To date, a total of 4,884 extrasolar planets have been confirmed in 3,659 systems, with another 8,414 additional candidates awaiting confirmation. In the course of studying these new worlds, astronomers have noted something very interesting about the “rocky” planets. Since Earth is rocky and the only known planet where life can exist, astronomers are naturally curious about this particular type of planet. Interestingly, most of the rocky planets discovered so far have been many times the size and mass of Earth.
Of the 1,702 rocky planets confirmed to date, the majority (1,516) have been “Super-Earths,” while only 186 have been similar in size and mass to Earth. This raises the question: is Earth an outlier, or do we not have enough data yet to determine how common “Earth-like” planets are. According to new research by an international team led by Rice University, it may all have to do with protoplanetary rings of dust and gas in an early solar system.
The team consisted of researchers from the Department of Earth, Environmental and Planetary Sciences (EEPS) at Rice University, Houston, Texas. They were joined by astrophysicists from the Laboratoire d’Astrophysique de Bordeaux (LAB), the Southwest Research Institute (SwRI) in Boulder, Colorado, and the Max-Planck Institute for Astronomy (MPIA) in Heidelberg, Germany. Their findings were shared in a paper that recently appeared in the journal Nature Astronomy.
This spectacular image from the SPHERE instrument on ESO’s Very Large Telescope is the first clear image of a planet caught in the very act of formation around the dwarf star PDS 70. Credit: ESO/A. Müller et al.
Solar System Architecture
According to the most widely-accepted theories of planetary formation (Nebular Hypothesis), solar systems begin as clouds of dust and gas. In time, this cloud will begin to swirl and coalesce to the point that it will undergo gravitational collapse at the center and give birth to a new sun. The remaining nebular material will settle into rotating rings due to the star’s angular momentum, which will slowly accrete to form planetesimals (and eventually planets).
These rings will typically have gap-like structures in them, which will profoundly impact the orbits of future planets. As the Rice team theorized, something happened during this period in the Solar System’s history to prevent Earth from growing to become a Super-Earth. Rice University astrophysicist André Izidoro, who led the research, explained in a Rice University press release:
“If super-Earths are super-common, why don’t we have one in the solar system? 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.”
According to precious models of planetary formation, protoplanetary disks become less dense as a function of distance from the star. However, computer simulations have shown that planets are unlikely to form in a smooth disk. “In a smooth disk, all solid particles — dust grains or boulders — should be drawn inward very quickly and lost in the star,” said Andrea Isella, an associate professor of physics and astronomy at Rice and a co-author on the study. “One needs something to stop them in order to give them time to grow into planets.”
The sharpest image ever taken by ALMA shows the protoplanetary disk surrounding the young star HL Tauri. Credit: Andrea Isella/Rice University
When dust particles move faster than the gas around them, causing them to pick up a “headwind” that causes them to move very quickly towards the star. At pressure bumps, gas pressure increases, causing gas molecules to move faster while solid particles are slowed down, allowing them to accumulate. Such bumps, said Isabella, were observed in protoplanetary disks by astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) 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. 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.”
Their model included the formation of three high-pressure bumps within the Sun’s young protoplanetary disk 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 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 any case, the three pressure bumps in the Rice simulations coincided with the sublimation lines of silicate, water, and carbon monoxide – i.e., the line where these become vapor. Being the nearest ring to the Sun, the silicate ring is where Mercury, Venus, Earth, and Mars would form. The middle ring appeared at the “Snow Line” between Mars and Jupiter, while the farthest ring coincided with the Kuiper Belt.
An illustration of three distinct, planetesimal-forming rings that could have produced the planets and other features of the solar system. Credit: Rice University/Rajdeep Dasgupta (et al.)
To test this model, Izodoro and his team ran hundreds of supercomputer simulations to recreate the formation of the Solar System. The results proved to be a vast improvement over previous models of Solar System formation. Not only did their simulations produce rings like those seen in many young star systems, but they also faithfully reproduced several features of the Solar System that previous models failed to produce. This included:
- An Asteroid belt between Mars and Jupiter containing objects from the inner and outer Solar System
- The locations and stable orbits (nearly circular) of Mercury, Venus, Earth, and Mars
- An accurate assessment of masses for the rocky inner planets
- The chemical composition between the planets of the inner and outer Solar System
- A Kuiper Belt of small bodies, asteroids, and comets beyond the orbit of Neptune
This simulation effectively showed how dust accumulates in protoplanetary disks as they cool and their sublimation lines migrate towards the Sun. Over time, this would give rise to planetesimals, which would then come together to form planets. In previous studies, astronomers assumed that planetesimals could form if dust were sufficiently concentrated, but no known mechanism existed to explain how dust might accumulate. Said Izodoro:
“Our model shows pressure bumps can concentrate dust, and moving pressure bumps can act as planetesimal factories. We simulate planet formation starting with grains of dust and covering many different stages, from small millimeter-sized grains to planetesimals and then planets.”
A labeled version of four of the twenty disks that comprise ALMA’s highest resolution survey of nearby protoplanetary disks. Credit: ALMA (ESO/NAOJ/NRAO) S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello
One of the improvements Rice team’s model offers over previous simulations is how it accurately predicted the mass of Mars. In previous simulations, Mars was predicted to be significantly more massive; in some cases, up to be ten times more massive than Earth. In contrast, the Rice team’s model correctly predicted the true mass of Mars (about 10% of Earth) because it showed that Mars was born in a low-mass region of the disk. It also addressed why Earth-sized planets appear to be in the minority of rocky exoplanets.
According to some of their simulations, a delay in the appearance of the middle ring led to the formation of super-Earths, indicating 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,” Izodoro added. “So the time when this middle-pressure bump formed might be a key aspect of the solar system.”
Last, but not least, the model provides a compelling explanation for two mysteries that have to do with the chemical makeup of the Solar System. First, there’s the marked difference in composition between the rocky planets of the inner Solar System and the gas/ice giants and icy bodies of the outer Solar System. Second, there’s the fact that both types of objects are present in the Main Asteroid Belt in between the inner and outer Solar System.
Regarding the first mystery, the team’s simulations showed that the middle ring could account for the chemical dichotomy by preventing material from the outer Solar System from entering the inner system. Regarding the second, the simulations predicted the correct location of the Asteroid Belt and showed it was fed objects from both the inner and outer regions. By looking at the protoplanetary disk as a series of rings rather than a uniform disk, the Rice team’s model provides a nuanced answer to these unresolved questions.