8000 degrees Celsius in space, a planet hotter than the sun!

Produced by: Science Popularization China

Author: Fang Hefei (Royal Observatory, Edinburgh, UK)

Producer: China Science Expo

When we think of hot celestial bodies in the universe, we all think of stars other than the sun. But are those celestial bodies that have not become stars necessarily calmer? Perhaps we need to think before we give an answer. The celestial body WD 0032-317B recently discovered by scientists has a surface temperature of about 8,000 degrees Celsius . In other words, the surface temperature of the sun is nothing compared to it.

How did such high temperatures come about? Let us uncover the mysteries of celestial bodies.

Death of a star, birth of a white dwarf

In the universe, nuclear reactions are mainly fusion : light elements such as hydrogen experience hundreds of millions of degrees of high temperature, aggregate into heavier elements and release energy. All stars, including the sun, generate energy in this way. After fusion stops, the remaining matter is not radioactive and is relatively much "cleaner".

The sun, along with all other stars, is powered by a reaction called nuclear fusion. (Image credit: NASA/SDO/AIA)

If nuclear fusion could be replicated on Earth, it could provide a virtually unlimited supply of clean, safe and cheap energy to meet the world's energy needs.

For stars, this process is called the star's death phase . During the normal life of a star, nuclear fusion is concentrated in the core area. After tens or even tens of billions of years of long burning, the hydrogen elements in the center have been consumed, and heavier elements are gradually generated: helium, and even carbon, oxygen...

As the fuel is consumed, the core of the star no longer generates outward heat pressure and gradually collapses under its own weight, shrinking inward until it shrinks to a certain extent. The repulsive force of the electrons inside the debris gradually increases, and the gravity caused by the mass of the debris itself just cancels each other out. At this point, the huge core of the star has shrunk into a small ball . The remnants of the nuclear reaction of the dead star form a new celestial body: a white dwarf .

White dwarf (Image source: Veer Library)

White dwarfs are not light in mass, but small in size. Take WD 0032-317 for example, it is 40% of the mass of the sun, but less than 3% of the sun in diameter, making it tens of thousands of times denser than the sun. As a result, the surface area available for heat dissipation is very small, and the residual high temperature after the star dies can be maintained for a long time.

We assume that the white dwarf is just one million years old, which is exactly the age of WD 0032-317 estimated by theory. Although this short cooling time is negligible compared to the billions of years of life of a star, it still retains a high temperature of 37,000 degrees Celsius, far higher than the temperature of 5,500 degrees Celsius on the surface of the sun, equivalent to the level of a blue giant.

The actual observed radiation change curve of WD0032-317.

As white dwarfs and brown dwarfs orbit (diagram above), the radiation flux of white dwarfs observed on Earth changes periodically (Image source: Reference [1])

Although white dwarfs do not generate energy through radioactive decay, the energy generated purely by high temperatures is also astonishing. The heat energy is mainly emitted in the form of ultraviolet and visible light, such as the carcinogenic ultraviolet rays that shine through the ozone hole on the earth, which are enough to break up biological macromolecules.

The planets around WD 0032-317 endured this fiery embrace.

Brown dwarf: the planet with the largest temperature difference between day and night known so far

WD 0032-317B, "WD" stands for "white dwarf". Adding a "B" after the number indicates the second object in this system - the brown dwarf WD 0032-317B.

Artist's concept of a brown dwarf (Image credit: NASA)

If we compare it to the celestial bodies in the solar system, brown dwarfs are a bit like Jupiter . They are also gas condensed, but they are heavier than Jupiter, with a mass roughly between 13 and 80 Jupiters.

WD 0032-317B is 79 times heavier than Jupiter. If it were heavier, the internal pressure caused by its own gravity would increase the temperature and ignite a nuclear reaction in the core region, and it would become a star .

Therefore, although WD 0032-317B is a "general" among gas giant planets, it is actually a "dwarf" in the star family .

Not only that, it is also a "tidally locked" planet . The gravitational pull of the white dwarf causes its orbit to change, and its rotation and revolution periods gradually become synchronized, just like the moon.

Therefore, only one side of WD 0032-317B always faces the white dwarf, which is called the "day side" . The energy released by the white dwarf is poured into this side, which is always as bright as day. On the back side, the brown dwarf blocks the light of the white dwarf and never shines directly, so it is called the "night side".

