When 'Earth-like' planets aren't necessarily very much like Earth

NASA's Transiting Exoplanet Survey Satellite (TESS) has found two planets similar in size to Earth and within the habitable zone of star TOI 700 . We might temper our excitement though, as this is not enough to conclude that the environments of these planets are otherwise very much like Earth. 

In science fiction, alien planets are frequently Earth-like. Space explorers land or beam down to the surface of another world and can freely breathe the air; the temperatures are comfortable and the gravity is just like home.

Conveniently, the alien inhabitants frequently speak English, making it much easier for filmmakers to depict. But it turns out, in reality, most alien planets that have been found so far are very un-Earthlike.

Thanks to orbiting telescopes such as TESS and its predecessor Kepler, the number of exoplanets — planets circling stars other than our own Sun — has passed 5,000 and counting. Most are truly alien worlds — gas giants like Jupiter or super hot like Mercury. That's why astronomers get excited when planets that seem to be more like Earth are found within what's known as the habitable zone of their star.

The habitable zone is a region around a star where the planet is just the right distance so it is neither too hot nor too cold, meaning liquid water could potentially exist on its surface.

This region is also referred to as the Goldilocks zone, and Earth resides in this zone around our sun. And on Earth, where there is water, there is life.

Two of the four planets that have been seen orbiting a dwarf star with the unimaginative name TOI 700, about 100 light years away in the southern southern sky, within its habitable zone.

They're about the size of Earth and believed to be rocky, another requirement for them to be Earth-like worlds. But there are also major differences, which suggest their surface conditions could be quite unusual. 

First of all, the dwarf star is smaller, cooler and dimmer than our sun, so the habitable zone is in a closer range. One of the two planets only takes 28 Earth days to complete its orbit, while its companion's year is 37 days long.

WATCH: NASA video about the discovery of the newest Earth-sized planet

When planets are that close to their stars, they are often gravitationally locked, which means one side always faces the star while the other faces outer space. Our moon is gravitationally locked to Earth, which is why we only see one side of it in our sky.

That means these planets would have two very different hemispheres. The sun would never set on the daylight side and the far side would be in permanent darkness. This could cause two extreme environments, where the sunny side could be as hot as summer in the Sahara desert, while the dark side remains permanently frozen like an Arctic winter. 

Perhaps a narrow band of moderate temperatures might lie in the twilight zone between the two extremes, where rivers and lakes could exist. But the weather patterns on these worlds would certainly be very different from what we experience on Earth. There would be no seasons, and wind circulation would follow very different patterns.

Astronomers have yet to find Earth 2.0, an exact replica of our planet, orbiting a star like our sun. Finding it is a huge challenge, because the sun is a much brighter type of star than a dwarf, so small rocky planets are lost in the glare in our telescopes.

Perhaps the new James Webb Space Telescope will find one. One of its exciting capacities is the ability to look at exoplanets and see their atmospheres, which could reveal the presence of water vapour and other gases. This could help tell us whether an exoplanet is more Earth-like than just the being the right size and in the right place.

In the meantime, perhaps future Hollywood films could do a better job of depicting alien worlds as they truly are — alien.