It rains only one day a year at night, but that's not all the wonder of the planet.

WASP-121b is probably the most thoroughly studied exoplanet. It was discovered in 2015, with a mass of 1.18 times that of Jupiter, a radius of 1.81 times that of Jupiter, and an orbit period of 30.6 hours. In 2017, astronomers even found water in its stratosphere. Today, abnormal data in spectral analysis show that the planet will even rain gems.

Of course, that doesn't mean you should look for opportunities to get rich on this star. As you may have noticed, the orbit period of WASP-121b is only 30.6 hours. Unfortunately, its host star WASP-121 is a F6 star, which is close to the sun's G2 star. So, according to Kepler's third law, the shorter the period, the smaller the radius of the orbit, and the WASP-121b is very close to the host star. According to the observation, its surface temperature can even reach more than 3000 Kelvin. WASP-121b can't make you rich, it can only make you die.

Lingxing method

So far, nearly 5000 exoplanets have been identified, of which more than 300 are gaseous planets with orbital periods of less than 10 days, which astronomers call "hot Jupiter". However, even among the more than 300 "hot Jupiter", the environment of WASP-121b is so extreme that astronomers even plan to create a new classification of "superhot Jupiter" for it.

Astronomers mainly use the transit method to find exoplanets. If the exoplanets revolve around the plane of the star to face us, then when the exoplanets turn in front of the star, they will block part of the starlight and cause the brightness of the star to decline. We can find the exoplanet.

When a planet obscures a star, the light emitted by the star penetrates the planet's atmosphere, resulting in a change in the star's spectrum, which allows us to analyze the types of elements contained in the planet's atmosphere. "although thousands of exoplanets have been discovered, we can only study a small part of the atmosphere because of the challenges of observation," said Thomas Mikal-Evans, an astronomer at the Max Planck Institute for Astronomy in Germany. The extremely high temperature of WASP-121b makes its own brightness so high that it is even possible for astronomers to directly study the light emitted by the planet itself.

Extreme day and night

It is such a short distance that makes the tidal effect on this planet very strong. This puts the planet in a tidal lock-the rotation period is the same as the rotation period, and there is always only one face to its host star. This creates a huge temperature difference on the WASP-121b, with a hemispheric temperature of 3000 Kelvin during the day and only 1500 Kelvin in the night hemisphere. The temperature difference between the two hemispheres is so strong that astronomers are curious to know the impact of such an extreme environment, but the brightness of its night hemisphere is only about 1 × 10 of that of the daytime hemisphere, which makes it more difficult to observe.

On Earth, water molecules participate in the water cycle in solid, liquid and gas states, while in the diurnal hemisphere of WASP-121b, 3000 Kelvin of water vapor may even cause water molecules to decompose to form hydrogen and oxygen atoms, and only in the night hemisphere can 1500 Kelvin's "low temperature" allow it to "condense" again into water vapor.

The 1500-degree temperature difference between the two hemispheres of the star creates a permanent pressure difference across the planet, creating extreme westerly winds orbiting the entire planet. "the wind on WASP-121b is much faster than it is on Earth and could sweep the planet in 20 hours," said Tansu Daylan, an astrophysicist at the Massachusetts Institute of Technology.

As the most thoroughly studied exoplanet, astronomers have found vanadium, iron, chromium, calcium, sodium, magnesium, nickel, aluminum and titanium in its atmosphere. In the atmosphere of the night hemisphere, however, aluminum and titanium disappear. The team believes that aluminum and titanium have been brought deep into the planet's atmosphere in the night hemisphere. Aluminum can easily combine with oxygen to form alumina. Given that the WASP-121b atmosphere contains a variety of metal elements, these alumina are likely to carry "impurities" of vanadium, iron, chromium or titanium, which are the conditions for the formation of rubies and sapphires.

This means that it is probably raining gems-or "hail"-in WASP-121b 's night hemisphere. after all, the high winds that can sweep the planet in just 20 hours and the heat of 1500 Kelvin will leave you in no mood to "get rich" there.

For researchers, however, they prefer to find carbon monoxide. Carbon monoxide is the most common molecule in the nebula except hydrogen. Because of its asymmetric structure, carbon monoxide is more easily observed and is often used by astronomers to study how stars form. Mikal-Evans, who is looking for traces of carbon monoxide on WASP-121b with the James Webb Space Telescope, said: "the abundance of carbon and oxygen in the atmosphere will provide us with clues about the formation of such planets."