The conventional theory on planet formation, known as the core accretion model or also the nebular hypothesis in its broader context, describes how planets are assembled from the material leftover from the birth of a star. A dense knot in a giant molecular cloud of gas and dust collapses under the influence of gravity, to form a protostar. As it collapses, the conservation of angular momentum causes the infalling material to flatten into a rotating circumstellar disk. Once the star ignites and begins to sustain nuclear fusion, the gas from the inner disk is blown away by the radiation. Material begins to coalesce in the circumstellar disk, which becomes a protoplanetary disk, where worlds are assembled.
The conventional picture is that grains clump up into pebbles, that then gradually form boulders, mountains and eventually protoplanets. In the inner disk, terrestrial worlds such as Earth or Mars are made, composed mostly of silicates and metals. In the cold outer regions, beyond the frost line, where water, ammonia and methane can freeze into ices, there is more material available for larger worlds to form. Once an accreting core crosses between five and ten Earth masses, the gravity becomes strong enough to retain large amounts of the lightest gases, including hydrogen and helium, leading to the formation of gas giants such as Saturn or Jupiter. If the disk dissipates before much gas is captured, you end up with ice giants such as Uranus or Neptune, with thicker mantles of ice and rock, and thinner gas envelopes.
There is just not enough material in the inner disks to form gas giants. However, gas giants can migrate inwards or outwards because of gravitational interactions. While there are no gas giants close to the Sun, such worlds are common in other systems, and are known as Hot Jupiters. There are no worlds in the Solar System straddling the mass gap between Earth and Neptune. However, exoplanet surveys have discovered that most Sun-like stars host at least one world that sits squarely in this mass gap, but these worlds are divided into two groups, termed as Super Earths and Sub Neptunes.
Super Earths and Sub Neptunes
Most Super Earths are about 1.2 times the size of the Earth, while most Sub Neptunes are 2.4 times the size of the Earth. Super Earths are terrestrial worlds that are larger than the Earth. Sub Neptunes have a low density and are enveloped by volatile gases. Scientists have predicted in theory that some Sub Neptunes could be water worlds with global liquid water oceans, enveloped by atmospheres rich in hydrogen and helium. These are known as Hycean planets, but none have been discovered so far. It is generally believed that once a planet grows to more than twice the size of Earth, the gravity is sufficient to trap and retain hydrogen and helium.
Now, astronomers have spotted a world that does not fit into this neat little picture. GJ 523 b orbits an orange dwarf or K-dwarf star, slightly smaller and cooler than the Sun, at a distance of about 86.6 lightyears from the Earth. It is an unusually massive rocky world, containing about 23.5 times the mass of the Earth, packed into a radius of 2.5 times that of Earth. According to conventional theories of planet formation, such a world simply should not exist. The gravity on GJ 523 b is 3.6 times stronger than the Earth, sufficient to trap gas. The lack of a thick atmosphere on such a planet is surprising. GJ 523 b is also in a polar orbit, not on the orbital plane. Something kicked it out of alignment, hinting at a dynamic and violent past. Astronomers have proposed classifying this category of planets as ‘Mega Earths’.
The processes that could have led to the formation of GJ 523 b
Scientists do not know exactly how GJ 523 b formed, or how it moved into a polar orbit, but they have proposed some possible mechanisms. The world may have originated as a cold gas giant protoplanet, then pushed into a polar orbit by the gravitational influence of an undiscovered massive world in the system. The primordial gas envelope would have been stripped away as it moved closer to the host star. The system is only 170 million years old, which is not sufficient time to entirely strip away the atmosphere of even a small gas giant, which is the major hurdle to this theory.
The high orbital obliquity may hold a clue, suggesting that the circumstellar disk may have itself influenced the exoplanet. As the disk dissipated and a gap opened, a rapid nodal precession could have pushed or ‘excited’ the planet into its polar orbit. Alternatively GJ 523 b could have formed in a disk that was already misaligned with the rotation of the star due to turbulence or inclined magnetic fields. Another proposed mechanism is that the planetary core could have grown rapidly by accreting pebbles, growing to a massive size. The heat in the young system would have prevented the gas envelope from contracting and accreting more gas. By the time the planet cooled sufficiently to accrete gas, the gas could have already dissipated, leaving behind a stripped Mega Earth.
Yet another possibility is that GJ 523 b formed as a typical planet with a thick atmosphere, but lost it through catastrophic events. This could be in the form of successive impacts of bodies of roughly equal mass, that could have stripped away the primary atmosphere, while simultaneously increasing the core size of the world. The heat from these impact or impacts could have prevented the planet from accreting gas before it dissipated from the disk. Such collisions between planetesimals is believed to be common in the infancy of a star system. Future observations may help narrow down the processes responsible for the unique properties of GJ 523 b.
Life on GJ 523 b
The dominating force on the Mega Earth would be the immense gravity and extreme heat. The proximity to the host star also results in a surface temperature of about 265°C, with the polar orbit inducing radical seasonal shifts, as the poles would be directly pointed at the star for most of the orbits, causing violent atmospheric circulation and temperature swings every few days. At these temperatures water on the surface would exist mostly as vapour, or in a supercritical state, a high-pressure phase where water behaves as both a gas and a liquid. There is a narrow possibility of a liquid water ocean if the atmosphere is precisely a specific mass fraction, a rare scenario where a liquid ocean could exist on the surface before transitioning into supercritical layers and high-pressure ice at increasing depths.
The high gravity and density means that the mountains would not be very tall. If there is an ocean, then most of the surface would be peppered with archipelagos. Any life forms would have to be extremely robust. The cost of locomotion would be extremely high, and they might perceive time as passing much slower than life forms on Earth. The current high jump record on Earth has been unbroken for over 30 years, and stands at 2.45 metres, set by Javier Sotomayor in 1993. On GJ 523 b, this would be only 68 cm high, ignoring the more challenging run up and take off. The system is very young, and the host star is still in its tempestuous youth. It is flaring frequently, making GJ 523 b an environment hostile to life as we know it.
If life forms do find a way to survive the environment, and an advanced civilisation emerges in a few billion years, they would need a rocket over 12 times the height of the Great Pyramid of Giza to launch a mission similar to Apollo. These Mega Earthlings would suffer even more than the Super Earthlings.
A pre-print paper has been posted on Arxiv. Note that the research is not peer reviewed yet.
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