Astronomers have measured WD 0032-317B. After their analysis, they found that the day side of WD 0032-317B has reached nearly 8,000 degrees Celsius . In front of it, the sun seems dim - the surface temperature of the sun is 5,500 degrees Celsius, which is more than 2,000 degrees Celsius lower than it!

Temperature estimation diagram for the day and night sides of WD 0032-317B.

The purple and gray curves represent the energy distribution on the night side and the day side, respectively. The sky blue curve represents the energy distribution of the white dwarf, and the black curve represents the overall energy distribution. (Image source: Reference [1])

On the night side, because the white dwarf is not illuminated, the surface temperature of the brown dwarf suddenly drops to about 2000 degrees Celsius. This is the planet with the largest temperature difference between day and night known so far, reaching as much as 6000 degrees Celsius.

The gaseous brown dwarf revolves around a white dwarf in the universe, and its surface is swept by heat waves that are hotter than the surface temperature of the sun. Humans obviously cannot survive on such a planet, and they cannot even get close to it. The surface temperature of the brown dwarf is far higher than the boiling point of steel, and even diamonds will boil on the day side.

Because of this powerful energy, brown dwarfs are likely to experience violent internal turbulence and even eject their own material into space. Although this has not been confirmed for WD 0032-317B, a similar object, KELT-9b, has been observed to have this phenomenon. At 4,300 degrees Celsius on the dayside, it shows a comet-like outflow of material.

Schematic diagram of the evaporation of planetary material caused by the release of energy from a white dwarf (Image source: EarthSky official website)

White dwarfs and brown dwarfs: a perfect combination for astronomical research

Of course, this observational study of WD 0032-317B is not just a curiosity in the universe, it also has important implications for the study of planetary evolution.

Brown dwarfs are about the size of Jupiter and very hot, so they can serve as analogs to gas giant planets that orbit very close to their stars - "hot Jupiters" .

Hot Jupiters are currently a hot topic in the field of planetary evolution. They are usually difficult to detect because they orbit hot and bright massive stars, obscured by the star's glare.

Schematic diagram of the size comparison between the two hot Jupiters and Jupiter and the Earth (Image source: Chinese Academy of Sciences)

Previously, people could only indirectly detect hot Jupiters through gravitational effects , that is, detecting the corresponding stars producing weak motion anomalies under the gravitational effect of hot Jupiters. However, these stars themselves often rotate rapidly and release a large amount of stellar wind material, which makes it difficult to measure whether the motion is abnormal.

The white dwarf-brown dwarf combination has particular advantages as a substitute for the star-hot Jupiter .

First, the white dwarf is small , so the brown dwarf can be close enough to it and be heated enough .

Secondly, the white dwarf has a small luminous surface area and is much fainter than the star of a hot Jupiter, so it will not obscure the brown dwarf around it , making it more likely to image the brown dwarf directly without the need for indirect detection.

In addition, the WD0032-317 and WD0032-317B system itself provides new information about the final evolution of stars. According to observations, brown dwarfs should be several billion years old. However, white dwarfs have very small masses, which means that their progenitors should have a longer lifespan . Researchers believe that brown dwarfs may have participated in the evolution of white dwarfs and accelerated the death of their progenitors.

Conclusion

Twenty years ago, WD 0032-317B also entered the field of vision of astronomers, but was ignored. **At that time, its night side was facing the Earth and was not conspicuous enough in the telescope. **Perhaps it was because scientists at that time did not have sufficiently advanced observation instruments, or perhaps it was because the exploration of the universe was not easy.

Twenty years later, WD 0032-317B has re-entered people's field of vision. I believe that in the future, with the help of high-resolution telescopes, scientists will be able to build a three-dimensional atmospheric model of WD 0032-317B and reveal how such a large temperature difference between the day and night sides is distributed, thus helping us further understand similar gas giant planets.

More stories about the hot planet WD 0032-317B are on the way, and more discoveries are waiting for us about the stars in the universe.

References:

[1] Na'ama Hallakoun, Dan Maoz, Alina G. Istrate, et al. An irradiated-Jupiter analogue hotter than the Sun.

[2] Elizabeth Gamillo. Astronomers find a brown dwarf that's hotter than the Sun.

[3] Atmosphere Models of Brown Dwarfs Irradiated by White Dwarfs: Analogs for Hot and Ultrahot Jupiters. Joshua D. Lothringer and Sarah L. Casewell. 2020 ApJ 905 163. DOI: 10.3847/1538-4357/abc5bc