The Missing Planets: The Enigma of the Solar System's Lost Super-Earths

Scientific Summary

This article explores the enigma of the absence of super-Earths in our Solar System, while they are common around other stars. Material clues, such as high-pressure diamonds (formed at >20 GPa) found in meteorites (ureilites, Almahata Sitta), suggest the past existence of these massive rocky planets (2-5 Earth masses). According to the Grand Tack model, the early migration of Jupiter would have acted as a gravitational barrier, ejecting or destroying these super-Earths. Their absence may have favored the exceptional orbital stability of the Solar System, allowing the evolution of life on Earth.

Why are there no super-Earths in our Solar System?

The most likely reason is that our Solar System once possessed them, but they disappeared. Mineralogical evidence, such as diamonds formed under extreme pressure in certain meteorites, indicates they existed. Their disappearance would be due to the early migration of Jupiter (Grand Tack model). This gas giant would have created a gravitational barrier, ejecting the super-Earths out of the Solar System or causing them to fall into the Sun. This absence may have been beneficial, offering a unique orbital stability conducive to the emergence of life on Earth.

The enigma of the lost super-Earths

What if our solar system once hosted giant worlds that have now disappeared? Extraterrestrial diamonds found in meteorites reveal the past existence of super-Earths, massive rocky planets that may have orbited the young Sun before being ejected into interstellar space.

The forgotten worlds of the young solar system

Super-Earths are among the most common exoplanets observed in the Milky Way. Yet, our solar system lacks them. This apparent absence, despite nature favoring their formation, suggests they once existed before disappearing. The most convincing material evidence comes from certain meteorites containing high-pressure diamonds, formed inside massive planetary bodies now lost.

Diamonds formed under extreme pressures

Analyses of ureilite meteorites have revealed diamond crystals several tens of microns in size, with metallic inclusions (Fe, Ni, Cr) formed at over 20 GPa. Such pressures can only be achieved in rocky planets several times the mass of Earth, far beyond the capabilities of a simple asteroid.

Considering Earth's average density, a pressure of 15 to 20 GPa corresponds to a body of about 2 to 5 Earth masses, i.e., a super-Earth. These diamonds thus testify to the existence of a planetary mantle subjected to internal conditions comparable to those of Uranus or Neptune.

N.B.:
Diamond-bearing ureilites may represent the only mineralogical witnesses of the lost super-Earths of the early solar system. Their internal structures testify to pressures inaccessible to simple asteroids, supporting the idea of a planetary population that disappeared before the stabilization of current orbits.

Ejection models and Jupiter's barrier

Simulations by Sean Raymond and Alessandro Morbidelli show that Jupiter acted as a gravitational barrier, preventing the inward migration of super-Earths in the solar system. This interaction led to their ejection or destruction. The phenomenon is described in the context of the Grand Tack model, where Jupiter migrates to 1.5 AU before moving outward, destabilizing planetary embryos.

The Grand Tack model

The Grand Tack model is a dynamic hypothesis proposed by Alessandro Morbidelli and Sean Raymond, describing the early migration of Jupiter and Saturn in the primitive nebula. According to this model, Jupiter first migrated toward the Sun to ≈1.5 AU before "turning around" due to the resonant effect of Saturn. This movement would have disrupted inner planetary embryos, ejected potential super-Earths, and limited the final mass of Mars. The term "tack" comes from the sailing maneuver of tacking, illustrating the gravitational change of direction of the two giants.

A super-Earth reaching an ejection speed greater than 42 km/s could have become an interstellar planet, permanently leaving the solar system.

The Almahata Sitta meteorite: witness to a lost world

The Almahata Sitta meteorite, which fell in Sudan in 2008, contains high-purity diamonds, confirmed by spectroscopy. The metallic inclusions it contains require formation at pressures of 20 to 25 GPa. According to Farhang Nabiei (EPFL, 2018), these diamonds come from a parent body the size of Mercury or a super-Earth of several Earth masses.

How to read history in diamonds?

Scientists use several techniques to date and characterize these extraterrestrial diamonds:

A solar system of rare stability

The absence of super-Earths may have favored the gravitational stability of the solar system. Without these intermediate masses, the current planets occupy nearly circular orbits, avoiding destructive resonances. This prolonged stability allowed the slow and continuous evolution of life on Earth, an exceptional scenario in exoplanetary statistics.

Distribution of observed stellar system types

Distribution of stellar system types in the Galaxy
System type	Observed proportion	Gravitational structure	Physical comments
Single system (one star)	≈ 45%	A single central star	Stable and common for low-mass stars, like the Sun.
Binary system	≈ 40%	Two stars in mutual orbit around their barycenter	Can cause planetary perturbations but also favors matter exchange.
Tertiary system (triple)	≈ 10%	Two close stars accompanied by a third more distant star	Conditional stability: requires strict orbital hierarchy to avoid gravitational ejection.
Multiple system (≥ 4 stars)	≈ 5%	Nested orbits around several secondary barycenters	Very unstable in the long term; often result from the initial fragmentation of a molecular cloud.

Stellar multiplicity rate by spectral type
Spectral type	Average mass (M_☉)	Multiple system rate (approx.)	Physical implication
O–B (massive)	≈ 8–40	≈ 80–100%	Formation in unstable cores, strong cloud fragmentation, very high probability of binarity and close multiples.
A–F	≈ 1.5–2.5	≈ 60–75%	Moderate fragmentation; frequent multiple systems but more hierarchical.
G (solar-type)	≈ 1.0	≈ 45%	Mixed: a substantial fraction of binaries but a significant number of single stars.
K	≈ 0.6–0.9	≈ 30–40%	Fewer companions; protoplanetary disks often more stable.
M (red dwarfs)	≈ 0.1–0.5	≈ 20–30%	Dominant population in the Galaxy; low multiplicity results in a majority of solitary stars.
All types (weighted average)	—	≈ 40–45%	Weighted average value by the initial mass function (IMF): the large proportion of red dwarfs lowers the overall average.

N.B.:
The often-read claim that "80% of stars are binary" is correct for observed **massive** star populations (O–B), but it is misleading if extended to all stars in the Galaxy. The Galaxy is numerically dominated by red dwarfs (type M), which have a low multiplicity rate, giving a weighted average of ≈40–45% multiple systems at the galactic level.

FAQ: All about the disappeared super-Earths

What is a super-Earth and how do we know they existed in our Solar System?

A super-Earth is a rocky planet whose mass is between 1 and 10 times that of Earth. We know they could have existed in our Solar System thanks to material evidence: diamonds formed at extreme pressures (>20 GPa) have been found in meteorites. Such pressures can only be reached inside massive planetary bodies like super-Earths, not in simple asteroids.

What is Jupiter's role in the disappearance of super-Earths?

According to the Grand Tack model, Jupiter migrated towards the Sun before turning back. This movement would have created a gravitational barrier that would have destabilized the orbits of the super-Earths. These would then have been either ejected into interstellar space (becoming rogue planets) or sent crashing into the Sun, thus disappearing from the Solar System.

What is the link between the absence of super-Earths and life on Earth?

The absence of these massive planets contributed to the exceptional gravitational stability of our Solar System. The current planets, including Earth, have nearly circular and regular orbits, avoiding destructive resonances. This long-term stability is considered a key factor that allowed the slow and continuous evolution of life on our planet.