Why are there "two-sided planets" hidden in the universe?

Produced by: Science Popularization China

Author: Shu Bo (popular science creator)

Producer: China Science Expo

In ancient Roman mythology, the two-faced god Janus has two faces, one looking forward to the future and the other looking back to history. When the ancients carved Janus gazing at the distant sky, they probably didn't expect that two thousand years later, astronomers would discover some strange two-faced planets in the universe. Like Janus, they show two completely different faces when viewed from different directions.

Statue of Janus in the Vatican Museums

(Image source: Wikipedia)

Half hydrogen, half helium white dwarf

The first two-faced star is a special white dwarf. The main component of a star is the lightest element in the universe: hydrogen, which combines heavier elements such as helium in nuclear fusion and releases the energy of burning. When a smaller star like the sun runs out of hydrogen in its core, it will expand into a red giant star that burns helium and produces two heavier elements, carbon and oxygen. When the red giant eventually extinguishes, it will leave behind a remnant composed mainly of carbon and oxygen, which is a white dwarf.

However, according to observations, the outer atmosphere of a white dwarf is still composed mainly of hydrogen or helium, just like an ordinary star. According to statistics, 80% of the atmosphere of a white dwarf is mainly hydrogen, and 20% is mainly helium. It seems that the atmosphere of a white dwarf has different compositions, just like the different "blood types" of the human body.

However, humans do not only have four blood types: A, B, AB, and O. There are also rare "panda blood types" such as Rh negative blood, and the same is true for white dwarfs.

In 2023, astronomers observed a white dwarf with a rare atmosphere, numbered ZTF J203349.8+322901.1 . It is located in the direction of Cygnus, more than 1,000 light-years away from us, and rotates once every 15 minutes.

A double-sided white dwarf, the side with the hydrogen atmosphere is smooth, while the side with the helium atmosphere is filled with rice-like bubbles.

(Image source: California Institute of Technology official website)

When scientists detected the atmospheric composition of this white dwarf, they were surprised to find that its atmosphere seemed to be divided into two sides. One side was mainly composed of hydrogen and had a smooth surface, while the other side was mainly composed of helium and was filled with rice-shaped bubbles.

Where does the strange atmosphere of this white dwarf come from? Scientists speculate that the atmosphere of a white dwarf is not calm but is constantly changing . It is the interference of certain factors that makes it become a special state.

At first, the atmosphere of a white dwarf is mainly hydrogen. This is because hydrogen and helium are not mixed together randomly. Helium is heavier than hydrogen, so it will slowly sink to the bottom to form a helium layer, while the hydrogen layer still floats on the outer layer of the atmosphere. When we observe a white dwarf, it appears that its atmosphere is mainly hydrogen.

Schematic diagram of an extremely low-mass white dwarf

An extremely low-mass white dwarf is a type of helium white dwarf with important scientific value, and its mass is generally less than 0.3 solar masses.

(Image source: Caltech/IPAC)

Later, the effect of temperature on the composition of the white dwarf atmosphere became apparent. White dwarfs come from extinguished stars and can no longer emit light or heat, leaving only the residual temperature that slowly cools down. When the temperature drops to around 30,000°C, the hydrogen layer and the helium layer below gradually mix together, and the white dwarf atmosphere at this time changes from being mainly hydrogen to being mainly helium.

But the above changes all occur as a whole. This white dwarf is half hydrogen and half helium, and it is speculated that the magnetic field is likely to be the cause.

A star is a huge ball of plasma flame. The electromagnetic activity inside is very intense, which can generate a powerful magnetic field and affect the temperature of the star's surface.

Take the sun, for example, which has many areas on its surface where the magnetic field is very strong. There, the convection from the sun's interior to the surface is suppressed by the magnetic field , forming some low-temperature areas that appear black on Earth. These are sunspots .

The magnetic field has two poles, so sunspots often appear in pairs.

(Photo source: Hong Kong Astronomy Museum)

Even after a star burns out and becomes a white dwarf or a neutron star, the magnetic field will not disappear but will continue to be around.

The scientists who discovered the double-sided white dwarf speculated that the asymmetric magnetic field might be the culprit .

The magnetic field on one side is stronger, which suppresses the "rhythm" of the helium layer rolling up, so a hydrogen-type atmosphere appears here, and the surface appears calm and smooth; while the magnetic field on the other side is weaker, and the helium layer rolls up much more smoothly, and half of the atmosphere here turns into helium type first, and the surface is also covered with bubbles.

