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As a kid in the 1990s, I chose Saturn as my favorite planet because of its rings. They\u2019re so perfectly circular and solid-looking, like a disk you might see slicing through a sphere in a math textbook. Yet up close, they\u2019re sparse, a bunch of pebbles and rocks in free-fall. How surreal it would be to be among them, clinging to a boulder, silently tumbling. (I also respected my sister\u2019s choice of stripe-y, spotty Jupiter.)<\/p>\n
Perhaps we liked to consider other planets because they\u2019re alternative realities, refreshingly exotic manifestations of the same laws of physics. These voluminous balls of rock, gas and ice fueled our imaginations, but at the same time they grounded us, their alien landscapes bringing Earth into relief.<\/p>\n
Around the time these important deliberations were happening in a bedroom in rural Texas, astronomers were beginning to detect planets orbiting other stars. The list of known planets has since grown from nine in our solar system (Pluto counted back then) to include more than 6,300 beyond it, known as exoplanets. Thousands more exoplanet candidates await confirmation. Probably about 100 billion others are expected to exist in our galaxy alone.<\/p>\n
These newfound planets are too many light-years away to capture images of with current telescopes, barring a few blurry exceptions. Scientists discover them sight unseen, inferring their presence from their gravity or their shadows.<\/p>\n<\/div>\n<\/div>\n
Lately, though, with tools like the James Webb Space Telescope, they are gleaning from the teensiest signals knowledge of exoplanets\u2019 compositions and atmospheres, their origin stories, even the possibility that they might host life.<\/p>\n
Exoplanets expand our minds about what reality is like even further. Imagined possibilities abound. Like my sister and me, many scientists have developed favorites.<\/p>\n
Ren\u00e9 Heller, an astrophysicist at the Max Planck Institute for Solar System Research in G\u00f6ttingen, Germany, is most taken with KOI-456.04. It is a \u201cvery likely\u201d exoplanet that he and colleagues spotted in 2020 when reanalyzing data from the Kepler Space Telescope, a NASA spacecraft that detected thousands of exoplanets between 2009 and 2018. Scientists are 85 percent certain KOI-456.04 exists.<\/p>\n
Heller deduced the possible presence of KOI-456.04 from variations in the intensity of light coming from a distant, sun-like star. At regular intervals, the starlight\u2019s intensity would briefly and ever-so-slightly drop. Though the light change could be a meaningless statistical fluctuation, KOI-456.04 is probably a transiting planet periodically passing in front of the star, each time blocking a bit of its light. That would make it the fourth planet around the star.<\/p>\n<\/div>\n<\/div>\n
If confirmed a planet, then KOI-456.04 is arguably the most Earthlike world known so far. It would have 1.9 times Earth\u2019s radius and receive about the same amount of heat and light from its star as we do from our sun. Water would flow on its surface. It would probably have plate tectonics. If planets of similar sizes and circumstances undergo similar evolutions, it may be bursting with plant photosynthesizers and other kinds of alien life. \u201cFascinating, right?\u201d Heller said.<\/p>\n
Among confirmed exoplanets, popular favorites are the Trappist-1 worlds \u2014 seven rocky planets that orbit in synchrony around a small red star 40 light-years away. A few lie in the star\u2019s \u201chabitable zone,\u201d the region where temperatures allow water to exist in liquid form, a prerequisite for life.<\/p>\n
The question of the moment is whether the Trappist-1 planets have atmospheres. Everyone hopes they do, because air is probably needed to sustain life. Being blanketed by gases regulates a planet\u2019s temperature, shields the surface from damaging radiation and keeps water from floating off into space. It also provides the chemical fuel for biological activity, like the air we breathe.<\/p>\n
