Astronomy · Planets

Mercury

The smallest planet, closest to the Sun.

Order from Sun: First planet
Type: Terrestrial (rocky) planet
Size: Smallest planet in the Solar System
Atmosphere: Only an extremely thin exosphere

NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington · NASA media (public domain)

Gallery

Mercury in Bronze
2014
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington · NASA media (public domain)
Source archive ↗

The innermost planet

Mercury orbits closer to the Sun than any other planet and is the smallest of the eight. It is a rocky, terrestrial world with a large metallic core relative to its size.

A world of extremes

With essentially no atmosphere to trap or redistribute heat, Mercury experiences enormous differences between its sunlit and night sides. Its cratered, Moon-like surface preserves the scars of ancient impacts.

Hard to observe

Because it never strays far from the Sun in our sky, Mercury is best seen low on the horizon during twilight, and only briefly. Spacecraft have provided most of our detailed knowledge of it.

Knowledge connections

How this connects across Asteria Star — scientific, cultural, and astrological links are kept separate.

Scientific connections

Part ofSolar System
Associated withCore
Associated withImpact Cratering History
Associated withSpin–Orbit Coupling

Mission connections

TargetsMESSENGER
TargetsMariner 10
Target ofMariner 10
Target ofMESSENGER
Target ofBepiColombo

Has featureCaloris Planitia
Has featureDiscovery Rupes

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Fast facts

Order from Sun: First planet
Type: Terrestrial (rocky) planet
Size: Smallest planet in the Solar System
Atmosphere: Only an extremely thin exosphere

Key points

The innermost and smallest planet.
Has almost no atmosphere, so temperatures swing wildly between day and night.
Its surface is heavily cratered, recording a long history of impacts.

Data & provenance

Stable ID: planet:mercury
Type: Planet
Domain: Scientific connections
Relationships: 11
Graph version: 1.0.0

Dataset membership

Planets

Open data

In the graph export: graph.json · graph.jsonld

Planned API: GET /api/v0/entities/planet:mercury

Scientific quality

Complete

Reviewed2 sourcesv1.0.0

6 of 6 applicable dimensions complete

Description: Complete
Source coverage: Complete
Relationship coverage: Complete
Review coverage: Complete
Citation coverage: Complete
Image coverage: Complete
Timeline coverage: Not applicable

Scientific entity. Dimensions that cannot apply to this object are excluded rather than counted as gaps. See the evidence framework and authority dashboard.

Provenance & sources

review 2026.1

Asteria Scientific Review Process · verification: sourced · accurate

High evidenceconfirmed
Mercury is the smallest planet and the closest to the Sun, with only a tenuous exosphere and the largest day-to-night temperature swing of any planet.
Source: NASA (NSSDCA) — NASA Planetary Fact Sheet · Public domain (US Government work)
- NASA Planetary Fact Sheet, Planetary Fact Sheet

Reviewed by the internal Asteria Scientific Review Process — not an external institutional review. See the evidence framework and authority dashboard.

Scientific citations

Real, source-backed references — primary papers first, then datasets and institutional sources. Formatted through the citation engine; nothing is fabricated.

DatasetApproximate Positions of the Planets

NASA Jet Propulsion Laboratory (Caltech)

Cite (APA / BibTeX)

NASA Jet Propulsion Laboratory (Caltech) (n.d.). Approximate Positions of the Planets. NASA Jet Propulsion Laboratory (Caltech). https://ssd.jpl.nasa.gov/planets/approx_pos.html

@misc{cite:jpl-approx-positions,
  title        = {Approximate Positions of the Planets},
  organization = {NASA Jet Propulsion Laboratory (Caltech)},
  year         = {n.d.},
  url          = {https://ssd.jpl.nasa.gov/planets/approx_pos.html},
  note         = {Public-domain Keplerian orbital elements and rates (Standish) — the method behind the computed planet positions, rise/set, and visibility.}
}

DatasetMercury Fact Sheet

NASA Goddard — NSSDCA

Cite (APA / BibTeX)

NASA Goddard — NSSDCA (n.d.). Mercury Fact Sheet. NASA Goddard — NSSDCA. https://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html

@misc{cite:factsheet-planet-mercury,
  title        = {Mercury Fact Sheet},
  organization = {NASA Goddard — NSSDCA},
  year         = {n.d.},
  url          = {https://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html},
  note         = {Physical and orbital parameters for Mercury.}
}

Institutional pageMercury — NASA Solar System Exploration

NASA

Cite (APA / BibTeX)

NASA (n.d.). Mercury — NASA Solar System Exploration. NASA. https://science.nasa.gov/

@misc{cite:nasa-planet-mercury,
  title        = {Mercury — NASA Solar System Exploration},
  organization = {NASA},
  year         = {n.d.},
  url          = {https://science.nasa.gov/},
  note         = {NASA overview of Mercury.}
}

Institutional pagePlanets — NASA Science

NASA

Cite (APA / BibTeX)

NASA (n.d.). Planets — NASA Science. NASA. https://science.nasa.gov/planets/

@misc{cite:nasa-planets,
  title        = {Planets — NASA Science},
  organization = {NASA},
  year         = {n.d.},
  url          = {https://science.nasa.gov/planets/},
  note         = {NASA overview of the planets: how the naked-eye planets appear and move through the sky.}
}

Sources & references

The primary and reference sources this topic draws on.

NASANational Aeronautics and Space Administration
Mission data, planetary science, space telescopes, and public-domain imagery.
Most NASA-produced imagery is in the public domain; individual items are checked for usage terms before publication.
NASA JPLJet Propulsion Laboratory / Solar System Dynamics
Orbital data, ephemerides, and small-body parameters for planets, asteroids, and comets.

Mercury

Gallery

The innermost planet

A world of extremes

Hard to observe

Knowledge connections

Scientific connections

Mission connections

Related

Venus

Earth

Mars

Jupiter

BepiColombo

Mariner 10

MESSENGER

You might also explore

Mercury

Gallery

The innermost planet

A world of extremes

Hard to observe

Knowledge connections

Scientific connections

Mission connections

Related

Related entries

Venus

Earth

Mars

Jupiter

Related missions

BepiColombo

Mariner 10

MESSENGER

You might also explore