Posted by Stand on Mars and you are inside the habitable zone. Look up at Venus from Earth and you are looking at a planet that, by some definitions, sits on the inner edge of it. Neither one could support you for a minute. That small embarrassment is the best way into the question of which habitable planets deserve the name, because it shows that orbit is only the first filter. Whether a world can actually keep water liquid for billions of years depends on things that have little to do with distance: how heavy it is, what its interior is doing, how much water it started with, and what kind of star it was unlucky or lucky enough to be born around.
Astronomers can measure almost none of this directly for planets around other stars. They can, however, learn it from the three rocky planets they know best, and then go looking for exoplanets that pass the first few tests.
The Mass Problem, Seen From Mars
Mars is about a tenth of Earth’s mass. That number explains a great deal of its history. A small planet cools faster, and as the Martian core cooled, the dynamo that generated its global magnetic field shut down, probably around four billion years ago. Lower gravity also makes it easier for gas to leak away. NASA’s MAVEN orbiter, which reached Mars in 2014, has measured atoms being stripped from the upper atmosphere by the solar wind and estimated that the loss was much heavier in the Sun’s turbulent youth. The rivers and lakes recorded in Martian rock needed a thicker blanket of air than Mars has today. Most of it is gone.
So mass matters in both directions. Too little, and a planet struggles to hold an atmosphere and to stay geologically alive. Too much, and it may grab and keep a thick envelope of hydrogen and helium from its birth nebula, becoming a small gas planet rather than a rocky one. In 2017 the California-Kepler Survey team, led by Benjamin Fulton, found a curious gap in the sizes of known planets between roughly 1.5 and 2 Earth radii. Planets tend to fall on one side or the other. Below the gap they are mostly rock; above it, most carry gas. The gap gives a rough upper size limit for anything that might resemble Earth, although plenty of individual planets refuse to behave.
Then there is the magnetic field, which turns out to be a messier argument than school textbooks suggest. Venus has no global field of its own and still holds an atmosphere ninety times as massive as ours. Some researchers now think a magnetic field helps less than once believed; others think it still decides how much water survives. That debate is not finished.
A Thermostat Made of Rock
Earth has stayed wet for something like four billion years while the Sun has brightened by roughly a quarter or more. Without some kind of regulation, that should not have worked. The leading explanation, worked out by James Walker, Paul Hays and James Kasting in 1981, is the carbonate-silicate cycle. Rain pulls carbon dioxide out of the air and dissolves rock, rivers carry the products to the sea, and the carbon ends up locked in seafloor sediments. Plate tectonics drags those sediments down into the mantle, and volcanoes eventually breathe the carbon back out. When the climate warms, weathering speeds up and draws down more carbon dioxide. When it cools, weathering slows and volcanic gas builds up again.
A slow thermostat, but a thermostat.
Whether a given rocky exoplanet has plate tectonics is something nobody can observe today. Models disagree about whether larger rocky planets would have more vigorous tectonics or would seize up under their own gravity. Venus seems to lack Earth-style plates altogether, and whether that is a cause or a consequence of its runaway greenhouse is still argued. Measurements of Venus’s atmosphere since the Pioneer Venus mission found far more deuterium relative to ordinary hydrogen than on Earth, a sign that a great deal of water was once there and escaped. Venus may have been a habitable planet early on. We do not know.
How Much Water Is Too Much
Liquid water is the headline requirement, but the quantity matters as well. A planet with very little water can dry out completely once its climate tips. A planet with far too much might be covered by an ocean hundreds of kilometres deep, with high-pressure forms of ice at the bottom sealing the water away from the rock beneath. That would cut the seafloor chemistry that may have helped start life on Earth and could interrupt the carbon cycle described above. Such ocean worlds are a real possibility in planet formation models, especially for planets that formed farther out and migrated inward. They might still be habitable in some form. They would not look like Earth.
Earth, for comparison, is quite dry. Its oceans make up a tiny fraction of a percent of its mass.
Picking the Right Kind of Star
Most known rocky planets in temperate orbits circle red dwarfs, because those small stars are common and their planets are easier to find. The catch is that red dwarfs are often violently active in youth. Sun-like stars are gentler but shorter-lived and harder to search. Between them sit the orange K dwarfs, which some researchers have started calling the best compromise. They are calmer than red dwarfs, they burn for tens of billions of years, and they are numerous. In 2020 Dirk Schulze-Makuch, René Heller and Edward Guinan went further and argued that a slightly larger, slightly warmer and older planet around a K star might be even better for life than Earth, a category they called superhabitable. It is a provocative idea, and a useful reminder that Earth was never proven to be the ideal.
Kepler-442b is one example of the type: a planet about a third larger than Earth, orbiting a K dwarf roughly 1,200 light-years away, announced in 2015. It is too far for detailed study with current telescopes. That is the frustrating pattern of this field.
Some Names Further Down the List
The best-known candidate habitable planets soak up most of the attention, but several quieter worlds show what the search actually produces. TOI-700 d, announced in January 2020, was the first Earth-sized planet in a temperate orbit found by NASA’s TESS mission, which watches nearby, bright stars instead of distant ones. It orbits a red dwarf about 100 light-years away in the constellation Dorado, and in 2023 the same system yielded TOI-700 e, a planet a little smaller than Earth also in the temperate band. A system with two such planets lets researchers compare siblings, which may prove more informative than any one planet on its own.
Ross 128 b, reported in 2017 by a team using the HARPS spectrograph, orbits a red dwarf about 11 light-years away that appears unusually quiet for its class. Teegarden’s Star b, found in 2019 by the CARMENES survey, is roughly Earth’s minimum mass and sits around a tiny star about 12 light-years from us. Neither transits from our point of view, so their sizes and atmospheres remain unknown.
Kepler-1649c has a story that says a lot about how data are handled. The Kepler spacecraft recorded it years earlier, but an automated vetting program had dismissed the signal. When a team re-examined the rejected cases, they found a planet about 1.06 times Earth’s radius receiving roughly three quarters of the starlight Earth gets, announced in 2020. Some candidates are hiding in old data, waiting for someone to look again.
None of these is known to have an atmosphere, an ocean or a working carbon cycle. Each one is a set of numbers, mostly radius or mass and orbit, plus a great deal of hope.
Reading the List With Care
When a news story calls a planet Earth-like, it usually means the planet is about Earth’s size and gets about Earth’s sunlight. That is two traits out of the many discussed above. The rest require atmospheric spectra, and for small rocky planets those measurements are only now beginning, mostly with JWST, and are sometimes contradictory. The coming generation of giant ground telescopes, such as ESO’s Extremely Large Telescope, should push the work further, especially for the nearest systems.
Habitable planets are therefore best treated as a ranked list of questions, not a list of answers. Some names will rise as data arrive; others will turn out to be bare rock or deep ocean or something nobody planned for. If you want to follow that sorting process as it happens, SETIworld covers new exoplanets and the disputes around them in more depth, and it is a good place to keep reading.
