Searching for a new Earth

Is the earth special?

Lessons 1 to 3 add up to one idea: nothing about our place in the universe looks special. The sun is an ordinary star in an ordinary galaxy. The physics we found on earth turns out to hold in distant galaxies, and the chemistry we found in a laboratory turns out to be the chemistry of the oldest stars. If both physics and chemistry are universal, there is no reason to think the earth is special in anything else. Whatever happened here, we should expect that it can happen elsewhere, at least in principle. Some astronomers call this working assumption the principle of mediocrity.

That raises a question that no amount of thinking can settle: is the earth’s life unusual, or one example of something common? So far we have exactly one planet with life, and one example proves nothing. To test the idea we need another earth. This lesson is about the search for it.

Next door

The nearest places to look are the planets and moons of our own solar system, and by now probes have visited every planet. They found surprises. Mars is cold and dry, but its surface is scarred by dry riverbeds, and in 2021 the American rover Perseverance landed in Jezero crater, a former lake with a delta, where rivers once poured in. In 2025 NASA reported a potential sign of past life in a rock the rover drilled in 2024, though non-living explanations remain open. (The “canals” that astronomers around 1900 thought they saw were an illusion and a mistranslation, from the Italian canali, “channels.”) Jupiter’s moon Europa is covered by an icy shell, about 30 kilometers thick by the Juno spacecraft’s measure, and measurements suggest an ocean beneath it, more water than all the earth’s seas. Saturn’s small moon Enceladus sprays jets of water into space. Saturn’s big moon Titan has a crust of water ice under a thick atmosphere, with lakes of liquid methane and ethane; the European Huygens probe landed on it in 2005, and a NASA helicopter-like craft called Dragonfly is planned to arrive in 2034.

All of these are hints of water, or of the chemistry of life, and none is a promise. No other place in our solar system is known to be inhabited, and none seems to have a surface where life like ours could easily thrive. For another earth we must leave the solar system and look at other stars.

Catching a shadow

Most planets around other stars are far too faint to see directly; the glare of the star drowns them. Astronomers usually find them indirectly. In 1995 the Swiss astronomers Michel Mayor and Didier Queloz found the first planet around a sun-like star by watching the star wobble back and forth as an unseen companion tugged on it. The other main method, and the one that has found the most planets, is the transit. If a planet’s orbit happens to be edge-on to us, then once per orbit the planet crosses the face of its star and blocks a sliver of the light. The star dims, very slightly, and returns to normal.

That is what the Kepler space telescope was built to catch. NASA launched it in 2009, named for Johannes Kepler, and pointed it at one patch of sky in the constellations Cygnus and Lyra with about 150,000 stars in it. For four years it stared, like a fisherman watching a float, measuring the brightness of each star every half hour. The dip from an earth-sized planet crossing a sun-like star is only about eight thousandths of a percent, and lasts about thirteen hours once a year. Try it.

In January 2011 the Kepler team announced Kepler-10b, a planet about one and a half times the width of the earth. It was the first rocky planet Kepler found (the French-led CoRoT-7b, in 2009, had been the first shown to be rocky anywhere). It was not a friendly one: it circles a sun-like star once every 20 hours, so close that its surface is hot enough to melt rock. But it showed that rocky worlds not much bigger than the earth exist, and a gold rush began. The number of confirmed planets around other stars rose from a handful in the 1990s to 6,375 as of 1 October 2026. Kepler found 2,787 of them and its second mission, K2, another 549, together more than half.

The Goldilocks zone

Finding earth-sized planets is not the same as finding earths. What would make a planet a good candidate for life? Astronomers begin with a few plain conditions. There must be liquid water on the surface, so the planet can be neither too hot nor too cold. It should usually be rocky, with a solid surface to hold the water. The star should live long enough, billions of years, for life to develop, which rules out the heaviest stars, which burn out in a few hundred million years. And the planet should have an atmosphere. The range of orbits where a planet can have liquid water is called the habitable zone, or, jokingly, the Goldilocks zone: not too hot, not too cold, just right. In our solar system it begins somewhere between Venus and the earth and ends near Mars, with the earth inside it. The Chinese-born American astrophysicist Su-Shu Huang (黄授书) introduced the term in 1959.

