Summary in seconds: The potential for life in our solar system includes two planets, Venus and Mars, as well as seven ocean worlds—the moons of the gas giants: Europa, Ganymede, Callisto, Io, Titan, Enceladus, and Triton—and one asteroid, Ceres. This article will cover the two primary planets, Venus and Mars, and the dwarf planet Ceres.
In the previous article of this series (Part Four – Life in Our Backyard), I listed a number of celestial bodies in our solar system that have the potential to harbor life, either now or earlier in their history. This list includes two planets: Venus and Mars; seven moons, which revolve around three giant gaseous planets: Jupiter (Europa, Ganymede, Callisto, and Io), Saturn (Titan and Enceladus), and Neptune (Triton); and a dwarf planet (Ceres) in the asteroid belt. In this article, I will cover the two primary planets: Venus and Mars, and the dwarf planet Ceres.
First: The Two Primary Planets
1. Venus, a Tantalizing Target
Venus is often called our “sister planet.” Venus shares a similar size and structure with Earth; however, its surface is hot enough to melt lead, has a crushingly heavy atmosphere, and an extremely volcanic geology. Venus began its existence much as Earth did, perhaps even with globe-spanning oceans. But the two planets took very different paths. A runaway greenhouse effect likely boiled off Venus’s oceans and turned the planet into a perpetual inferno—the hottest world in the solar system.
Yet Venus also exerts an irresistible pull for astrobiologists1, with the necessary ingredients and the planetary environments that it might require. Venus is a kind of negative to Earth’s positive; by studying what went so very wrong, we might learn what it takes to get life right.
“Venus gives us an example of an alternative evolution for planets,” said Vikki Meadows2. “The planet’s divergent path includes ‘loss of habitability, loss of water on the surface, sulfuric acid clouds, and a dense carbon dioxide atmosphere,’” Meadows said. “It’s also a warning—how terrestrial planets die.”
The presence of dark streaks in Venus’s clouds, where temperatures and pressure are more congenial, prompt an intriguing question: “Could they be wind-whipped bands of microbial lifeforms?” A recent study3 even suggested the presence of one potential sign of life: a gas called phosphine, in the Venusian atmosphere. Bacteria on Earth produce it. For now, this possibility remains in the “unlikely but possible” column, scientists say; only further investigation will offer a definitive answer.
2. Mars, Potentially Habitable at Some Point
Decades of investigating Mars by orbiters and studying its surface by rovers have confirmed that Mars was once wet, with rivers, lakes, and perhaps even oceans, and like Earth, was potentially habitable. Planetary scientists4 think that Mars’s climate was similar to Earth’s almost 3.5 billion years ago, with liquid water on its surface. Then, most of its atmosphere was stripped away by solar wind and radiation. Mars’s minimally active core ceased to generate a protective magnetic field, and its surface became forbiddingly cold and dry, even as it was bombarded with radiation. Astrobiologists speculate that some form of life might be thriving beneath the surface of Mars or in the frozen polar caps.
Scientists working with the European Space Agency’s Mars Express orbiter reported that, in the summer of 2013, the spacecraft detected methane within Gale Crater. Around the same time, NASA’s Curiosity rover measured a marked rise of methane in the air that lasted at least two months5.
Ceres, a Dwarf Planet
Ceres is a dwarf planet located in the asteroid belt between Mars and Jupiter. For a long time, its surface was barely visible, even with powerful telescopes. In 2015, NASA’s Dawn probe6 entered orbit around Ceres and discovered that its surface is made of water ice and hydrated minerals. While Ceres likely does not have a liquid water ocean inside, brines (salty liquids) flow through its outer layer, which could potentially support microbial life.
However, there are major challenges to habitability. The surface is extremely cold, averaging about -105°C (-157°F), and Ceres lacks a thick atmosphere to shield it from radiation or support liquid water on the surface. Additionally, energy sources like hydrothermal activity or chemical reactions, which are essential for life, are still uncertain.
In my next article, I will cover the remaining habitable celestial bodies in our backyard, seven moons that revolve around three giant gaseous planets: Jupiter (Europa, Ganymede, Callisto, and Io), Saturn (Titan and Enceladus), and Neptune (Triton).
References and Sources
1. Astrobiologists: scientists who study how life begins.
2. Meadows, Vikki: an astrobiologist who heads the Virtual Planetary Laboratory in NASA’s Nexus for Exoplanet System Science.
3. Greaves, Jane; et al. “Phosphine Gas in the Cloud Decks of Venus.” Nature Astronomy, 14 September 2020.
https://www.nature.com/articles/s41550-020-1174-4
4. Voytek, Mary; Billing, Linda; New, M.H. “Astrobiology: Interdisciplinary Research Providing a Vision for Planetary Exploration.” ResearchGate, Planetary Sciences, December 2009.
5. “Something on Mars Is Producing Gas Usually Made by Living Things on Earth.” The New York Times, 1 April 2019.
https://www.nytimes.com/2019/04/01/science/mars-methane-gas.html
6. Northrop Grumman. “Dawn, Engineering Breakthroughs and Dawn Spacecraft.” Northrop Grumman, 2025.