanks to this team of University of Hawaii astronomers, we are now getting closer to an answer of how these puffy, hot, giant planets form.
Gas giant planets are mostly made up of hydrogen and helium–the two lightest atomic elements in the Universe–and they are at least four times the diameter of Earth. There are no hot Jupiters in our own Solar System, and these alien and exotic planets hug their parent stars in scorching orbits–hence, their designation as “hot Jupiters”. These gas giant planets sport masses that are similar to the two gas giant planets inhabiting our own Solar System–Jupiter and Saturn. However, these enormous hot Jupiter worlds tend to be much larger than the two gaseous behemoths that orbit our Star in the chilly outer region of our Solar System, far from the golden light and heat of our Sun. Some of the hot Jupiter exoplanets are puffed up to enormous sizes that make them even larger than the smallest stars.
The first batch of exoplanet discoveries, that came at the end of the 20th century, were dominated by hot Jupiters. This is because they are the easiest exoplanets to discover by astronomers using the original radial velocity (Doppler) method and the transit method. The Doppler method favors the discovery of hot giant worlds hugging their parent-stars fast and close, because these behemoth worlds provide the greatest tug on their stellar parents. The Doppler method searches for a tiny repetitive wobble that indicates the presence of a giant world close to its star. The transit method, on the other hand, searches for an almost imperceptible blotting of a fiery star’s brilliant light when a planet travels in front of its glaring face. The larger the planet, the more stellar light is blocked.
Even though hot Jupiters dominated exoplanet discoveries at first, the occurrence rate differs by a factor of 2-3 between Doppler planet surveys and transit planet surveys.
Other nagging mysteries remain. For example, hot Jupiters are much too massive to be born close to their parent-stars in tight, roasting orbits. This is because of a lack of planet building materials close to their stellar hosts. One possible solution to this puzzle is that hot Jupiters are born further out, where there is sufficient material to build such gigantic planets, but then travel inward to their current broiling positions.
Several scenarios have been proposed to explain what triggers this inward migration. Some scientists think that an imbalance occurring in the protoplanetary accretion disk itself is the true culprit. Other scientists, however, suggest that the orbits of hot Jupiters are excited to an extremely high eccentricity. The high eccentricity causes the migrating hot Jupiter to wander towards the central star–approaching its fiery stellar parent so close that the orbital energy of the hot Jupiter is tidally dissipated. The tidal energy dissipation shrinks and circularizes the orbits of hot Jupiters. However, what triggers the high eccentricity is another mystery, and a source of considerable debate. Some astronomers propose that planet-planet scattering is the true underlying cause, while others think that the perturbation of a companion star is the true trigger.
Exoplanet “Roasters”
When the first hot Jupiters were detected about a generation ago, they were generally thought to be “oddballs” because we do not have anything like them in our own Solar System. However, as more and more of these bizarre, exotic, and puffed-up giant worlds were spotted over the last two decades, in orbit around distant stars beyond our own Sun, it began to look like our own Solar System is the true oddity.
Ever since the historic discovery of the first exoplanet in orbit around a Sun-like star, back in 1995, planet-hunting astronomers have been detecting a previously unknown, and well-hidden, treasure trove of weird, wild, and wonderful distant worlds. Some of these remote planets display an almost eerie similarity to the familiar planets inhabiting our own Solar System–while others are so exotic that their existence in nature both surprised and baffled their discoverers.
Hot Jupiters hug their parent-stars so closely that a “year” for them lasts only a few days. One of the most famous hot Jupiters, 51 Pegasi b, discovered in 1995, was the first exoplanet to be discovered circling a main-sequence (hydrogen-burning) star on the Hertzsprung-Russell Diagram of Stellar Evolution. 51 Pegasi b has an orbital period of about 4 days. This initial discovery of a hot Jupiter proved to be a surprise for planet-hunting astronomers who did not think that such close-in, giant, gas-laden worlds could really exist in nature. The mystery surrounding the formation of this very alien form of exoplanet has plagued the astronomical community for more than twenty years.
Even though the discovery of literally thousands of exoplanets has now become “business as usual” for astronomers on the hunt for these remote worlds, this has not always been the case. Indeed, the search for planets belonging to the families of stars beyond our own Sun, historically proved to be extremely challenging–as well as frustrating. At last, back in 1992, the first batch of truly weird exoplanets to be validated were detected in orbit around a very small, dense, and rapidly spinning stellar corpse termed a pulsar. Dr. Alexander Wolszczan of Pennsylvania State University, after carefully observing radio emissions flowing out from a compact millisecond pulsar with the unexciting name of PSR B1257 +12, made the determination that it was being circled by several very exotic little worlds. A pulsar is only about 12 miles in diameter–and it is really the collapsed core of what was once a massive main-sequence star. This strange, dense, and tiny stellar “oddball” is all that is left of a star that has finished burning its necessary supply of hydrogen fuel, and has “died” in the horrific, brilliant, and explosive tantrum of a supernova blast.
51 Pegasi b was discovered three years later by Dr. Michel Mayor and Dr. Didier Queloz of Switzerland’s Geneva Observatory. This discovery was quickly confirmed by a team of American planet-hunting astronomers using the Lick Observatory’s three-meter telescope poised at the summit of Mount Hamilton in California.
