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Solar System Series

Pluto

Planet or not, it is in our solar system: the world with an icy heart.

Author: Juan Carlos Euyoque Aguilera · October 7, 2026 · CC BY 4.0

Image of Pluto (NASA/JPL, public domain)
NASA/JPL illustration, public domain. Not to scale.
2,376.6 ± 3.2 km
Diameter
248 Earth years
Year
5
Moons
1,600 km
Tombaugh Regio

Pluto was the ninth planet for 76 years, until the International Astronomical Union (IAU) reclassified it as a dwarf planet on 24 August 2006. The decision is still debated: this piece presents what the definition says and what its critics answer, without declaring a winner.

On 14 July 2015 NASA's New Horizons spacecraft visited it — the first and only close visit — and found a geologically living world: a convecting nitrogen glacier shaped like a heart, mountains of water ice, and a thin atmosphere with a blue haze.

Measured data [DATO]

DataValueStateSource
New Horizons flybyClosest approach 14-Jul-2015 at ~12,500 km, at ~49,600 km/h; launched 19-Jan-2006; the first and only spacecraft to visit Pluto up close[DATO] observedScience News Explores; Spaceflight Now
Diameter2,376.6 ± 3.2 km — smaller than the Moon (3,476 km)[DATO] observedNASA JPL; Wikipedia (Pluto), a partir de datos de radio-ocultación REX de New Horizons
Tombaugh RegioBright heart-shaped region ~1,600 km wide; named for Clyde Tombaugh, who discovered Pluto in 1930[DATO] observedNASA (artículo de imagen)
Sputnik PlanitiaThe heart's western lobe: a glacier of convecting nitrogen ice (polygonal cells); nearly crater-free surface less than ~100 million years old[DATO] observedNASA (PIA19841)
AtmosphereThin and nitrogen-dominated (~1 Pa / 10 μbar at the surface); blue haze in layers up to ~160 km high[DATO] observedScientific American (hallazgos de New Horizons)
Moons5 known: Charon (1978, James Christy), Nix and Hydra (2005, Hubble), Kerberos (2011), Styx (2012)[DATO] observedSpace.com / NASA
Charon1,207 km in diameter (~half of Pluto); the system's barycenter lies in the space between the two: a double world[DATO] observedSpace.com / NASA
IAU 2006 definition (what it says)24-Aug-2006, Prague, Resolution 5A: a planet = (1) orbits the Sun, (2) massive enough for hydrostatic equilibrium, (3) has cleared its orbital neighborhood. Pluto meets 1 and 2, not 3 → dwarf planet[DATO] observedWikipedia (IAU definition of planet)
Counter-argument (opinion)[OPINION] Stern and other planetary scientists propose a geophysical definition: a planet is any body round by its own gravity that does not fuse elements — it would include 100+ bodies, including the Moon. Criticism of criterion 3: it depends on where a body is, not just on what it is[MODELO] modeledPhys.org (definición geofísica de planeta, 2017)

Truths [DATO]

  • [DATO] The only spacecraft to visit it up close is New Horizons (flyby of 14-Jul-2015).
  • [DATO] It is 2,377 km in diameter: smaller than the Moon.
  • [DATO] Its 'heart' (Tombaugh Regio) hosts a convecting nitrogen glacier: Sputnik Planitia.
  • [DATO] It has a thin nitrogen atmosphere with a blue haze.
  • [DATO] It has 5 moons; with Charon it forms a double world (the barycenter lies outside Pluto).
  • [DATO] Its year lasts 248 Earth years.

Marked myths [MITO]

[MITO] Pluto stopped being a planet because it turned out to be too small.

Size is not a criterion in the IAU's 2006 definition. Pluto meets two of the three criteria (it orbits the Sun and is round by its own gravity); it fails only the third: it has not 'cleared its neighborhood' in the Kuiper Belt.

Fuente: Wikipedia (IAU definition of planet).

[MITO] The 2006 vote settled the debate: astronomers agree.

It was a controversial decision (Resolution 5A, Prague, 24-Aug-2006). Scientists like Alan Stern (New Horizons principal investigator) propose the geophysical definition and call for revisiting it; the debate remains open. No winner is declared here: both sides are presented.

Fuente: Phys.org (definición geofísica, 2017); Wikipedia (críticas a la definición IAU).

[MITO] Pluto is a dead, frozen rock with no change.

New Horizons found an active world: Sputnik Planitia's nitrogen glacier convects like a slowly boiling liquid, there are water-ice mountains kilometers high and an atmosphere with a blue haze; parts of the surface are less than 100 million years old.

Fuente: NASA (PIA19841); SpaceDaily (resumen del sobrevuelo).

Open questions [PREGUNTA]

  • [PREGUNTA] What keeps Pluto geologically active if it has no giant neighbor flexing its interior?
  • [PREGUNTA] Is there a liquid water ocean beneath its ice crust?
  • [PREGUNTA] Will the IAU ever revisit its planet definition?

The chain in brief

  1. [DATA] 1. Ninth planet for 76 years: Discovered by Clyde Tombaugh in 1930, it was the ninth planet until the IAU reclassified it as a dwarf planet on 24 August 2006.
  2. [DISPUTED] 2. A definition that did not settle the debate: The IAU definition requires a cleared orbital neighborhood, which Pluto does not meet; the geophysical definition proposed by Stern and others would include more than 100 bodies, including the Moon. The 2006 vote did not leave astronomers in agreement.
  3. [DATA] 3. The New Horizons visit: On 14 July 2015 the New Horizons spacecraft flew by at ~12,500 km: the first and only spacecraft to visit it up close.
  4. [DATA] 4. A living world with an icy heart: Its bright heart-shaped region, Tombaugh Regio (~1,600 km wide), hosts Sputnik Planitia: a convecting nitrogen glacier whose surface is less than ~100 million years old.
  5. [DISPUTED] 5. What remains to be learned: What keeps Pluto geologically active and whether there is a liquid-water ocean beneath its ice crust remain open; with Charon, its largest moon, it forms a double world.