The astronomy world has been abuzz for several years regarding the discovery of so many mini-Neptunes – planets that seems to be similar in characteristics to Neptune, but smaller.

A Mini-Neptune, also called an ice dwarf or transitional planet, is a planet less massive than Neptune but resembling Neptune in that it has a thick hydrogen–helium atmosphere, probably with deep layers of ice, rock or liquid oceans, likely made of water, ammonia, a mixture of both or heavier volatiles that are able to remain liquid in the conditions within the planet.

An ice dwarf is a planet with a rocky core that has accumulated a thick envelope of hydrogen, helium, and other volatiles, having, as a result, a total radius between 1.7 and 3.9 Earth radii (1.7–3.9 REarth). The term is used in a three-tier, metallicity-based classification regime for short-period exoplanets, which also includes the rocky, terrestrial-like planets with less than 1.7 REarth and planets greater than 3.9 REarth, namely the ice giants and gas giants, we will likely find worlds that sit between these and call them gas dwarves!!

We have none of these in the solar system, there is a significant gap between Earth, the most massive of the terrestrial planets, and Uranus which is some 14.3 times the mass of Earth and 4 times the radius of Earth, that is a large void between the sizes of the planets of the inner solar system, and those of the outer solar system – which seems to be unusual in those exoplanets we have so far identified and the objects of interest that need further confirmation.

The Solar system appears to be an oddball, but is it really? We have found several stars with 5 planets or more, such as Trappist 1, and it is likely we are only finding those whose orbital parameters lend themselves to our search techniques and technology limits – it is highly likely more planets will be found that reside in longer orbits around many stars showing that in complement at least – our solar system is just boringly normal – which is actually good for us.

Mercury is a bit of an anomaly in our solar system, not least because Mercury has the most eccentric orbit of the major planets, only Pluto (no debate here about planethood) has a more eccentric orbit unless you include comets, some asteroids, scattered disk objects and likely a large number of Oort cloud objects, an example of which is Sedna, having an eccentricity of 0.854, for comparison, Earth has an eccentricity of only 0.016.


Mean Diameter: 4,880km

Mean Density:   5.427g/cm³

Inclination: 7.005° to the ecliptic

Aphelion:  0.4666 AU (69,816,900 km)

Perihelion: 0.3074 AU (46,001,200 km)

Orbital eccentricity: 0.205630

The core of Earth forms around 17% of the bulk of the planet, but Mercury’s core is a whopping 85% of the bulk of the planet, making it the largest core, relative to the size and mass of the planet, in the solar system.

Image 1: Comparison of the relative sizes of the terrestrial planet cores.

Image courtesy of NASA

Clearly something happened to Mercury in the early days of the solar system that changed its overall makeup, but what was this, was it struck by a large object in a glancing blow that stripped off much of its outer surface? This has been speculated upon, but this idea has its detractors.

We know the early solar system was a chaotic and violent place, Venus had one or more events that slowed the planet down to the point where a year is shorter than a day, and it rotates backwards, current theories for the formation of the Moon posit that a mars mass planet collided with the proto-Earth, which they believe to have been around 10% more massive, and this collision not only stripped mass from proto-Earth, but resulted in the formation of the Moon. This theory has its problems, there is still a lot of missing mass, but it does fit the best models at this time and helps to clear up some questions – even though it opens the door to many others.

However, what if Mercury formed as a mini-Neptune, it had a nice heavy core, it pulled in other material and perhaps grew to several times Earth mass with a large outer envelope of volatiles – we have no evidence for this, but it is not impossible. The Sun was a very much dimmer star in the early days of the solar system, in fact it is possible that the planets formed before the nuclear furnace at the heart of the Sun really kicked into life, so planets with volatiles in their outer envelopes may have been prevalent, but as the Sun awoke, the energy expended increased exponentially, the Sun became extremely active, we believe it went through a period typified by the T-Tauri stars. This is a dramatic period in the lives of young proto stars where they vary dramatically in brightness, thus output, it is believed the energy of the star at this stage is from gravitational collapse and matter falling onto the surface of the star as nuclear fusion is not believed to have started. This period may last up to 100 million years, and for a planet with an extended envelope, this period could well be the undoing of its life as a mini-Neptune.

In the early days, when our Sun was in the T-Tauri phase, some 4.56 billion years ago (approximately), it would not have been burning hydrogen in its core. It would have been some 15-20% larger in diameter than today and may even have had jets emanating from it.

Image 2: The T Tauri star HH-30 shows ‘bullets’ of material propagating along a jet.

NASA, A. Watson (UNAM), K. Stapelfeldt (JPL), J. Krist (STScI) and C. Burrows (ESA/ STScI)

The material that falls onto the star provides much of the material for the jets that have been observed being ejected from these stellar objects, the speculation is that the material is from the accretion disk from which planets will eventually form, but if they have already formed, the material being blasted off the stellar mass at the heart of the system would, very quickly, strip any volatiles within the envelopes of planets in the firing line or with insufficient mass to resist the blast.

We see material blasted off our Sun as Coronal Mass Ejections (CME), at the T-Tauri phase, and likely once the Sun moved onto the main sequence and settled down, the CMEs from the Sun could very easily have stripped any planetary atmospheres. We know that the solar wind would have severely depleted Earth’s atmosphere of most of the lightest volatiles were it not for the magnetic field, mercury, Venus and mars do not have this luxury.

Earth has an atmosphere that reaches out more than 644,000km (400,000 miles), so far in fact that the Moon orbits, technically, inside the atmosphere, and technically, no human has ever left the atmosphere – although this would be very picky, the atom density near the Moon is around 0.2 atoms cm³, which is a really good vacuum by Earthly standards, however, what if this was once significantly denser than we see today, perhaps when Earth first formed the region now called the geocorona may have been millions of times more dense than today, Earth’s atmosphere may have been significantly larger, and Earth may have been a mini-Neptune or an ice dwarf.

We have no evidence this was the case; we know that the atmosphere of Earth was markedly different from today for the first 1-2 billion years, but how far it reached out beyond the surface and at what density is purely speculation – and that must be remembered, this is simply thinking out loud and not a scientific fact.

However, some people speculate that by the 2050’s the number of confirmed exoplanets may be counted in the tens of millions based on the current rate of detection and the speed of technological improvement to this end. Once we have millions of planets around other stars, perhaps in hundreds of thousands or even millions of planetary systems, we may detect enough planets at various evolutionary points to make more informed guesses as to whether the terrestrial planets in the solar system could have started out significantly different than those we have today



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