According to an article Theia (from the Giant impact hypothesis) was an asteroid and according to The Wikipedia article about Theia Theia was an Earth trojan (which is an asteroid). Which is dubious, so why it is dubious?. It feels very clear to me that Theia was a planet since it was 0.40 the Earth's mass, according to this ScienceDirect article.

In the Wikipedia article clicking the dubious - discuss hyperlink, someone said that Theia should be an asteroid since it has not cleared it's neighborhood according to IAU's 3rd rule. But why couldn't it have cleared it's neighborhood, being 0.40 M⊕ / Earth's Mass while Mercury is only 0.055 M⊕ ?

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    $\begingroup$ It obviously did not "clear its neighbourhood"... or it could not have collided with Earth. Wether asteroid, planetesimal, proto-planet, dwarf-planet... it's just a word for the same or very similar things at that time in the solar system. $\endgroup$ Dec 2, 2022 at 15:13
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    $\begingroup$ A dubious link indeed - in actual, reputable science riding this semantic question of 'what is a planet' is not really done - those are mostly armchair experts. Ignore. It's a $0.4 m_{\oplus}$ object. $\endgroup$ Dec 2, 2022 at 15:24
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    $\begingroup$ I am not sure if the IAU definition of planet can be applied to early solár systém bodies. $\endgroup$
    – Leos Ondra
    Dec 2, 2022 at 17:16
  • $\begingroup$ FWIW, the total mass of the main asteroid belt is calculated to be ~3% that of the Moon. $\endgroup$
    – PM 2Ring
    Dec 3, 2022 at 4:56
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    $\begingroup$ Please don't directly paste google search results. Doing so can result (and has resulted) in problems. Instead, visit the found link and paste that page's address. I've fixed that link. $\endgroup$ Dec 4, 2022 at 12:58

3 Answers 3


The IAU definition of "planet" is at best irrelevant here. The IAU defines a planet as a body that has cleared its neighbourhood. But in the early solar system the 8 bodies that are now classified as planets were in the process of clearing their neighbourhoods.

The classification of certain solar system bodies as "planets" is not arbitrary in the stable solar system in which we live now. It becomes arbitrary if you go back far enough in time. We can say that we can model the formation of the moon as being the result of a collision between a "proto-earth" and a body with about 40% of the mass of Earth.

Such a body would be far more massive than any current asteroid. It would be planet-sized. The semantics of "is it an "asteroid" or "planet"" are not relevant to that period in the solar system's history.

  • $\begingroup$ The only meaningfull definition is the geophysical one (you can categorize a body based on Its properties without reference to neighborhood). $\endgroup$
    – Leos Ondra
    Dec 2, 2022 at 19:52
  • $\begingroup$ @LeosOndra Many astronomers disagree with what you wrote, including the IAU. There are no crisp boundaries with regard to properties such as roundness. On the other hand, there is a huge gap between the one object in the solar system classified as a star and everything else in the solar system, and another huge gap between the eight objects in the solar system classified as planets and the other smaller bodies. The gap between stars and non-stars gets a bit fuzzy outside of the solar system, and determining roundness outside of the solar system is beyond science's current capabilities. $\endgroup$ Dec 4, 2022 at 13:46
  • $\begingroup$ Maybe the IAU definition is useful for solár systém. But if I understand what you wrote, it would ve enough to define the bodies in solar system by enumeration - stars: Sun, planets: Mercury, ..., Neptune. $\endgroup$
    – Leos Ondra
    Dec 4, 2022 at 16:50

Was Theia a planet or an asteroid?

Neither. It was, if the giant impact hypothesis is correct, a planetary embryo. Not yet a planet, but bigger than any known asteroid.

It feels very clear to me that Theia was a planet since it was 0.40 the Earth's mass, according to this ScienceDirect article.

That's one article, and it hasn't been cited very often, which suggests it might not be that good. It's best to view recently published peer reviewed articles as potentially suspect. Getting an article published in a peer reviewed scientific journal is where science starts, not ends. It's been estimated that many (maybe even most) peer reviewed papers are wrong. This is a much bigger problem in the soft sciences, but it's a problem with models of star systems as well.

