Mars

Lore & Background

Mars formed approximately 4.5 billion years ago. Its geological history is divided into three primary periods: the Noachian (4.5 to 3.5 billion years ago), marked by impacts, valley formation, and possible water oceans; the Hesperian (3.5 to 3.3–2.9 billion years ago), dominated by volcanic activity and flooding; and the Amazonian (3.3–2.9 billion years ago to present), which continues today. The planet experiences seasons due to its axial tilt of 25 degrees, similar to Earth's 23.5 degrees. Mars has two small, irregular natural satellites: Phobos and Deimos. A 2023 study suggests Mars may have had a ring system 3.5 to 4 billion years ago, possibly formed from a moon 20 times more massive than Phobos, with Phobos as a remnant.

Reader's Guide

Mars has been observed throughout history as a red wandering star visible to the naked eye, acquiring diverse cultural associations. The first flight to Mars was Mars 1 in 1963, but communication was lost. The first successful flyby was Mariner 4 in 1965, and Mariner 9 became the first spacecraft to orbit another body in 1971, the same year as the first uncontrolled impact (Mars 2) and first successful landing (Mars 3). Probes have been active on Mars continuously since 1997, and at times more than ten probes have simultaneously operated in orbit or on the surface—more than at any other planet beyond Earth. The possibility of past or present life on Mars remains an active area of scientific investigation, with some possible traces needing further examination. Mars is an often proposed target for future crewed missions, though none are currently planned.

Did You Know?

• Mars has the tallest volcano in the Solar System, Olympus Mons, at 21.9 km (13.6 mi) tall.
• Valles Marineris is one of the largest canyons in the Solar System, 4,000 km (2,500 mi) long.
• The Martian surface is red-orange due to iron(III) oxide and goethite.
• Mars experiences seasons because of its axial tilt of 25 degrees.

The Reclassification Storm

When the IAU General Assembly convened in August 2006, the question of what counts as a planet had become one of the most heated debates in modern astronomy. The initial draft proposal would have listed Charon, Eris, and Ceres alongside the traditional eight, a move that drew immediate objections from the astronomical community. An alternative framework, crafted by Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi, introduced an intermediate category for objects massive enough to assume a spherical shape yet unable to dominate their orbital neighborhoods. Under this revised scheme, Pluto, Ceres, and Eris were all removed from the planet roster. The fallout was immediate and cultural: scientists took to bumper stickers and T-shirts to voice dissent. Mike Brown, the very person who discovered Eris, embraced the eight-planet model. Meanwhile, Alan Stern, leading NASA's Pluto mission, rejected the IAU definition outright, arguing that orbital mechanics should not dictate an object's identity. He continued calling Pluto a planet and extended that label to Ceres, Eris, and large moons. NASA itself adopted the new guidelines in 2006, but the philosophical split among scientists persisted well beyond the vote.

Geological Wonders Beyond the Classical Planets

For decades, dwarf planets were dismissed as inert rocks drifting through space. That assumption shattered in 2015, when two separate spacecraft campaigns revealed a startling truth: these small worlds can be geologically alive. NASA's Dawn mission, orbiting Ceres, and the New Horizons flyby of Pluto both confirmed that processes once thought exclusive to the eight major planets—internal differentiation, possible cryovolcanism, complex surface geology—were at work on bodies a fraction of Earth's size. This finding transformed how planetary geologists view the outer Solar System. Where astronomers might catalog a dwarf planet as a data point in an orbital table, geologists see a laboratory for understanding how small bodies evolve, retain heat, and reshape their surfaces over billions of years. The expectation of geological activity is now a baseline assumption rather than a surprise, and it has made dwarf planets one of the most compelling targets in planetary science. The implication is profound: the boundary between planet and not-a-planet does not correspond to a boundary between geologically interesting and geologically boring.

The Roster and the Unknowns

Astronomers broadly agree on at least nine objects that qualify as dwarf planets, listed roughly from largest to smallest: Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus. A tenth candidate, Salacia, sits in a gray zone where its status remains genuinely uncertain. Among these ten, only two—Pluto and Ceres—have been visited by spacecraft, giving scientists direct measurements of their surfaces and compositions. Seven others (Eris, Haumea, Makemake, Gonggong, Quaoar, Orcus, and Salacia) possess at least one known moon, and that gravitational relationship allows researchers to calculate the parent body's mass and, by extension, its density. Those numbers feed into geophysical models that attempt to reconstruct each world's internal structure. Sedna stands alone as the one dwarf planet that has neither been visited nor has any confirmed satellite, leaving its mass poorly constrained and its interior essentially a mystery. Beyond these ten, some astronomers propose additional smaller bodies as dwarf-planet candidates, but no consensus exists on whether they truly meet the threshold.

From Ceres to Eris — A Shifting Sky

The story of what counts as a planet is, in many ways, a story of discovery outpacing taxonomy. In 1801, astronomers spotted Ceres between Mars and Jupiter, and for decades it was listed as a planet. As more similar bodies appeared, the planetary count swelled to twenty-three by around 1851, prompting the adoption of the word asteroid and the distinction between minor and major planets. Pluto's 1930 discovery restored a comfortable nine-planet model, but the 1978 finding of its moon Charon revealed that Pluto was roughly one-twentieth the mass of Mercury and only one-fifth the mass of Earth's Moon—far smaller than anyone had guessed. The 1990s brought the Kuiper belt, a population of objects sharing Pluto's orbital quirks, and Pluto began to look less like a lonely planet and more like the largest member of a new family. Then Eris appeared in January 2005, apparently slightly larger than Pluto, and the old taxonomy simply could not absorb it. The cascade of discoveries made reclassification not just possible but inevitable.