Hoptunes are a population of exoplanets recently discovered jointly by a number of institutions including the Planetary Project Team of the School of Astronomy and Space Science, Nanjing University, and the Kavli Institute for Astronomy and Astrophysics, Peking University.
The astronomers made the discovery by using the Guo Shoujing Telescope of the National Astronomical Observatories of China (NAOC). The telescope is also known as the Large Sky Area Multi-Object Fiber Spectroscopic Telescope or LAMOST.
The discovery was reported in the article titled “LAMOST telescope reveals that Neptunian cousins of hot Jupiters are mostly single offspring of stars that are rich in heavy elements,” printed on the January 9, 2018, issue of PANS, a journal of the American Academy of Sciences. The journal also highlighted the title of this article on its cover as one of the best on that issue.
Research fellow Dong Subo, Peking University, and Associate professor Xie Jiwei, Nanjing University, are both the first authors and corresponding authors of this paper.
Other members of the research team include Professor Zhou Jilin, Nanjing University, Professor Zheng Zheng, University of Utah, and Research fellow Luo Ali, NAOC, Chinese Academy of Sciences.
Fig.1 . Imaginary picture of the Hoptunes. This picture shows a member of the newly discovered “Hoptunes” population (lower right), a member of the first discovered exoplanet population “hot Jupiters” (lower left), and in the background, the diverse planetary systems discovered by the Kepler satellite. (Edited and drafted by Ma Jing, Beijing Planetarium)
“Lonely” but metal-rich hot Jupiters
The discovery of the first exoplanet 51 Pegasi b in 1995 was a milestone in the history of astronomy because it broadened the horizon of humankind and revealed a pretty unique population of exoplanets. Since then, exploring exoplanets has always been an internationally hot topic and frontier of research.
Such exoplanets are called hot Jupiters because they are about the size of the Jupiter and they have high temperature on the surface as a result of proximity to their host stars: it is less than 1 tenth of the Sun-Earth distance while the Jupiter-Sun distance is more than five times the Sun-Earth distance.
For the past two decades since the discovery of the first exoplanet, scientists have found and studied over a hundred hot Jupiters, but their formation and origin remain elusive.
Among the over 1,000 exoplanets that have been discovered, the population of hot Jupiters is characterized by some important properties. First, they are rare. About one out of 100 stars hosts one hot Jupiter.
Second, their host stars are predominantly more metal rich (it means they have a higher proportion of chemical elements which are heavier than hydrogen and helium) than the Sun.
Moreover, they are “lonely” for the dearth of other planets on nearby orbits in their systems.
In a word, hot Jupiters are rare and mostly single children of the stars that are rich in heavy elements. Such characteristic properties offer important clues to their formation and origin.
Fig. 2 . Guo Shoujing Telescope (a.k.a. LAMOST) at Chinese Academy of Sciences’ Xinglong Station of NAOC, Hebei Province
The LAMOST: Surfing in the sea of stars and planets
Is there any other planet population sharing key similarities with hot Jupiters? The answer will undoubtedly offer fresh clues about the origin of hot Jupiters.
Over the past few years, NASA’s Kepler satellite has achieved great success in finding thousands of exoplanets. This brings an opportunity to explore the formation of hot Jupiters.
The key step is to determine the basic properties of these planetary systems such as planet radius and the host star metallicity, and the measuring of these properties requires spectroscopic observation of fixed stars. However, most telescopes are not powerful enough because the Kepler satellite has identified more than 200,000 stars and they need observation with an extremely high spectrum acquiring rate.
China’s LAMOST (Fig. 2.), with a unique design, can observe the spectra of thousands of celestial objects at the same time in a wide field of view and is the telescope with the highest spectrum acquiring rate. This makes the telescope unique in the study of exoplanet science.
In recent years, LAMOST has observed tens of thousands of stars in Kepulatian area, including host stars of over 1,000 planets, and obtained their spectra.
The team found that compared with other high-precision methods such as stellar seismology and high resolution spectra, the spectra obtained through LAMOST are reliable in measuring the basic properties of stars. High-precision LAMOST star data can make high-precision measurements of key properties such as the planet radius and make it possible to accurately classify and systematically explore hundreds of exoplanets.
Fig. 3 . The discovery of Hoptunes. The three metallicity subsamples show the distribution of the planets’ orbital period (Perid) and radius (RP) as affected by the increase of the metal abundance ([Fe/H]), from bottom to top. The planets in single- and multiple-transiting planetary systems are plotted in blue and red dots, respectively. The dark green horizontal line denotes the empirical lower boundary of hot Jupiters, and the magenta lines denote the empirical boundaries of Hoptunes. LAMOST observations offer precise star radii and host metallicities, and the star radii can be used to determine planet radii (while Kepler photometric observations can only give the ratio of planet radius to star radius).
Hoptunes – Cousins of hot Jupiters and new members of the planet family
Using accurate stellar parameters of LAMOST, the research team studies the orbital period and the radius distribution for the planets with different metallicity (Fig. 3).
They discover that in the metal-poor sample ([Fe/H] < -0.1), almost all the planets lie in the right and bottom part of period- radius plane. With the increase of metallicity, planets gradually appear in other areas in the plane; among the new planets are hot Jupiters, mostly “single children” of stars that are rich in heavy elements (the radius of the planets in the part above the dark green line in Fig.3 is ten times larger than that of the Earth).
Besides, the team was surprised to find that as the metallicity increases, some planets also appear in the magenta frame, and they appear together with hot Jupiters. Most of them are “lonely” single-transiting systems just like hot Jupiters.
The team calls the new planet population Hoptunes. The Hoptunes are about the size of the Neptune, with a radius about four times that of the Earth. Their sizes, however, vary widely, ranging from twice to six times the size of the Earth.
It is not yet clear whether their physical composition is the same as the Neptune in the solar system. That is why they are not called hot Neptunes.
By analyzing the LAMOST data, the research team eventually found the cousins of hot Jupiters, Hoptunes, which are as “rich” and “lonely” as hot Jupiters. Like hot Jupiters, Hoptunes are hosted by about one out of 100 solar-type stars.
Through accurate radial-velocity (RV) observation into the stellar system of Hoptunes in the future, the team is confident that it will measure the mass of Hoptunes and further study their composition and formation.
Over the last two decades, people have put forth many theories about the mechanisms of the formation and development of hot Jupiters, but there has been no consensus on their origins.
Based on the LAMOST data, the research team has newly discovered characteristics shared by the planet populations Hoptunes and hot Jupiters, and this has provided key clues and a brand-new research direction for exploring the mystery of the origins of hot Jupiters and other short-period planets.
This study has been supported by the National Natural Science Foundation of China, LAMOST Distinguished Scholars of Chinese Academy of Sciences, and the Special Fund for National Excellent Doctoral Dissertations.