Of course, new theories were proposed to explain these “oddball’ hot Jupiters. Some astronomers suggested that these “roasters” were really enormous molten rocks; while still others proposed that they were gas-giant planets that had been born about 100 times further away from their parent-stars. According to this latter theory, hot Jupiters were ruthlessly thrown about 100 times closer to their stellar parents as a result of near-collisions with other sibling worlds. Alternatively, a binary stellar companion of their host star may have been the culprit behind this tragic kick towards their fiery, roiling stellar parent.
One theory put forward suggests that hot Jupiters are born at a distance from their star that is approximately the same as that of our own Solar System’s banded behemoth, Jupiter’s, distance from our Sun. Alas, these ill-fated giant worlds slowly lose energy as a result of their unfortunate dance with the protoplanetary accretion disk, which is a disk of gas and dust surrounding their parent-star, from which planets eventually emerge. The newborn giant planet, as a result, spirals into the well-lit and seething-hot inner regions of its planetary system, coming in from its much colder and very remote place of birth.
Hot Jupiters are likely doomed giants, destined to come to a final, fiery, and truly miserable end within the furious furnaces of their glaring parent-stars. However, until that final, fatal moment, these very unfortunate “roasters” orbit their host stars fast and close.
These puffy “roasters” are actually a mixed bag, displaying some diversity in their attributes. However, these exoplanets do share certain characteristics. All hot Jupiters have very low densities, large masses, brief orbital periods around their parent-stars, and almost circular orbits. Hot Jupiters also are likely to possess extreme and exotic atmospheres because of their brief orbital periods, relatively long days, and tidal locking.
A Dynamic Duo Of Alien “Oddballs” May Solve The Puffy Planet Puzzle
The enormous size of these puffy “oddballs” is probably related to the heat that flows in and out of their bloated atmospheres. Several theories have been devised to explain hot Jupiters’ fluffiness. “However, since we don’t have millions of years to see how a particular planetary system evolves, planet inflation theories have been difficult to prove or disprove,” Samuel Grunblatt explained in a November 27, 2017 University of Hawaii (IfA) Press Release.
In order to solve this longstanding puffy planet puzzle, Grunblatt sifted through data obtained by NASA’s K2 Mission to go on the hunt for hot Jupiters in orbit around red giant stars. These big, crimson, and relatively cool members of the stellar zoo are in the end stages of their “lives”, and are themselves growing significantly more bloated over their orbiting hot Jupiters’ “lifetime”. Taking into consideration a theory proposed by Dr. Eric Lopez of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, Grunblatt decided to go on the hunt for hot Jupiters orbiting red giant stars. This is because, according to Dr. Lopez’s theory, these stellar red giants should be greatly inflated if direct energy input from the parent-star is the primary process responsible for inflating these puffy hot Jupiters.
The hunt has now detected a dynamic duo of twin planets, each orbiting their parent star with a period of approximately 9 days. Using stellar oscillations to calculate precisely the radii of the planetary twins and their parent-star, Grunblatt’s team found that the duo are 30 percent larger than Jupiter. Observations using the W.M. Keck Observatory atop Mauna Kea in Hawaii revealed that, despite their jumbo sizes, the two planets are only half as massive as Jupiter. Due to a remarkable stroke of good luck, the pair of planets are near-twins in respect to their orbital periods, masses, and radii.
Using models to track the evolution of the puffy pair and their parent-star over time, the team calculated the planets’ efficiency at absorbing heat from their star, and then transferring it deep down into their secretive interiors–thus causing the whole planet to become increasingly more and more bloated in size, while decreasing in density. The IfA team found that these planets probably required the increased radiation emanating from their red giant star in order to inflate. However, the quantity of radiation absorbed was lower than the astronomers had suspected.
In astronomy, it is considered to be premature to come to strong conclusions based on only two examples. However, in this case, the results have begun to rule out some of the explanations of puffy planet inflation, and are also consistent with a scenario whereby planets are directly influenced by the heat flowing out from their parent-stars. The accumulating scientific evidence suggests that stellar radiation alone can, indeed, change both the size and density of an orbiting planet.
Our own Sun will become a red giant star when it approaches the end of that long stellar road. Right now, it is a small middle-aged star of about 4.56 billion years of age, and so it has about another 5 billion years to go before it reaches its grand finale. Because our Sun is doomed to become a bloated red giant as it comes to the end of its “life”, it’s important for astronomers to quantify the effects its evolution will have on the rest of our Solar System. “Studying how stellar evolution affects planets is a new frontier both in other solar systems as well as our own. With a better idea of how planets respond to these changes, we can determine how the Sun’s evolution will affect the atmosphere, oceans, and life here on Earth,” commented Grunblatt in the November 27, 2017 University of Hawaii, IfA Press Release.
And, so, the quest to discover gas giant planets in orbit around red giant stars continues, since additional systems could conclusively distinguish between differing puffy planet scenarios. Grunblatt and his team have been awarded time with NASA’s Spitzer Space Telescope so that they can measure the sizes of these twin bloated planets more accurately. Also, the hunt for planets orbiting red giants will continue with the NASA K2 Mission for another year, and NASA’s Transiting Exoplanet Survey Satellite (TESS), scheduled to launch in 2018, will observe hundreds of thousands of red giants across the entire sky.
