Other Earths

The odds of there being other planets like Earth in the Universe are heavily stacked in our favor. If every planet in our galaxy were the size of a basketball, you could stretch them in a line from Earth to Mars and back again. If you included every planet in every galaxy, that line would stretch to the edge of the observable Universe. Among those, chances are there will be worlds of a similar size to ours, orbiting stars like our Sun in a similar position. 

The only problem is finding them. To date, we have found more than 7,000 planets orbiting other stars (known as exoplanets), but our efforts have focused on relatively easy-to-discover worlds. That includes large gaseous planets like Jupiter or rocky worlds orbiting small and dim stars known as red dwarfs. 

Hunting for worlds truer to our own planet is often much more difficult because these stars are large and the planets are comparatively small. Moreover, if we wanted to find planets exactly like Earth on year-long orbits, we would need to watch the star for a long time to see the planet swing around multiple times and confirm its existence. That’s one of the major reasons why, to date, we have never found such a world. No exact Earth 2.0 replica has ever been seen. 

That might be all set to change in the coming years. A new European Space Agency (ESA) telescope launching next year, called PLATO (Planetary Transits and Oscillations of Stars), is expected to find dozens of Earthlike planets in the habitable zone of Sunlike stars, the position from the star where temperatures could be just right for liquid water to exist on the surface, like on our planet.

At the same time, new ground-based telescopes that should aid in the hunt are coming online. Further afield, all eyes are on NASA’s thrilling Habitable Worlds Observatory (HWO), set to launch in the 2040s with a clear goal in mind: produce images of Earthlike planets around other stars and look for signs of life. 

“We are actually at a very special time,” says Vikki Meadows, PhD, an astrobiologist at the University of Washington who also runs NASA’s Virtual Planetary Laboratory, which studies exoplanet habitability. “For millennia, we have wondered: Are we alone in the Universe? It’s only now [that] we have both the scientific knowledge and the technology to try and answer that question.” 

We hunt for exoplanets in a number of ways. One is by watching for wobbles in a star’s position caused by the gravitational tug of an orbiting planet, known as the radial velocity method. Another is by watching for the dip in a star’s light as a planet passes in front, known as the transit method. This method is relatively simple and allows you to watch many stars at once for dips in light but relies on the system being oriented to us in just the right way for a transit to occur. 

Our Milky Way galaxy is home to an estimated 400 billion stars, the majority of which likely host at least one planet and often more. PLATO will use 26 cameras to stare at more than 200,000 stars in our galaxy, many of which will be bright Sunlike stars, for at least two years. In doing so, it will be capable of detecting many small rocky worlds transiting these stars. 

Were it looking at our Solar System, the telescope would see “Mercury, Venus, and Earth,” says Ana Heras, PhD, an astronomer at ESA in the Netherlands and project scientist for PLATO. 

While the exact number of rocky planets the telescope will find isn’t known for certain, Heras says the team estimates they might find “tens of exoplanets” similar to Earth, orbiting stars like our Sun on similar orbits, among many other planets. 

How plentiful Earthlike planets are in the galaxy remains an open question, but so far, rocky worlds in the habitable zones of smaller red dwarf stars seem to be “pretty common,” says Meadows. One of the most notable examples is the TRAPPIST-1 system about 40 light-years away, where seven rocky planets orbit a red dwarf star, three in its habitable zone. Because red dwarf stars are smaller and dimmer than our Sun and more prone to extreme flaring events, it is not yet known how hospitable they are to life.

Efforts to examine the atmospheres of these planets to look for hints of life are ongoing using telescopes like the James Webb Space Telescope. These efforts will be aided by large new telescopes being built on Earth, like the Extremely Large Telescope, set to begin observations from Chile in 2029. These telescopes are looking in the atmospheres of planets for hints of gases like carbon dioxide, oxygen, and methane, which could be biosignatures for life. 

We will need more advanced telescopes to learn more about truer Earth analogs like the worlds PLATO may find. In Europe, a proposed fleet of spacecraft called LIFE (Large Interferometer For Exoplanets) could hunt for life on dozens of Earthlike worlds, although the idea remains conceptual. More concrete is NASA’s HWO, which is currently in the planning stage ahead of a proposed launch in the 2040s. 

HWO would use an advanced coronagraph and a large mirror to block out the light of Sunlike stars and directly image planets in orbit around them. The goal is to produce images of at least 25 Earth analogs and pick apart their atmospheres to look for signs of life. The telescope might even see the reflected light of vegetation and oceans. “With direct imaging, we can go all the way down to the surface,” says Meadows. “We will see these pale blue dots.” 

Until then, astronomers will continue to pick apart and probe planets less similar to Earth but still with the potential for habitability. Perhaps the first truly Earthlike planet discovered won’t be much like Earth at all but rather a seemingly inhabited world around a much different star than our own.