You should not be taking that 0.4 Earth masses as a given. It's a conjecture. Most scientists lean toward a Mars-sized object, or about 0.1 Earth masses. The science is not settled. Nor should you be taking the conjecture in the wikipedia article on Theia that states that Theia formed in the vicinity of the Earth-Sun L4 or L5 point as a given. The wikipedia article does write as if this is a fact. That also is a conjecture.

In fact, the giant impact hypothesis itself is a conjecture. (Hypothesis and conjecture are near synonyms.) The giant impact hypothesis is by far the most widely accepted scientific explanation of the Moon's origin, but there are other explanations out there. In addition, there are many different variants of the giant impact hypothesis.

Take everything you read in wikipedia with a giant grain of salt, and everything published in scientific journals with a smaller grain of salt.

But why couldn't it have cleared it's neighborhood, being 0.40 M⊕ / Earth's Mass while Mercury is only 0.055 M⊕ ?

Theia obviously couldn't clear its neighborhood because the presumably larger port-Earth was in Theia's orbital neighborhood. The larger proto-Earth also had not yet cleared its neighborhood up until the time of the giant impact because Theia was in the proto-Earth's orbital neighborhood. Impacting one another is one of the multiple mechanisms by which planet sized objects clear their neighborhood.

The IAU definition of a planet pertains to the solar system after it had formed and more or less settled down into its current state. Conjectures abound regarding the mechanisms by which the solar system formed and then settled down. Some have hypothesized a fifth giant planet that was ejected from the solar system. Some have hypothesized that Uranus and Neptune switched places. Some have hypothesized that Jupiter dove deep into the solar system, to about where Mars is now, only to move back out to where it is now. This last one, the grand tack model, is fairly widely accepted, but it is not settled science.

Regarding the origin of the Moon, there is a lot that remains unsettled. The giant impact hypothesis explains a number of mysteries, but leaves several unsolved mysteries. Another yet to be resolved mystery is when the Earth got its water. If this happened mostly after the giant impact such as the late heavy bombardment, it wouldn't be surprising that the Earth and the Moon exhibit different hydrogen isotope ratios.


If the hypothetical Theia had 0.40 the mass of Earth, it would definitely be in the category of planetary mass objects, or planemos.

A planemo is a geophysical planet, a planetary mass object.

A body massive enough to pull itself into a spheroid shape by its own gravity, but not massive enough to become a brown dwarf (which is about 13 times the mass of Jupiter or about 4,131.4 times the mass of Earth).

The composition of a world will determine what mass is sufficient to make it gravitationally rounded. An iron object can remain irregularly shaped at a much higher mass than an icy object would become rounded at.

Planetary mass objects in our solar system have masses as low as 0.0001 the mass of Earth. So the hypothetical Theia would definitely be a planetary mass object and a planet by the the geophysical definition of a planet.

One reason why the International Astrophysical Union adopted a definition of planets which required them to have "cleared" their orbits of other objects was to avoid having to recognize many new Trans Neptunian Objects (TNOs) of planetary mass as planets, which was considered undesirable.

But even mighty Jupiter hasn't cleared its orbit of all other objects. In fact its orbit has quite a lot of asteroids in the L4 and L5 positions, the Trojan asteroids. Venus, Earth, Mars, Uranus, and Neptune also have Trojan asteroids. So, at best, there could be only two planets in our solar system which have truly cleared their orbits: Mercury and Saturn.

So think of Theia as a planetary mass object, a planemo, a geophysical planet, or a plain planet, if you want. Those seem like accurate terms for an object of that mass, whatever its orbital characteristics.

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    $\begingroup$ I think you've misspelled "planemo" several times. But that's also semantics. It's all just protoplanets. Nobody in the planet formation literature cares about those labels, because they are not meaningful in the early phases. $\endgroup$ Dec 3, 2022 at 0:52
  • $\begingroup$ Isn't also the geophysical definition of planet / brown dwarf tied to their origin / history? For brown dwarf we assume hydrogen / helium body. Would silicate / iron planet become brown dwarf oř a star if we add enough material? $\endgroup$
    – Leos Ondra
    Dec 13, 2022 at 19:44

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