With these criteria in hand, astronomers have found promising worlds. In April 2014 came Kepler-186f, the first earth-sized planet found in the habitable zone of another star, about 580 light-years away. (The star is a red dwarf, smaller and cooler than the sun, so its habitable zone is much closer in.) In 2016 came Proxima b, a planet at least as massive as the earth, in the habitable zone of Proxima Centauri, only 4.2 light-years away, the nearest star to the sun. In 2017 came TRAPPIST-1, a small, dim star 40 light-years away with seven earth-sized planets, packed in orbits of between one and a half and nineteen days, three of them in the habitable zone. A Chinese reader might think of the seven Calabash Brothers, the cartoon heroes of the 1980s. As Thomas Barclay, one of Kepler-186f’s discoverers, put it: “Being in the habitable zone does not mean we know this planet is habitable.” These systems are a start.

Life in extreme places

So how demanding is life? Look at the earth. In Yellowstone National Park, some springs in the Norris Geyser Basin are near boiling and as acid as lemon juice, with a pH around 2, yet microbes such as Sulfolobus live in them. Around deep-sea vents, some microbes grow at up to 122 °C (252 °F) under pressure, and life has been found in lakes under Antarctic ice and in rock nearly three kilometers underground. For most of the earth’s history life was microscopic; animals arrived only in the last eighth of it. A planet that could host microbes is a far easier target than one that could host people. A rocky planet, some water, some carbon-based molecules, and a source of energy, whether starlight, tides, or heat from below, may be enough. In this reading, single-celled life is probably common in the galaxy.

Are we alone?

The harder question is whether there are other civilizations. In 1961 the radio astronomer Frank Drake, who in the spring and summer of 1960 had listened for signals from two nearby stars, gathered a small meeting at Green Bank, West Virginia, and wrote down the factors that decide how many technological civilizations might be broadcasting in the galaxy: how many stars form each year, what fraction have planets, how many planets per system can support life, what fraction of those develop life, what fraction of that becomes intelligent, what fraction builds technology that can be detected, and how long such a civilization lasts. Multiply them. The result, N, is the Drake equation. It was never meant to give a number. It was meant to show what the question depends on, and how little we know.

This is why the question is wide open. Careful estimates of the odds range from “the galaxy is full of civilizations” to “we are probably alone.” A 2018 analysis found that when the huge uncertainties are carried through honestly, there is a substantial chance that no one else is out there. The physicist Enrico Fermi put the other side in a lunchtime question in 1950, as colleagues later remembered it: if they are common, where is everybody?

Headlines sometimes run ahead of the evidence. In 2023 the James Webb Space Telescope found methane and carbon dioxide, but no water, in the atmosphere of K2-18b, a planet about two and a half times the width of the earth, 124 light-years away, in the habitable zone of a red dwarf. In 2025 a team reported a faint hint of dimethyl sulfide, a gas that on earth is made almost only by living things, though it has also been found in a comet and in interstellar space; other teams reanalyzing the data found that it was not significant. As of 2026, whether K2-18b is a watery “Hycean” world or a gas-rich mini-Neptune with no surface at all is still debated, and most independent studies favor the second. Simply put: no one has yet found life, or even an agreed biosignature, beyond the earth.

The next telescopes, and the long future

The tools are improving. The James Webb telescope, launched on Christmas Day 2021, sees in the infrared, where the chemical fingerprints of planetary atmospheres are clearest. NASA’s Roman Space Telescope launched on 30 August 2026; with Europe’s PLATO and China’s Earth 2.0, due in March 2027 and 2028, it is forecast to raise the count of known planets into the tens of thousands. On the ground, the European Southern Observatory’s Extremely Large Telescope, with a mirror 39 meters wide, plans its first test observations for 2029, and the Giant Magellan Telescope is under construction in Chile. The Thirty Meter Telescope, planned for Mauna Kea, has been stalled by protests from many Native Hawaiians, who consider the summit sacred; in 2025 the U.S. National Science Foundation chose to back the Giant Magellan instead, and in September 2026 the TMT turned down an offer to move to Spain’s La Palma. Astronomers are asking how to balance science with respect for a place that matters to others. NASA’s proposed Habitable Worlds Observatory, meant for the 2040s, would photograph earth-like planets directly and read their air.

There is also a reason to hurry. The sun is slowly brightening. In a billion years or more, depending on the model, it will be hot enough to boil away the oceans, and in about five billion it will swell into a red giant. “The Wandering Earth,” the Chinese film from Liu Cixin’s story, imagines taking the planet with us. A quieter answer is the one this course gives: we do not know what will happen, but we may well be able to find somewhere to go, and exploring the universe may be part of what human life is for. Since Galileo raised his telescope in 1609, telescopes have supplied the evidence that moved us out of the center: first the earth, then the sun, then our galaxy. They have told us where most of what we are made of came from. The next chapter of the story is about whether we have company.