Producer: Popular Science China
Producer: Zhao Fei, Fan Zhou (National Astronomical Observatory, Chinese Academy of Sciences)
Producer: Computer Network Information Center, Chinese Academy of Sciences
Do there exist many worlds, or is there but a single world?
This is one of the most noble and exalted questions in the study of Nature.
——Saint Albertus Magnus (c. 1206–1280)
How many are there in the world? Or is there only one? This is one of the noblest and most exciting questions in the study of nature.
- St. Albertus Magnus (1204–1280 AD)
The night sets on the galaxy, and the sky is full of stars.
In today's continuous development of science and technology, we know that there are still many stars such as the sun in the universe, and most of them also have planets around.
Image credit: veer gallery
Planets are derived from the evolution of star children in the protoplanetary disk, and planets have a variety of forms and types depending on the formation environment and evolution trajectory. For a long time, it was widely believed that exoplanets were similar to planets within the solar system. It wasn't until 1995 that the discoveries of Swiss astronomers Mayor and Queloz broke the shackles of humanity's thinking in the search for planets. They discovered a completely new planetary form, hot Jupiter.
Hot Jupiter is very different from the eight planets in the solar system, they are as massive as Jupiter, but their orbital period is extremely short, less than 10 days, while Jupiter's orbital period is 12 years.
Recently, a team of astronomers from MIT discovered a hot Jupiter with an orbital period of only 16 hours, a hot behemoth that has set a new record for the "shortest new year" for a gas giant planet. The study was published in The Astronomical Journal.
Super-hot Jupiter, artistic imagination (Credit: ESA/ATG medialab, CC BY-SA 3.0 IGO)
Planetary Travel Guide: Hot Jupiter
So far, humans have discovered more than 4,800 exoplanets, of which hot Jupiter accounts for only 9%, while hot Jupiters around Sun-like stars are even rarer, accounting for only 0.5%.
(Image source: Author, Tool: Python.) Data downloaded from The Extrasolar Planets Encyclopaedia database)
Exoplanet distribution map (The abscissa is the orbital period of the planet, and the ordinate is the minimum mass of the planet. The data points in the figure represent 4800 discovered exoplanets, and the different colors represent different detection methods. The yellow shaded area is the range of hot Jupiter, and the planet in the upper left corner is TOI-2109 mentioned here. ）
For astronomers, such bizarre planets are crucial, they are very different from the giant planets in the solar system that we are familiar with, and the study of them is expected to reveal the origin and formation of planetary systems, thereby deepening human understanding of the birth and evolution of life.
The discovered hot Jupiter, numbered TOI-2109b, is located in the constellation of Bucher, about 855 light-years from Earth, orbiting an F-shaped main sequence star. The planet has a radius of 1.35 times that of Jupiter, but is 5 times more massive than Jupiter, making it a "behemoth." However , it orbits very close to the Sun , with an orbital radius of only 0.018 AU ( astronomical unit , 1 AU = solar-Earth distance ), about 1/22 of the orbital radius of Mercury.
Figuratively speaking, this picture is equivalent to us standing in God's perspective, rubbing Jupiter a little larger, and then throwing it directly into the innermost orbit of Mercury 1/22.
According to Kepler's third law, the smaller the orbital radius of a planet, the shorter the orbital period. So, let's say we're standing on this hot Jupiter, there are only 16 hours a year.
Faced with this wonderful scene, we must have many questions: Why is it so different from the planets of our solar system? How was it born? Why does it appear in such a close orbit? To answer these questions, we first have to know, where does hot Jupiter come from?
Attack on Hot Jupiter: From past lives to this life
When it comes to gas giants, many people think of Jupiter and Saturn. They are mainly composed of two elements, hydrogen and helium. In their center are solid cores composed of rock or ice, the middle layer is composed of hydrogen and helium in the liquid metal phase under high pressure, and the outer layer is a large amount of hydrogen and helium that gradually transition from liquid to gaseous. Since the center is so small, we can simply think of a gas giant planet as a giant ball of gas.
Hot Jupiters are relatively close in composition to gas giants in the Solar System, but their formation process varies greatly from each other. On the question of the origin of hot Jupiter, there are currently two main theories in academia, namely the "migration" hypothesis and the "in situ formation" hypothesis, and the former is the most widely accepted.