Half Black Half White

Double-sided white dwarfs are too far away from us, so whether the current explanation is correct requires observing more similar white dwarfs in the future.

In fact, two-faced strange celestial bodies like Janus are not very far away. Not far from us in the solar system, there is a half-black and half-white celestial body. It is Saturn's third largest satellite - Iapetus .

The two sides of Iapetus

(Image credit: NASA)

Iapetus is a sphere with a diameter of nearly 1,500 kilometers. Its iconic appearance is that it has two sides with distinct black and white colors, one side is black and the other side is white , which is quite similar to the "Death Star" in the movie "Star Wars".

More than 300 years ago, when astronomer Cassini discovered this satellite, he noticed something strange because the brightness of Iapetus could vary by five or six times when observed at different times. He guessed that this was because one side of Iapetus was very bright, while the other side was very dark. Later generations used more advanced telescopes to observe and proved that his guess was correct.

So where do the black and white tones of Iapetus come from?

Let's talk about white first. Satellites like Iapetus that are far from the sun are rich in ice, and white is the color of ice . And black? It comes from organic compounds on the surface of Iapetus, which are baked by direct sunlight and turned black, forming dark areas.

The junction of the two sides of Iapetus

(Image credit: NASA)

But why are the light-colored areas and the dark-colored areas concentrated together, each occupying half of the planet?

The mainstream explanation in the scientific community now is that it’s all because Iapetus rotates too slowly .

It takes 79 days for Titan to rotate once. During this period, the temperature difference between the sunlit side and the shady side becomes increasingly larger, forming a "thermal feedback" effect.

Common sense tells us that black clothes absorb heat more easily than white clothes, and the material on the surface of Titan is no exception.

Areas covered by dark-colored matter absorb heat more easily than areas covered by light-colored matter, so the ice there sublimates into gas, flows into the atmosphere from the ground, then drifts to the cold, shady side, condenses on the ground and turns back into ice, making the area whiter.

Various celestial bodies in the solar system

(Photo source: Veer Gallery)

At first, only a small amount of ice on the surface of Iapetus was transferred, and the color difference between different places on the planet's surface was also very small. But after billions of years of changes, the dark side became darker and darker, while the other side was "painted" by the drifting ice and became whiter and whiter. Iapetus thus grew a "yin-yang face".

In fact, in the solar system, Titan is not the only celestial body that is dyed by matter transferred from the atmosphere. For example, Charon, the largest satellite of Pluto , is a satellite that is dyed by Pluto's transplanetary process.

Pluto and Charon have very different colors. Pluto is covered with a red substance called "Thorin", so most of its surface is red; while most of Charon's surface is white, but its poles are red like Pluto.

Pluto and its moon Charon, as photographed by the New Horizons probe in 2015

(Image source: Wikipedia)

The frozen material on Pluto's surface turns into gas under the sun, so it also has a thin atmosphere. The distance between Pluto and Charon is less than 20,000 kilometers, which is only about one twentieth of the distance between the earth and the moon. Many materials in Pluto's atmosphere drifted to the neighboring planet, including tholin. This red material froze at the poles of Charon and dyed it red.

Conclusion

It seems that, whether it is the white dwarfs far away in the sky or the solar system close at hand, there are always all kinds of strange and bizarre celestial bodies in the universe waiting for us to uncover the secrets behind them.

Humans who have mastered modern science are like Janus, looking at both the past and the future, learning from historical knowledge while exploring new frontiers. We will surely make even more amazing discoveries in the future.

References:

[1] Kong Xiao, Luo A-li. Searching for white dwarfs in sky survey projects[J]. Progress in Astronomy, 2019, 37(03): 287-303.

[2]Strange two-faced dying star 'Janus' baffles scientists in cosmic oddity, Space.com, By Monisha Ravisetti published July 20, 2023.

[3]Caiazzo, Ilaria, et al. "A rotating white dwarf shows different compositions on its opposite faces." Nature (2023): 1-6.

[4]Lapetus: Saturn's Yin-Yang Moon, Space.com, By Nola Taylor Tillman published June 28, 2016.

[5]Scientists Explain Pluto's Red-Headed Moon, By Nola Taylor Tillman, SPACE.com on September 14, 2016.