Trappist-1 is an \u201cM dwarf\u201d star with only about one-tenth the radius \u2014 and one-thousandth the volume \u2014 of our sun, and is therefore much dimmer and colder. This means its habitable zone is closer than if it were a hotter, sun-like star. Planets in such tight orbits carve deeper dips in the starlight, making them easier to study.<\/p>\n<\/div>\n<\/div>\n
However, being so close-in also means these planets experience intense stellar radiation that could strip away the molecules from their skies. The Webb telescope has seen enough transits of Trappist-1b, the planet nearest the star, for astronomers to conclude that it\u2019s almost certainly airless, a bare rock. Its neighbor, 1c, is looking disappointingly bare, too.<\/p>\n
Unperturbed, Victoria Meadows, principal investigator of NASA\u2019s Virtual Planetary Laboratory, pins her hopes for habitability on the fourth rock from the star, Trappist-1e. Not only is it orbiting farther out; Meadows likes planet \u201ce\u201d because it is also Earth-size and warm enough to hold onto its liquid water with only a thin sky.<\/p>\n
Meadows and her colleagues hope Trappist-1e will contain \u201cbiosignature gases\u201d \u2014 a combination of chemical ingredients that could point to biological activity. \u201cI think it\u2019s the first planet where we might be able to potentially look for a sign of life,\u201d she said.<\/p>\n
One group of exoplanets that \u201cdoesn\u2019t get enough attention,\u201d according to Paul Robertson, a planet-hunting astronomer at the University of California, Irvine, is the Kepler-444 system.<\/p>\n<\/div>\n<\/div>\n
\u201cMaybe I\u2019m a hipster, but I think it\u2019s underappreciated,\u201d he said.<\/p>\n
These five Earth-size globes orbit close to their star, perhaps so close that they\u2019re deadly hot. What intrigues Robertson is that the star\u2019s chemical makeup suggests the system is 11 billion years old, more than twice the age of the sun and Earth \u2014 and 80 percent of the age of the universe.<\/p>\n
\u201cI like to think about how there are planets out there that are billions of years older than the solar system,\u201d Robertson said. \u201cIf you can have habitable conditions consistently on those planets, then what might be out there?\u201d<\/p>\n
Life might turn up in surprising places. William Bains of Cardiff University in Wales is struck by recent studies of WD 1856 b, a Jupiter-size, gaseous planet discovered in 2020 orbiting the remains of its dead star. The star is now a \u201cwhite dwarf,\u201d a stellar core left behind after the dying star ballooned into a red giant, engulfing everything around it, before fizzling out. Somehow, the planet survived; scientists think it must have migrated inward from farther away.<\/p>\n
Bains thinks about WD 1856 b\u2019s moons. If, like Jupiter and Saturn, the gas giant started out with large, icy moons, then, Bains wonders, \u201cwould it now be circled by ocean worlds? And if so, could life have evolved there?\u201d Bains noted that \u201cShroud,\u201d a 2025 sci-fi book by Adrian Tchaikovsky, features an alien civilization living in a similar scenario.<\/p>\n<\/div>\n<\/div>\n
Imagining extraterrestrial civilizations helps reveal the parts of our earthly experience that are universal. It\u2019s mind-bending to consider all the different ways matter forms big balls, and to try to imagine such places. Planets that are nothing like Earth help us know ourselves by the contrast. We see how lucky we are to live on such a livable planet, one so balmy, airy, awash in a life-giving solvent and with continents rising above the waves providing a place to stand, a platform for our telescopes.<\/p>\n
Astronomers will continue to search for the earthiest exoplanets they can find with modern-day telescopes. And they\u2019re building ever-better instruments. NASA\u2019s Habitable Worlds Observatory, planned for the 2040s, ought to be able to find Earthlike planets around nearby stars and will scrutinize their skies for gases linked to life.<\/p>\n
For Sara Seager, a planetary scientist who helped develop the techniques for probing exoplanet atmospheres, the possibilities for finding a planet with signs of life only keep growing. \u201cThere\u2019s no favorite planet because always the next one is better,\u201d she said.<\/p>\n<\/div>\n<\/div>\n