What is the Closest Earth-like Exoplanet?

Artist View of M dwarf Exoplanet Sun

Recently I was asked this question by a friend which made me stop and ponder it for a minute. Of course, my mind jumped to my older posts on what an 'Earth-like' planet is along with the definition of a "habitable" planet (the key differentiator being Earth-like doesn't necessarily mean habitable or terrestrial).

This isn't an easy question to answer since we don't fully understand what habitable or terrestrial might look like for different exoplanet stellar systems. For example, if we're going to limit the question to exoplanets around F, G, K and M dwarf stars due to stellar lifespan (for example, bright A5V stars only have a lifespan of about 750 million years), then one current estimate is there are 65 billion terrestrial planets around these 4 classes of stars in the Milky Way galaxy. However, since cool, low mass M dwarf stars comprise 76% of the total, terrestrial exoplanets are going to be in tight, short orbits, most likely tidally locked with low UV stellar radiation. That doesn't rule out Earth-like habitable exoplanets, but that's a large unknown at this point (something we'll talk about in a future post). 

So, to get back to the question, let's pick F, G and K main sequence stars first. Using that criteria, 82 Eridani in Eridanus (visible from the southern hemisphere) is going to be the best answer for those stellar candidates. At about 20 light years away, the two planets, 82 Eridani e and f orbit a G6V star (very similar to the Sun) with a periods of 147 and 647 days potentially within the star's habitable zone. As of this writing, both fall on the edges of that habitability zone with e on in the inner edge and f on the outer edge. Also, the detected exoplanets masses are at least 4x that of Earth's. Given that, this is probably the best case for now. 

In the northern hemisphere, if you look in the constellation of Cassiopeia, the orange star Gliese 892 (stellar type K3V - cooler than the Sun) may be the closest at 21 light years. However, the main problem is the candidate exoplanet's mass at over 10x Earth's. Also, that particular exoplanet hasn't been confirmed.

Honestly, there's not much beyond this since we're dealing with more distant, brighter stars (see my mention of the Malmquist bias in my previous post). If we go back to lower mass, cooler M dwarf stars then my favorite pick is Gliese 667 in the constellation of Scorpius at 23 light years away. It is a triple star system with the third star, Gliese 667 C, having detected exoplanets. Planet e with an orbit of about 62 days and mass of about 3x that of Earth's is within the star's habitable zone. There are many other M dwarf star systems with detected exoplanets. If you're interested, you can browse the Habitable Worlds Catalog or NASA's Exoplanet Catalog to see what potential exoplanets could be candidates.

Given how difficult this question is, there just isn't a good answer yet. There are still so many assumptions along with unanswered questions as to just what extraterrestrial life is, how it may evolve and what types of stellar systems would support it. All we can do is continue to gather more data and assume life on Earth as the initial standard to measure against.

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Just how many Earth-sized planets are there in the Milky Way?

Credit - ESO

Of course, at this point there really isn't an answer to this question. But, as more data is collected by exoplanetary surveys, the predictive precision increases. I seem to recall a statistic that one calculated very broad average is ~1.8 planets for every star in the Milky Way based on Kepler data. But, I can't find that value in published works.

This uncertainty has led me on a bit of a hunt recently to see where the current predictions are today. I'm not going to get to them all in this post and may just write up a more definitive article on it. But, we can play with a few numbers for Earth-sized planet.

For example, Petigura et al. 2013 estimate that 22% of Sun-like stars (spectral types G and K) harbor Earth-sized planets within their habitable zones. Quick calculations (using popular references) show this would be about 20% of all main sequence stars which is 90% of all stars in the Milky Way. Roughly that would be about 10¹¹ * 0.9 * 0.2 * 0.22 = 3.96 billion Earth-sized planets in habitable zones (which only works out to about 0.04 planets per star) in our galaxy. As a filter though, just how many of the 4B Earth-sized planets have water, proper atmospheres, just the right habitable distance, old enough, in the GHZ (see previous post), etc. are just some of the limits on extraterrestrial life

Since this simple estimate ignores other types of planets and associated systems it would be good to go back to the full estimate of average number of planets per stellar type in the galaxy. More digging is needed here.

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