Earth isn’t 'super' because the sun had rings before planets

"In the solar system, something happened to prevent the Earth from growing to become a much larger type of terrestrial planet called a super-Earth," said Rice University astrophysicist André Izidoro, referring to the massive rocky planets seen around at least 30% of sun-like stars in our galaxy.

Izidoro and colleagues used a supercomputer to simulate the solar system's formation hundreds of times. Their model, which is described in a study published online in Nature Astronomy, produced rings like those seen around many distant, young stars. It also faithfully reproduced several features of the solar system missed by many previous models, including:

• An asteroid belt between Mars and Jupiter containing objects from both the inner and outer solar system.
• The locations and stable, almost circular orbits of Earth, Mars, Venus and Mercury.
• The masses of the inner planets, including Mars, which many solar system models overestimate.
• The dichotomy between the chemical makeup of objects in the inner and outer solar system.
• A Kuiper belt region of comets, asteroids and small bodies beyond the orbit of Neptune.

The study by astronomers, astrophysicists and planetary scientists from Rice, the University of Bordeaux, Southwest Research Institute in Boulder, Colorado, and the Max Planck Institute for Astronomy in Heidelberg, Germany, draws on the latest astronomical research on infant star systems.

Their model assumes three bands of high pressure arose within the young sun's disk of gas and dust. Such "pressure bumps" have been observed in ringed stellar disks around distant stars, and the study explains how pressure bumps and rings could account for the solar system's architecture, said lead author Izidoro, a Rice postdoctoral researchers who received his Ph.D. training at Sao Paulo State University in Brazil.

"If super-Earths are super-common, why don't we have one in the solar system?" Izidoro said. "We propose that pressure bumps produced disconnected reservoirs of disk material in the inner and outer solar system and regulated how much material was available to grow planets in the inner solar system."

Pressure bumps

For decades, scientists believed gas and dust in protoplanetary disks gradually became less dense, dropping smoothly as a function of distance from the star. But computer simulations show planets are unlikely to form in smooth-disk scenarios.

"In a smooth disk, all solid particles -- dust grains or boulders -- should be drawn inward very quickly and lost in the star," said astronomer and study co-author Andrea Isella, an associate professor of physics and astronomy at Rice. "One needs something to stop them in order to give them time to grow into planets."

When particles move faster than the gas around them, they "feel a headwind and drift very quickly toward the star," Izidoro explained. At pressure bumps, gas pressure increases, gas molecules move faster and solid particles stop feeling the headwind. "That's what allows dust particles to accumulate at pressure bumps," he said.

Isella said astronomers have observed pressure bumps and protoplanetary disk rings with the Atacama Large Millimeter/submillimeter Array, or ALMA, an enormous 66-dish radio telescope that came online in Chile in 2013.

"ALMA is capable of taking very sharp images of young planetary systems that are still forming, and we have discovered that a lot of the protoplanetary disks in these systems are characterized by rings," Isella said. "The effect of the pressure bump is that it collects dust particles, and that's why we see rings. These rings are regions where you have more dust particles than in the gaps between rings."

Ring formation

The model by Izidoro and colleagues assumed pressure bumps formed in the early solar system at three places where sunward-falling particles would have released large amounts of vaporized gas.

"It's just a function of distance from the star, because temperature is going up as you get closer to the star," said geochemist and study co-author Rajdeep Dasgupta, the Maurice Ewing Professor of Earth Systems Science at Rice. "The point where the temperature is high enough for ice to be vaporized, for example, is a sublimation line we call the snow line."

In the Rice simulations, pressure bumps at the sublimation lines of silicate, water and carbon monoxide produced three distinct rings. At the silicate line, the basic ingredient of sand and glass, silicon dioxide, became vapor. This produced the sun's nearest ring, where Mercury, Venus, Earth and Mars would later form. The middle ring appeared at the snow line and the farthest ring at the carbon monoxide line.

Rings birth planetesimals and planets

Protoplanetary disks cool with age, so sublimation lines would have migrated toward the sun. The study showed this process could allow dust to accumulate into asteroid-sized objects called planetesimals, which could then come together to form planets. Izidoro said previous studies assumed planetesimals could form if dust were sufficiently concentrated, but no model offered a convincing theoretical explanation of how dust might accumulate.

"Our model shows pressure bumps can concentrate dust, and moving pressure bumps can act as planetesimal factories," Izidoro said. "We simulate planet formation starting with grains of dust and covering many different stages, from small millimeter-sized grains to planetesimals and then planets."

Accounting for cosmochemical signatures, Mars' mass and the asteroid belt

Many previous solar system simulations produced versions of Mars as much as 10 times more massive than Earth. The model correctly predicts Mars having about 10% of Earth's mass because "Mars was born in a low-mass region of the disk," Izidoro said.

Dasgupta said the model also provides a compelling explanation for two of the solar system's cosmochemical mysteries: the marked difference between the chemical compositions of inner- and outer-solar system objects, and the presence of each of those objects in the asteroid belt between Mars and Jupiter.

Izidoro's simulations showed the middle ring could account for the chemical dichotomy by preventing outer-system material from entering the inner system. The simulations also produced the asteroid belt in its correct location, and showed it was fed objects from both the inner and outer regions.

"The most common type of meteorites we get from the asteroid belt are isotopically similar to Mars," Dasgupta said. "Andre explains why Mars and these ordinary meteorites should have a similar composition. He's provided a nuanced answer to this question."

Pressure-bump timing and super-Earths

Izidoro said the delayed appearance of the sun's middle ring in some simulations led to the formation of super-Earths, which points to the importance of pressure-bump timing.

"By the time the pressure bump formed in those cases, a lot of mass had already invaded the inner system and was available to make super-Earths," he said. "So the time when this middle pressure bump formed might be a key aspect of the solar system."

Izidoro is a postdoctoral research associate in Rice's Department of Earth, Environment and Planetary Sciences. Additional co-authors include Sean Raymond of the University of Bordeaux, Rogerio Deienno of Southwest Research Institute and Bertram Bitsch of the Max Planck Institute for Astronomy. The research was supported by NASA (80NSSC18K0828, 80NSSC21K0387), the European Research Council (757448-PAMDORA), the Brazilian Federal Agency for Support and Evaluation of Graduate Education (88887.310463/2018-00), the Welch Foundation (C-2035) and the French National Centre for Scientific Research's National Planetology Program.