Finding a planet like Earth

When I think of an Earth-like exoplanet, I think of a planet with oceans, continents and a friendly atmosphere. In short, a planet like Earth orbiting a star like our own sun.

Sara Seager

Professor of Physics and Planetary Science

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Sara Seager

Professor of Physics and Planetary Science

17 May 2025

Key Points

Nearby planets like Proxima Centauri b and Kepler-186f may receive enough energy and have other conditions suitable for life.
Observing Earth-like planets is challenging. It requires sophisticated telescopes to be sent into space beyond the atmosphere's blurring effect.
The brightness of host stars also makes planets challenging to observe. Starshade, a specially designed screen, may help identify Earth-like planets by blocking light.

Searching for a second home

Photo by Vadim Sadovski

When I think of an Earth-like exoplanet, I think of a planet with oceans, continents and a friendly atmosphere. In short, a planet like Earth orbiting a star like our own sun. Currently, finding the so-called Earth twin seems unlikely to occur soon.

After all, most of our attention in astronomy is focused on small stars rather than more prominent sun-like stars. This is because finding a planet suitable for life orbiting these smaller stars is much easier. We think of these potential planets more like an Earth cousin than like an Earth twin.

Regardless, finding a planet like Earth could actually happen at any time. We have maybe a dozen candidates we're just waiting to observe with the James Webb Space Telescope or JWST. The next generation of large, ground-based telescopes will also help find and characterise more planets and determine if they are anything like Earth.

Proxima Centauri b

When you find a new type of exoplanet, you know there are more like it. It's like the tip of an iceberg – if you see the tip, you know there's a lot more going on. As such, the discovery of Proxima Centauri b was highly significant.

The astronomers who discovered the exoplanet did things a bit differently. Usually, when astronomers look for planets with the technique used to find Proxima Centauri b, they look at several stars over several years. However, this group decided to focus heavily on one star. This is risky because telescope time costs a lot of money, and putting all the resources on one star is uncertain. Yet, in this case, the risk paid off.

Indeed, the fact that our nearest star has an Earth-mass planet is quite significant. There are likely many more Earth-mass planets receiving the right amount of energy to host life around similar stars.

Similarly, the Kepler-186 system has five planets tightly packed together in orbits very close to one another. Like Proxima Centauri b, we think Kepler-186f also receives the right amount of energy to host life – when there's one, there's more. Overall, each of these discoveries was a significant milestone in exoplanets.

Observing Earth-like planets

Observing Earth-like planets is far from trivial. For instance, aliens with our technology orbiting a star not too far away wouldn't see Earth. If they had a Kepler space telescope and everything was aligned just right, they might be able to see Venus and Jupiter.

To find a planet like Earth, these aliens would need a more sophisticated telescope located in space above their atmosphere. Astronomers are designing telescopes like that, although we're not quite ready to build them. Nevertheless, if aliens had the kind of telescopes we're hoping to develop, then they'd be able to see our planet. To them, the Earth would be a pale blue dot.

The need for better telescopes

Photo by AstroStar

To find another Earth, we need to send a sophisticated space telescope above the blurring effects of Earth's atmosphere. This telescope must block out the starlight so that the planet's light can enter. This is because an Earth-like planet would appear faint compared to the brightness of the host star.

In the 1960s, an astronomer came up with the idea of using a giant, specially-shaped screen to accompany spacecraft. Today, we call this screen Starshade. The shape of Starshade is crucial. In general, it is flower-shaped with many petals. The tips of these petals are particularly fine. I have a miniature version of Starshade, and the petals are so sharp that they can draw blood.

This model version, as it turns out, was actually tested in a simulation in Death Valley. This test consisted of fake LED “stars” and “planets” and a camera representing a telescope. The Starshade was placed accordingly to block out the LED star, making the fake planets more visible.

The TESS spacecraft

Even though we're fixated on Earth and Earth-like planets, it's certainly possible that a much wider variety of planet types could host life. Potential planets could be bigger than Earth or have an atmosphere made of entirely different gasses that would still be friendly to life. We're not sure, to be honest, and that's part of the beauty of discovery.

In this regard, the Kepler space telescope was a game-changer. The TESS mission is sometimes considered a follow-up to Kepler. It consists of four cameras, each containing a glorified telephoto lens designed to limit distortions. The apertures are only about ten centimetres wide, and the cameras include seven lens elements. At the back of the camera, there's a 16-megabyte detector that's also custom-designed.

The cameras are housed in giant baffles which block light from the moon and other things. The cameras are bolted on to a platform that is, itself, housed in a spacecraft with another baffle.

The TESS orbit

The TESS spacecraft orbits the Earth on a two-week elliptical orbit that is in resonance with the moon. Simply put, this means the moon orbits the Earth once a month and TESS orbits in half that time. The moon has a stabilising effect on TESS's orbit in this way. This allows TESS to travel great distances from Earth to observe the night sky uninterrupted for weeks at a time.

Photo by Dotted Yeti

The TESS spacecraft receives energy from the sun via its solar panels. It also contains a computer data storage unit and a radio that sends data back to Earth. Eventually, astronomers receive the images for analysis.

Searching for another Earth

There are so many stars and so many planets. No matter how diverse the range of possibilities of planets is, based on the numbers there, there has to be another Earth somewhere.

Whether or not we find another Earth in the next decade or two depends on how common these planets are. If we can launch Starshade and a telescope, we'll be able to search the nearest several stars. Indeed, we will soon have the capability to find another Earth, but nature has to have cooperated in making those planets abundant.