The last decade has seen the discovery of a treasure trove of exoplanets. Indeed, almost 2,000 of these distant, alien planets, that belong to the families of stars beyond our own Sun, have been confirmed–so far! Many of these distant, exotic worlds belong to a strange class termed hot Jupiters, which are gas-giants like our own Solar System’s behemoth Jupiter, but are much hotter because they sport orbits that carry them within a “roasting” distance of their fiery stellar parents. But as common as hot Jupiters are thought to be, their formation history is still heavily shrouded in mystery–how did these gigantic, roasting worlds manage to get so close to their searing-hot and roiling parent-stars? That is the question! In March 2016, a team of astronomers using NASA’s infrared Spitzer Space Telescope (SST), announced that they have found an answer to this intriguing riddle.

When the very first batch of hot Jupiters were discovered a generation ago, they were generally considered to be “oddballs” because we do not have anything like them in our Sun’s family. However, as more and more of these exotic and very distant worlds were spotted over the last two decades (along with many other smaller exoplanets that also hug their parent-stars fast and close in roasting orbits), our Solar System began to look like the real oddity.

“We thought our Solar System was normal, but that’s not so much the case,” commented Dr. Greg Laughlin in a March 28, 2016 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Laughlin is an astronomer at the University of California, Santa Cruz, and co-author of the new SST study that investigates hot Jupiter formation. The study has been accepted for publication in The Astrophysical Journal Letters.

How did these weird and exotic worlds wind up so meltingly close to their searing-hot parent-star?

The SST found new clues by watching a hot Jupiter dubbed HD 80606 b, situated about 190 light-years from our planet. This brave new world is weird because it has a wildly eccentric orbit that resembles that of a comet, swinging it very close to its parent-star, and then hurling it out again to a considerably greater distance repeatedly every 111 days. One side of this “oddball” is thought to become dramatically hotter than the other as the tortured world closely approaches its star. Indeed, when the hot Jupiter is closest to its stellar-parent, the side facing the star rapidly reaches a broiling temperature of over 2,000 degrees Fahrenheit.

“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,” Dr. Laughlin continued to explain.

Searing Hot Denizens Of The Exoplanet Zoo

Ever since the historic discovery of the first exoplanet a generation ago, planet-hunting astronomers have been detecting a previously hidden treasure of wild, weird, and wonderful faraway worlds. Some of these distant worlds display an almost eerie likeness to the familiar planets in our own Solar System–while others are so different that they appear intriguingly bizarre.

Hot Jupiters hug their parent stars closely, circling them in roasting hell-like orbits–with a “year” lasting only a few days. Sometimes alternatively termed roaster planets, epistellar jovians, pegasids, or pegasean planets, these weird worlds were some of the first to be discovered. This is because they are the easiest planets to detect using the radial-velocity method, which was the first method used to successfully spot exoplanets. The radial-velocity method detects the oscillations that these planets induce in their parent-stars’ motions, and the relatively large and rapid motions induced by close-in giant planets in tight orbits are the easiest planets to detect using this technique. One of the most famous hot Jupiters is 51 Pegasi b, which was discovered in 1995, and it was the first exoplanet to be discovered in orbit around a main-sequence (hydrogen-burning) Sun-like star. 51 Pegasi b sports an orbital period of about 4 days. This first discovery of a hot Jupiter surprised planet-hunting astronomers who did not think that such close-in, giant, gaseous worlds could exist. The mystery surrounding this exotic form of exoplanet has plagued the astronomical community for over twenty years.

As of April 2, 2016, 2,107 exoplanets have been discovered and confirmed, dwelling within 1349 planetary systems, including 511 multiple planetary systems. In addition, there are free-floating “orphan” exoplanets that are not bound to any parent-star at all, but instead travel as solitary objects through the space between stars, without the comforting companionship of a stellar parent or planetary siblings. Alas, once these planetary “orphans” were members of a system, but they were booted out of their home as a result of the gravitational bullying of sibling planets, that cast them out into the cold, to travel lost and lonely through the darkness of interstellar space.

Even though the discovery of thousands of alien planets has become “business as usual” for astronomers on the hunt for these distant 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 a difficult endeavor–and often quite frustrating. At long last, in 1992, the very first batch of truly weird exoplanets were spotted circling a small, dense, and rapidly spinning stellar corpse called a pulsar. Dr. Alexander Wolszczan of Pennsylvania State University, after carefully watching radio emissions emanating from a compact millisecond pulsar with the colorless name of PSR B1257 + 12, made the historic determination that it was being orbited by several extraordinarily bizarre small 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 on the Hertzsprung-Russell Diagram of stellar evolution. This strange, dense, small stellar ghost is the leftovers of a star that has burned its necessary supply of hydrogen fuel, and has perished in the fiery explosion of a supernova blast.

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

New theories were promptly devised to explain these very strange hot Jupiters. Some astronomers suggested that these bizarre “roasters” were huge molten rocks; while others proposed that they were gas-giant planets born about 100 times farther away from their parent-stars. According to this latter theory, hot Jupiters were tragically hurled back towards their stellar-parent as a result of near-collisions with other sibling worlds, or possibly even a binary companion of their own star.

One theory proposes that hot Jupiters are born at a distance from their star that is comparable to that of our own Solar System’s Jupiter around our Sun–but then they slowly lose energy as a result of interactions with the protoplanetary accretion disk, composed of gas and dust, from which they formed. The neonatal giant, as a result, spirals into the well-lit and hot inner regions of its planetary system, far from its colder and more remote birthplace.

Hot-Jupiters may well be doomed giants, destined to experience a violent, fiery death within the furious furnaces of their glaring parent-stars. However, until that final, fatal moment, these unlucky “roasters” circle their stars fast and close–and they are very, very hot!

Actually, hot Jupiters are a diverse bunch. However, they do share certain defining properties:

–They possess large masses and brief orbital periods.

–Most have almost circular orbits.

–Many have unusally low densities.

–They likely harbor exotic, extreme atmospheres as a result of their short periods, relatively long days, and tidal locking.

–They appear to be more prevalent around F- and G-type stars, but less so around K-type stars. They are rarely found circling red dwarf stars. Red dwarfs are the smallest of true stars, as well as the most abundant type of star in our Milky Way Galaxy.

The prevailing view among astronomers is that hot Jupiters form further away from their star, and then migrate inward.