Exoplanets range in size from much smaller than Earth to larger than Jupiter. However, when we classify exoplanets by mass and orbital distance, we find that their mass is several times greater than that of Neptune (several tens of times that of Earth, and a small fraction of Jupiter’s mass), and that they orbit very close to the star. Very few planets have been discovered. This region is called the “hot desert of Neptune,” and it is still unknown why planets that meet these conditions are so rare.
A research team led by Luca Naboniello from the University of Rome Tor Vergata announced the results of their analysis of observational data and follow-up research from NASA’s (National Aeronautics and Space Administration) TESS (Transiting Exoplanet Exploration Satellite) space telescope. Discovery of “TOI-1853b”. Not only is the mass and orbit of TOI-1853b located in Neptune’s hot desert, but it was also found to have an unusually high average density of 9.7±0.8 g/cm3. TOI-1853b may have undergone a giant impact in the past, which could provide clues to understanding the rarity of Neptune’s hot desert planet.
■ An “incredibly dense” planet.
TESS, which was launched in 2018, is a transit law (※1) Many exoplanet candidates have been discovered. Nabonello and his team analyzed the TESS data and made additional observations, discovering one planet, TOI-1853b.
*1…When a planet crosses in front of a star, the apparent brightness of the star decreases because the planet hides part of the star. The transfer method captures this change. If the decrease in brightness is constant and periodic, it is likely a planet. As the amount of decrease in brightness is proportional to the apparent size of the planet, we can determine the diameter of the planet.
TOI-1853b orbits the K-type main sequence star TOI-1853, located about 540 light-years away in the Bootes constellation, in an orbit so small that it makes one rotation every 1.24 days. Therefore, the surface temperature of TOI-1853b is expected to reach a high temperature of 1200 degrees Celsius. Furthermore, the diameter of TOI-1853b, estimated from TESS observational data, is 3.46 ± 0.08 times the diameter of Earth (about 44,000 km), which is about 90% that of Neptune, making TOI-1853b “hot Neptune.” It is classified as “Hot Neptune” which means “hot Neptune”.
In addition to analyzing the TESS data, Nabonello and his colleagues also performed observations with the National Galileo Telescope installed at the Roque de los Mochachos Observatory (Canary Islands, Spain), using the radial velocity method. (※2) We analyzed the data obtained and estimated the mass of TOI-1853b.
*2…When a planet orbits a star, the star is pulled by the planet’s gravity and moves very slightly. When viewed from an observer on Earth, the star appears to move closer or further away. The wavelength of light from the star changes due to the Doppler effect due to this apparent motion. The degree of wavelength change depends on the speed of the star’s movement and the mass of the planet that attracts the star, so the mass of the planet can be estimated through the Doppler effect. This is called the radial velocity method.
The results revealed that the mass of TOI-1853b is 73.2±2.7 times the mass of Earth. The size of this planet is about 4.3 times the size of Neptune, and about twice the size of the ice giant planets discovered so far. Indeed, its size is close to the size of the gas giant Saturn (about 95.2 times the size of Earth), and it is located almost in the middle of the hot desert of Neptune. Therefore, TOI-1853b is an incredibly dense planet with an average density of 9.7±0.8 g/cm3.
This average density is an improbable value not only for a gas giant planet composed mainly of hydrogen and helium, but also for an icy giant planet composed mostly of slightly heavier material. Earth has the highest average density in the solar system, at 5.51 grams per cubic centimeter. TOI-1853b is one of the densest known planets, with its diameter and mass precisely measured.
For a dense planet like TOI-1853b to exist, it must be rich in heavy elements. Roughly speaking, TOI-1853b can be considered an enlarged version of Earth, which is mainly composed of rocks. However, according to the usual theory of planetary formation, if such a huge mass of rock were formed, it would attract and retain the light elements surrounding it with its own gravity, and it would be a planet like Jupiter, which is mainly composed of light elements. Therefore, it should not become a dense celestial body. .
■Was TOI-1853b created as a result of a giant collision?
Naboniello and his colleagues believe that TOI-1853b may have formed as a result of a giant collision between planets in the past. In this case, TOI-1853b would have originally formed as a giant rocky planet (super-Earth), rich in light materials such as water. It is assumed that such a huge rocky planet was formed as a result of another rocky planet colliding with the planet at some point in the past and ejecting lighter material from it.
The giant impact scenario presented by Nabonello and others is similar to the “giant impact theory” that created Earth’s moon, but the giant impact theory estimates the speed of the celestial impact at about 9.3 km/s. In contrast, the speed of the giant impact is estimated to have been higher. From 75 km/s. If the probability of such a massive collision occurring was low, this type of planet would be rare, which might explain Neptune’s hot desert.
However, Nabonello and his colleagues are also thinking about formations other than giant impacts. Given the traditional theory of planet formation, it is very likely that TOI-1853b was born as a gas giant planet rich in light material. If TOI-1853b had a highly elliptical orbit and got too close to the central star, the star’s heat would vaporize light material, and tidal forces would cause the orbit to approach a perfect circle. Then, as TOI-1853b is heated to increasingly high temperatures, it will eventually lose most of its light matter and only a dense, rocky core will remain, resulting in its current appearance. This scenario shows that TOI-1853b could become the dense object it is today, even if it had a less anomalous origin and formed according to the normal planetary formation theory. However, this would still require the formation of a gas giant planet with a fairly large rocky core, which could explain Neptune’s hot desert depending on its rarity.
If it formed in a large impact scenario, TOI-1853b would be made up of roughly half rock and water, making it likely surrounded by an atmosphere of water vapor. On the other hand, in the scenario where it formed by the evaporation of lighter material, TOI-1853b would be made almost entirely of rock, with a thin atmosphere of hydrogen and helium making up less than 1% of its mass. In both scenarios, the planet will have an interesting past that is difficult to predict using traditional planet formation theory, and we will also be able to get closer to why Neptune’s hot desert was formed.
Nabonello and his colleagues admit that observing is very difficult, but their next research goal is to narrow down TOI-1853b’s formation scenarios by analyzing the thin atmosphere present on TOI-1853b.
Written by Riri Aya
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