Artistic image of an exoplanet orbiting a red dwarf Image credit: NASA/ESA/STScI/G.Bacon
"Finding 5,000 exoplanets" is a highly publicized and important goal, and the total number of exoplanets we have identified is only a few hundred away from this goal. Even more noteworthy than the numbers, however, is the diversity exhibited by these exoplanets. Of the exoplanets discovered by astronomers, there are a small number of planets that are similar to those in our solar system, but most of which are unfamiliar to us: there are scorching gas giants that rotate around the host star every few days; there are Neptune-sized planets that are as dense as marshmallows, like a mabo; and there are hordes of paired asteroids, huddled together like sardines, orbiting small, cold stars. Compared to them, the solar system that we are familiar with and considered "typical" is unusual.
GJ 367b is the latest discovery of a bizarre exoplanet that challenges our established notions and emphasizes to us that there are many unknowns in the universe. It's so weird that it seems more appropriate to appear on the cover of a heavy metal album or in a science fiction plot than in the real world. An article published in Science on December 2, 2021, said the planet could be essentially a semi-molten iron ball, about three-quarters the size of Earth.
Kristine Lam of the German Aerospace Center (DLR) and colleagues discovered GJ 367b using NASA's Transiting Exoplanet Survey Satellite (TESS). It's a peculiar "sub-Earth" planet (a planet much less massive than Earth and Venus) that sits around a small red dwarf star that's only 31 light-years away. TESS observations showed that the planet was about 9,000 kilometers in diameter (about 1/3 larger than the diameter of Mars); subsequent observations from the European Southern Observatory's High-Precision Radial Velocity Planet search telescope (HARPS) showed that the planet had only half the mass of Earth. Combining the above observations, it can be concluded that the density of this planet is amazing - about 8 grams per cubic centimeter, close to the density of pure iron. Lam noted: "The planet is likely to contain about 80 percent iron, surrounded by a silicate mantle. This structure is similar to Mercury in our solar system. ”
However , Mercury is 58 million kilometers away from the Sun with an orbital period of 88 days , while GJ 367b is only 1 million kilometers away from the main star , with an orbital period of only 7.7 hours. This means that GJ 367b is bathed in the glow of the star, and the surface temperature could be as high as 1500 degrees Celsius, enough to melt rock and metal. "It may not be very pleasant to live here." Lam said.
Mercury Image source: unsplash
About 100 similar ultra-short-period rocky planets have been discovered before, but GJ 367b is the smallest and lightest of them. It is likely to be tidally locked by its proximity to the star, meaning it is always facing the star in the same hemisphere, just as the moon is to Earth. The side that is always facing the star (eternal day) will have extremely high temperatures, so it may be an ocean of magma. Alexanderre Santerne of the University of Aix-Marseille in France (who was not involved in the study, but he previously discovered another Mercury-like exoplanet) noted that at such temperatures, the silicate should be liquid, like a huge pool of magma; at the same time, the planet's side facing the star (eternal night) is much lower, meaning there should be solid rock there; at the dividing line between day and night, there should be transitional material between magma and rock. If the planet had an atmosphere, this temperature difference could cause winds. But most experts believe that because GJ 367b is so close to its host star, it may have had no atmosphere long ago.
How the planet formed is a mystery, and the answer could have important implications for our understanding of the formation of the solar system. Scientists don't think that planets can form very close to the stars, because the gravitational pull that causes GJ 367b to be tidally locked should have disrupted the planet's formation process in the first place. So similar planets may have formed farther away from the star and then migrated inward, a process that can sometimes lead to spectacular interstellar collisions. Mercury may have been shaped by similar giant impacts, and its former structure may have been more similar to Earth's. "The most plausible explanation is that some object hit Mercury, leaving behind an object that was mostly iron." Joshua Winn of Princeton University said he was the co-author of the discovery GJ 367b paper. He added: "But this claim is not convincing to us because there is no other evidence that a collision occurred." If we can figure out how these iron-rich ultra-short-period planets formed, we might be able to shed light on our understanding of Mercury's formation. ”
One possibility is that ultrashort-periodic rocky planets like GJ 367b may not have been formed by catastrophic collisions, but may have been remnants of iron nuclei after giant planets were baked dry by the gas on the surface of the star as they migrated. When astronomers looked for exoplanets, they found that among the planets with close orbits to the stars were both huge "hot Jupiters" and planets similar to GJ 367b. It is worth noting that in such an environment, Neptune-like planets between the two are missing. The reason for this may be that such planets are pushed towards the star by another planet in the system, and when they are close enough to the star, their hydrogen and helium atmospheres are stripped away, leaving only a rocky interior. Don Pollacco, a former professor at the University of Warwick in the UK, said: "GJ 367b was originally likely a larger planet, but it has been burned out, so what we see may be the high-density core of a planet whose surface has evaporated. ”
For Mercury, due to its relatively far distance from the Sun, it is unlikely that there will be such a bizarre origin story. With further research into Mercury and more observations and discoveries of ultra-short-period planets using next-generation devices such as the James Webb Space Telescope, we may learn more about the formation of such planets. On top of that, out of the thousands of known exoplanets, we're constantly discovering novel planets. Pollacco said: "We are trying to find something similar to the solar system outside the solar system, but what we have actually discovered, and will continue to discover, is a world that is very different from the solar system and beyond our imagination." ”
James Webb Space Telescope (JWST) Image credit: NASA
Written by Jonathan O'Callaghan
Translator: Wu Yujia
Reviewer: Zheng Yuhong
Source of introduction: Scientific American
This article is from: China Digital Science and Technology Museum