The migration hypothesis suggests that in the early days of star formation, the prototype of hot Jupiter was first formed by the aggregation of solid rocks, ice cubes, and gases farther beyond the snow line.
A "snow line" is a dividing line in which a volatile compound (e.g., water, ammonia, methane, carbon dioxide, etc.) condenses into solid ice particles at a specific location far enough from the central star during planet formation. Rocky planets with smaller masses are thought to form more easily within the snow line, while larger gas giants form outside the snow line.
Schematic diagram of the hot Jupiter migration hypothesis and migration route (Credit: NASA/JPL-Caltech)
At the beginning of its formation, hot Jupiter and Jupiter were born at similar orbital positions. However, due to the difference in the mass distribution environment of the protoplanetary disk and the initial angular momentum, Jupiter and hot Jupiter have produced very different evolutionary routes, and the prototype of hot Jupiter began to gradually migrate inward (type II migration). In the process, it swept the trail of material and small objects along the way, increasing in mass, and finally came to a location close to the star.
Astronomers further observed that TOI-2109b is currently in the process of tidal locking and orbital decay, and its orbit forms a 1:1 resonance ratio, always with only one side facing the star, while the orbital radius is further shrinking. Through the continuous accretion of matter, perhaps it will move towards the fate of being gradually swallowed up by the central star.
Explore the Path of Discovery: Discovered from masking signals
So how was the record-breaking planet discovered? This has to start with nasa's TESS (Transiting Exoplanet Survey Satellite) space telescope.
On May 13, 2020, TESS began recording transit data for the star TOI-2109, and for nearly a month afterward, astronomers analyzed periodic occultation signals from the star's light curve, confirming that a planet was moving around the star at a rhythm of one cycle every 16 hours.
Schematic diagram of mask detection method or transit detection method (Image source: Courtesy of the author.) Source: NASA/exoplanet-catalog, Wang et al., AJ, 162, 256, 2021)
Over the next year, astronomers from around the world used telescopes at different sites to conduct more detailed spectral tracking of the star. These follow-up observations confirmed TESS's initial discovery, confirming the gas giant planet with the shortest period.
Schematic diagram of the Doppler look-of-view velocity method (Image source: Courtesy of the author. Source: NASA/exoplanet-catalog, Wang et al., AJ, 162, 256, 2021)
At present, a total of 434 hot Jupiters have been detected, and their orbital periods are usually less than 10 days. This time, TOI-2109b set a new record, with a shorter orbital period and an extremely high surface temperature (about 3500 K), which is close to that of late-type stars and brown dwarfs. THErefore, TOI-2109b should be classified as a sub-branch, superheated Jupiter or ultrahot Jupiter.
Astronomers plan to use the just-launched James Webb Space Telescope (JWST) to observe the planetary system at a wider wavelength and higher resolution in the future, uncovering more mysteries about superheated Jupiter.
Records exist to break, planets, and so do the roads to scientific heights. The discovery of super-hot Jupiter TOI-2109b is both an elegant application of modern science and technology and theory, as well as a valuable example of academic international cooperation. Its existence has made us re-examine the models and theories of planet formation, enriching human understanding of the universe and bringing more possibilities to future research.
Resources:
1. Wong, I. et al. TOI-2109: An Ultrahot Gas Giant on a 16 hr Orbit. The Astronomical Journal 162, 256 (2021).
2. Howard, A. W. et al. Planet occurrence within 0.25AU of solar-type stars from Kepler. Astrophysical Journal, Supplement Series vol. 201 15 (2012).
3. Ricker, G. R. et al. Transiting Exoplanet Survey Satellite (TESS). in Space Telescopes and Instrumentation 2014: Optical, Infrared, and Millimeter Wave vol. 9143 914320 (SPIE, 2014).
4. Ida, S. & Lin, D. N. C. Toward a deterministic model of planetary formation. VI. Dynamical interaction and coagulation of multiple rocky embryos and super-Earth systems around solar-type stars. Astrophysical Journal 719, 810–830 (2010).
5. Gardner, J. P. et al. The James Webb space telescope. Space Science Reviews 123, 485–606 (2006).
6. Greene, T. P. et al. Characterizing transiting exoplanet atmospheres with jwst. The Astrophysical Journal 817, 17 (2016).