NASA finds seven earth size planets, but are they anything like Earth?

Friday, 24 February, 2017

After days of keeping us in suspense about a new discovery, NASA let the cat out of the bag, in the early hours of yesterday morning. The TRAPPIST optic robotic telescope, located in Chile, recently identified a dwarf star, about forty light years distant from Earth, that is host to seven planets around about the same size as Earth.

Come on now, you didn’t think they were going to announce that an alien civilisation had been found, did you?

This is still a significant discovery though. Particularly as three of the seven bodies orbiting TRAPPIST-1 – the star also takes its name from the Belgian operated telescope – are within its solar system’s so-called Goldilocks, or habitable zone, an area capable of supporting life, that is neither too hot, nor too cold.

It is this bit that is especially of interest, as it means these planets may habour water in liquid form, and, as a result, potentially life of some sort. And that is obviously an exciting prospect. But talk we may one day be able to emigrate there is well wide the mark, to say the least.

There is, you see, a big difference between a planet that is “earth-like”, and one exactly like Earth. Or a planet that could be called an Earth twin, or Earth analog. For example, Proxima b, an exoplanet within the habitable zone around Proxima Centauri, the nearest star to the Sun, is considered to be earth-like, as it is a rocky, or terrestrial planet.

It might have some sort of atmosphere, and possibly there could be liquid water on its surface. But Proxima b may be far from habitable, at least as far as humans are concerned. As Proxima Centauri is a red dwarf star, which are relatively cool, Proxima b would need to be quite close, to be within the habitable zone.

This sort of proximity however could mean Proxima b is tidally locked, meaning the planet’s rotational period matches the time it takes to orbit the star. This result here is only one side of the planet would ever face the star.

Therefore, the sunny side of Proxima b would be quite warm, whereas the night side would be extremely cold. The only spots that might be conducive to life, would be near the day-night terminator. In addition, the planet is also exposed to stellar wind pressures far greater than those that Earth experiences.

Not all that earth-like, after all. So while some form of life may manage to eke out an existence there, it would hardly be suitable for human occupation. The same conditions could well apply to the planets within the Goldilocks zone of TRAPPIST-1, given it to is a relatively cool dwarf star.

At the very least, they’re quite possibly tidally locked. If we’re looking for a new planet to settle on then, it needs to be an Earth twin. This is a planet, as the name suggests, that is identical in almost every way to ours. And if there are at least one hundred billion planets in our galaxy, the Milky Way, then it stands to reason some proportion must be virtually identical to Earth.

But an Earth twin candidate needs to be more than a terrestrial planet, orbiting within the habitable zone of its solar system though. In their book, Rare Earth, published in 2000, US paleontologist Peter D. Ward, and Donald Brownlee, a professor of astronomy, outlined the criteria necessary for a planet to be classified as an Earth twin:

The right distance from a star; habitat for complex life; liquid water near surface; far enough to avoid tidal lock; right mass of star with long enough lifetime and not too much ultraviolet; stable planetary orbits; right planet mass to maintain atmosphere and ocean with a solid molten core and enough heat for plate tectonics; a Jupiter-like neighbor to clear out comets and asteroids; plate tectonics to build up land mass, enhance bio-diversity, and enable a magnetic field; not too much, nor too little ocean; a large moon at the right distance to stabilize tilt; a small Mars-like neighbor as possible source to seed Earth-like planet; maintenance of adequate temperature, composition and pressure for plants and animals; a galaxy with enough heavy elements, not too small, elliptical or irregular; right position the galaxy; few giant impacts like 65 million years ago; enough carbon for life, but not enough for runaway greenhouse effect; evolution of oxygen and photosythesis; and, of course, biological evolution.

That’s an extensive list. Some astronomers think two percent of the Milky Way’s planets may be Earth twins, meaning there could be two billion such bodies. Given the exacting conditions required for their existence though, I think the actual number may be far smaller.

It could a very long time, therefore, before any announcement is made regarding the discovery of a truly earth-like planet, that is, an Earth analog. It also means we have to take greater care of our own Earth. Clearly we’re not going to be emigrating anywhere else in any hurry.

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