Introducing "Hoptunes", a New Class of Exoplanets that Could Help Solve the Mystery of Worlds in Scorching Orbits

(Originally published by the Kavli Institute for Astronomy and Astrophysics at Peking University)

March 15, 2018

In this exoplanetary collage, the left side is an artist's depiction of a hot Jupiter exoplanet in a tight orbit around its host sun. The right side depicts a newly described population of exoplanets, dubbed Hoptunes. These worlds range in size from a bit smaller to a bit larger than Neptune.
In this exoplanetary collage, the left side is an artist's depiction of a hot Jupiter exoplanet in a tight orbit around its host sun. The right side depicts a newly described population of exoplanets, dubbed Hoptunes. These worlds range in size from a bit smaller to a bit larger than Neptune. Like their bigger Jovian cousins, Hoptunes also encircle their stars in close, scorching orbits. The background displays some of the diversity of solar systems.(Credit: Composite image by Jin Ma at the Beijing Planetarium, using public domain and Creative Commons-licensed images with credits belonging to NASA/ESA/ESO; Danielle Futselaar and Franck Marchis, SETI Institute; NASA/JPL-Caltech; NASA's Goddard Space Flight Center; and NASA/SDO)

Among the most baffling worlds discovered so far in the universe are "hot Jupiters." These gas giants orbit their host stars far closer than the innermost planet in our Solar System, Mercury, orbits the Sun. Many astronomers think hot Jupiters could not have formed in such searing, star-kissed conditions, suggesting the planets somehow moved in toward their suns after initially taking shape.

Now a new study offers fresh insight into the planets' perplexing provenance, thanks to a newly described clutch of toasty worlds—dubbed Hoptunes—that are like hot Jupiters' smaller cousins. Led by Subo Dong of the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University and Ji-Wei Xie of Nanjing University, the study finds striking similarities between the two planetary types. Akin to their bigger brethren, Hoptunes often orbit stars with higher abundances of what astronomers call metals—elements heavier than helium. Hoptunes also tend to be loner worlds, again like hot Jupiters, hogging host stars all to themselves in single-planet solar systems.

Evidently, the processes that bring about Hoptunes likely extend to the rise of hot, giant planets, too, pointing to a shared, ultimately knowable origin.

"Understanding how hot Jupiters form has been a detective story for decades, and the discovery of Hoptunes adds important new clues to this ongoing investigation," said Dong, the Youth Qianren Research Professor of astronomy at KIAA. "Our study shows Hoptunes probably develop in similar conditions as hot Jupiters, which means we're zeroing in how those conditions permit scorching planets."

Dong coined the name "Hoptunes" for worlds that possess anywhere from two to six times the diameter of Earth. This size range goes a bit below and above the diameter of the planet Neptune, which has a diameter of four Earths—far less than the 9.5 and 11 Earths, respectively, needed to equal Saturn's and Jupiter's tremendous girths. The masses for Hoptunes remain unknown, however, so astronomers do not know which of them are rocky, like Earth, or mostly gaseous, like Neptune. Thus, Dong opted against broadly calling this planetary class "hot Neptunes," because some of them are likely more terrestrial than Neptunian in character.

The research team first got onto the trail of Hoptunes with Kepler, NASA's exoplanet hunting spacecraft. Kepler detects exoplanets through the slight dimming in starlight they cause when crossing the faces of their host stars.

The team dug deeper into a large set of close-in planets initially spotted by Kepler. In order to accurately measure the planets' sizes and the metal levels in their stars, the scientists turned to the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), located in northern China. Also known as the Guo Shoujing Telescope, it uses a technique called spectroscopy to break apart the light from stars, revealing their chemical makeup. Spectroscopy also indicates the strength of gravity at the surfaces of stars which, when cross-referenced with their color-coded temperature—hot stars shine blue, cool stars glow red—discloses their sizes. LAMOST can uniquely perform spectroscopy on thousands of stars simultaneously, providing astronomers with huge amounts of critical data.

"LAMOST is currently the world’s most efficient machine in mass-producing stellar spectroscopy," said Dong. "Using LAMOST, we were able to identify and characterize the solar systems and the host stars that harbor Hoptunes."

The similarities discovered between Hoptune- and hot Jupiter-hosting solar systems might support astronomers' working theories for how colossal worlds can form. Take the observed levels of metals, or metallicity, for instance. Some astronomers think higher metallicity means greater amounts of solid material available to form planets in the gassy, dusty disks surrounding young stars. Bit by bit, the materials in the disks glom together, growing into ever larger, rocky bodies. Particularly massive bodies with powerful gravitational pulls can capture deep atmospheres of gases, forming Jupiter-like worlds or, on the smaller side, Neptunes or Uranuses. Systems with low metallicities, however, struggle to generate big planets.

It is generally believed that giant planets need massive, solid cores to build up before they can accrete a large amount of gas. In close quarters to stars, not enough solid materials may be available to build up such suitably bulky cores. Therefore, hot Jupiters and gassy Hoptunes must somehow migrate toward their stars after initially forming. Yet the role that metal levels actually play in this migration remains unclear. One possibility is that disks with high metallicity could give birth to a large number of big planets, fostering violent gravitational interactions. This process might encourage some planets to migrate inward.

Finally, the migration process may also have something to do with why Hoptunes and hot Jupiters are usually the only planets in their respective solar systems. The inward movement of a large world can gravitationally kick out other planets, leaving behind just a single, bullying scorcher. Notably, the team also found that Hoptunes are somewhat less "lonely" than hot Jupiters, probably because their smaller sizes make them generally less capable of expelling their fellow planets.

To further unravel the origins of planets in tight orbits around their stars, Dong and colleagues are looking forward to soon having boatloads of new specimen worlds to study. The Transiting Exoplanet Survey Telescope (TESS), a spacecraft launching in March 2018 and led by the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology, should discover thousands of exoplanets around the closest, brightest stars. Many of the planets will be in tight orbits and, being nearby, quite amenable to detailed study.

"With TESS and other upcoming missions, we expect to find a lot more hot Jupiters and Hoptunes to study," said Dong. "I am especially looking forward to high-resolution spectroscopic studies of Hoptunes that could yield their masses, which could provide important evidence to crack the case of these roaster planets."

Other members of the research team and paper co-authors are Ji-Lin Zhou of Nanjing University, Zheng Zheng of the University of Utah, and Ali Luo of the National Astronomical Observatories of the Chinese Academy of Sciences. The research is funded, in part, by the National Natural Science Foundation of China, the Chinese Academy of Sciences, the Key Development Program of Basic Research of China, and the Foundation for the Author of National Excellent Doctoral Dissertation of People’s Republic of China.

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