astronomy

Mega-Earth

Mega-Earth is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Mega-Earth rather than just read about it. In short: A mega-Earth or massive solid planet is a type of terrestrial exoplanet that is very massive and dense — more massive than super-Earths. The term "mega-Earth" was coined in 2014, though it remained an informal category until a quantitative definition was proposed for it in 2026.

Key takeaways

Mega-Earth belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Mega-Earth to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mega-Earth from memory before moving on to harder problems.

Reference excerpt

A mega-Earth or massive solid planet is a type of terrestrial exoplanet that is very massive and dense — more massive than super-Earths. The term "mega-Earth" was coined in 2014, though it remained an informal category until a quantitative definition was proposed for it in 2026. Based on the measured radii and densities of known exoplanets as of March 2026, mega-Earths appear to be a distinct category of exoplanets that are between 2.1 and 5.0 Earth radii (R🜨) and have densities higher than Earth's (5.5 g/cm3). Based on their observed high densities, mega-Earths are inferred to be largely made of solid material, such as rock, metal, and ice. If the mega-Earth is rich in volatiles like water, it may harbor a supercritical ocean under a thin atmosphere of hydrogen, helium, and other gases. The existence of mega-Earths challenges conventional theories for planetary formation, as massive planets should accrete large amounts of gas in addition to solid material from the host star's protoplanetary disk. It has been hypothesized that mega-Earths may either be remnant cores of evaporated gas giants or white dwarfs or products of consecutive collisions between super-Earths. Mega-Earths as evaporated gas giants have been theorized to occur frequently around luminous massive stars and supermassive black holes.

Examples

Kepler-10c was the first exoplanet to be classified as a mega-Earth. At the time of its discovery, it was believed to have a mass around 17 times that of Earth (M🜨) and a radius around 2.3 times Earth's (R🜨), giving it a high density that implied a mainly rocky composition. However, several follow-up radial velocity studies produced different results for Kepler-10c's mass, all much below the original 17 M🜨 estimate. In 2017, a more careful analysis using data from multiple different telescopes and spectrographs found that Kepler-10c is more likely around 7.4 M🜨, making it a typical volatile-rich mini-Neptune and not a mega-Earth. K2-56b, also designated BD+20°594b, is a much more likely mega-Earth, with about 16 M🜨 and 2.2 R🜨. At the time of its discovery in 2016, it had the highest chance of being rocky for a planet its size, with a posterior probability that it is dense enough to be terrestrial at about 0.43. For comparison, at the time the corresponding probability for Kepler-10c was calculated as 0.1, and as 0.002 for Kepler-131b. Kepler-145b is one of the most massive planets classified as mega-Earths, with a mass of 37.1 M🜨 and a radius of 2.65 R🜨, so large that it could belong to a sub-category of mega-Earths known as "supermassive terrestrial planets" (SMTP). It likely has an Earth-like composition of rock and iron without any volatiles. A similar mega-Earth, K2-66b, has a mass of about 21.3 M🜨 and a radius of about 2.49 R🜨, and orbits a subgiant star. Its composition appears to be mainly rock with a small iron core and a relatively thin steam atmosphere. Kepler-277b and Kepler-277c are a pair of planets orbiting the same star, both thought to be mega-Earths with masses of about 87.4 M🜨 and 64.2 M🜨, and radii of about 2.92 R🜨 and 3.36 R🜨, respectively. PSR J1719−1438 b may be one of the most massive mega-Earths ever known, with a mass of about 330 M🜨 and a radius less than 4 R🜨, slightly more massive but smaller than Jupiter. It is a pulsar planet which is most likely composed largely of crystalline carbon but with a density far greater than diamond. However, as it is a likely remnant core of a former white dwarf companion of PSR J1719−1438, it is instead considered an ultra-low-mass carbon white dwarf or object per some definitions. A 2026 study defined a mega-Earth to be a planet with a radius between 2.1 and 5 Earth radii and a density greater than 5.5 g/cm3, and listed 13 "confirmed" examples with well-measured radii and densities: K2-263 b, HD 88986 b, HD 207897 b, Kepler-538 b, HIP 97166 b, TOI-2093 c, GJ 143b, TOI-815 c, K2-292 b, GJ 523b, TOI-332 b, TOI-1853 b, and Kepler-411 b. The 2.1 radius limit is chosen to be above the small planet radius gap that separates super-Earths from sub-Neptunes.

Origin

The discovery of mega-Earths had challenged planetary formation theories. Formation mechanisms and the occurrence of such objects remain subjects of ongoing research and debate.

Around massive stars A 2007 study had suggested the possibility of hypothetical solid planets up to thousands of M🜨 forming around massive stars (B-type and O-type stars; 5–120 M☉). The hypothesis proposed that the protoplanetary disk around such stars would contain enough heavy elements, and that high UV radiation and strong winds could photoevaporate the gas in the disk, leaving just the heavy elements. For comparison, Neptune's mass equals 17 M🜨, Jupiter has 318 M🜨. The most massive of those objects were assumed to be up to approximately 4,000 M🜨 (or 13 MJ) per the said upper mass limit used in the IAU's working definition of an exoplanet. However, this limit has been debated due to no precise physical significance, with many exoplanet catalogs including objects with heavier masses, such as up to 60 MJ. Despite the suggestion of the possibility of massive solid planets, it lacks supporting evidence for planetary formation theories and was primarily based on simulating mass-radius relationships for rocky planets, without investigating whether planetary formation theories support the existence of such objects. The 2007 study acknowledged that such massive exoplanets are not yet known to exist. More recent research has shown that the ratio of protoplanetary disk mass to stellar mass decreases rapidly for massive stars with initial masses above 10 M☉, falling to less than 10−4. Furthermore, no protoplanetary disks have been observed around O-type stars to date. Given these considerations, the formation and existence of massive solid planets around massive stars remain speculative and require further research and observational evidence.

Around supermassive black holes

Illustrations

Worked examples

Example 1 — a first encounter with Mega-Earth

Start with the simplest possible case. Write down what Mega-Earth claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mega-Earth before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mega-Earth ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mega-Earth

In research: Mega-Earth appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mega-Earth in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mega-Earth is common in secondary-school and first-year university syllabi. It links to neighbouring topics Giant planets, Mega-Earths, Terrestrial planets, so understanding it makes those chapters shorter.
In everyday life: Look for Mega-Earth outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Mega-Earth in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mega-Earth means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Mega-Earth out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mega-Earth in simple terms?

Why does Mega-Earth matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Mega-Earth?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Mega-Earth.