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Opinion: We have found Earth's Cousin outside our solar system ... but what does that mean?

The Kepler spacecraft been on a planet-hunting mission since 2009 – and it’s just found the first Earth-sized planet in a habitable zone. What are the implications?

Conor Farrell

A COUPLE OF weeks ago the announcement was made that “Earth’s Cousin” had been discovered by the Kepler spacecraft during its planet-hunting mission that has run since 2009. The planet, known as Kepler-186f, is the first Earth-sized planet discovered in its parent star’s habitable zone. Planets outside our own Solar System are known as extra-solar planets, or exoplanets for short.

The existence of such objects moved from speculation to reality in 1992, when the first confirmed exoplanet was discovered orbiting a type of dead star known as a pulsar. Since then, in the space of about 22 years, we have discovered a total of almost 1,800 confirmed exoplanets. The Kepler mission itself found 962 of these confirmed planets, and it has a further 2,900 or so planet candidates yet to be confirmed.

That’s a lot of planets out there, considering Kepler looked at only a quarter of a percent of the sky.

But what does it all mean? What types of exoplanets are there? What is the habitable zone? What’s actually meant by “Earth’s Cousin”?

Most life on Earth is dependent on the existence of liquid water, and we know that Earth has liquid water because it’s neither too hot nor too cold; it’s just the right distance from the Sun for water to exist in this phase. This is the habitable zone, which has, for obvious reasons, been nicknamed the Goldilocks zone.

Objects in the habitable zone of a star are the best bet for looking for life, but a planet in the habitable zone does not automatically mean there could be liquid water on it. There are a number of other parameters to consider when looking for life-harbouring exoplanets, for example:

  • What size is the planet? Its size affects its gravity, which in turn can affect development of life;

  • How old is its parent star? If the star is very young, it’s a lot less likely that life would have had time to arise on the planet;

  • Does water exist around the star in the planetary system? Liquid water can’t pop out of nowhere, and it must be present in some form near the exoplanet before it can arrive on the planet;

  • What is the planet made of? Earth’s iron core gives it a magnetic field which prevents harmful radiation reaching to the surface. Mars, however, is in the habitable zone but has no magnetic field, and also no life that we know of.

As you can see, there are lots of things to take into account. The exoplanet Kepler-186f has been called “Earth’s Cousin” rather than “Earth’s Twin” because, while it’s in the habitable zone of the parent star just like Earth, other conditions of the planet and its environment make it quite different to Earth.

Mind-blowingly different planets

Hunting for life-harbouring planets isn’t the only thing the Kepler mission does. Astronomers have found a whole host of planets that are mind-blowingly different to what we might be used to in our own Solar System. Take, for example, hot Jupiters. These planets are massive gas planets like Jupiter that orbit their stars extremely closely – even as close as about 2 million km (Earth is about 150 million km from the Sun, for comparison). The planet Bellerophon, discovered in 1995, is just under 8 million km from its parent star 51 Pegasi and is believed to have a surface temperature of about 1,000 degrees Celsius.

Things get even more mind-melting. There is a type of hot Jupiter known as “ultra-short-period Jupiters”. These planets orbit their stars in less that one day. That is, a year on such a planet lasts less than our 24 hours. The exoplanet SWEEPS-10 resides about 1.2 million km from the star, and zips around it in only 10 hours. There are even exoplanets that orbit stars very quickly, but rotate quite slowly, meaning a day there can last longer than a year!

There are a number of different methods used to detect planets around other stars. As it’s very difficult to point a telescope and just see a new planet, astronomers have to use clever tricks and accurate measurements to find exoplanets. The “easiest” way to find a planet is by the transit method: when a planet passes in front of a star, the star’s brightness will dip slightly. When astronomers see this happen at regular intervals, they’ve probably found a new planet.

When we do find a planet like Earth, definitely capable of allowing life to survive, it’s likely to be very, very far away. It would be nigh on impossible to get there now, but as science and technology advances over the decades and centuries we will eventually begin to develop starships, and humanity and civilisation will be able to spread throughout the galaxy as we find new homes.

Conor Farrell is an avid science enthusiast and studied physics with astronomy at Dublin City University. He has worked at Astronomy Ireland and Dunsink Observatory in the past, and loves to promote all things space-related to a wider audience. In his spare time he writes about science and current affairs, and can be followed on Twitter at @conorsthoughts.

Read more articles by Conor Farrell

Read: Scientists discover planet that is more like Earth than any other planet (but still not that similar)

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