We've learned many things from this vast catalog of alien worlds orbiting alien stars. But one small detail stands out like a sore thumb. We've found nothing else out there like our own Solar System.
This has led some to conclude that our home star and its brood could be outliers in some way – perhaps the only planetary system of its kind.
By extension, this could mean life itself is an outlier; that the conditions that formed Earth and its veneer of self-replicating chemistry are difficult to replicate.
If you're just looking at the numbers, the outlook is grim. By a large margin, the most numerous exoplanets we've identified to date are of a type not known to be conducive to life.
Most exoplanets we've seen so far orbit their stars very closely, practically hugging them; so close that their sizzling temperatures would be much higher than the known habitability range.
It's possible that as we continue searching, the statistics will balance out and we'll see more places that remind us of our own backyard. But the issue is much more complex than just looking at numbers. Exoplanet science is limited by the capabilities of our technology. More than that, our impression of the true variety of alien worlds risks being limited by our own imagination.
What's really out there in the Milky Way galaxy, and beyond, may be very different from what we actually see.
Expectations, and how to thwart them
Exoplanet science has a history of subverting expectations, right from the very beginning.
" If you go back to that world I grew up in when I was a kid, we only knew of one planetary system," planetary scientist Joni Honer of the University of Southern Queensland tells Science Alert.
" And so that was this kind of implicit assumption, and sometimes the explicit assumption, that all planetary systems would be like this. You know, you'd have rocky planets near the star that were quite small, you'd have gas giants a long way from the star that were quite big. And that's how planetary systems would be."
For this reason, it took scientists a while to identify an exoplanet orbiting a main sequence star, like our Sun. Assuming other solar systems were like ours, the tell-tale signs of heavyweight planets tugging on their stars would take years to observe, just as it takes our own gas giants years to complete an orbit.
Based on such lengthy periods of a single measurement, it didn't seem worth the trouble to sift through a relatively short history of observations for many stars to conclusively sift out a fellow main-sequence solar system.
When they finally did look, the exoplanet they found was nothing like what they were expecting: a gas giant half the mass (and twice the size) of Jupiter orbiting so close to its host star, its year equals 4.2 days, and its atmosphere scorches at temperatures of around 1,000 degrees Celsius (1800 degrees Fahrenheit).
Since then, we've learned these 'Hot Jupiter' type planets aren't oddities at all. If anything, they seem relatively common.
We know now that there's a lot more variety out there in the galaxy than what we see in our home system. However, it's important not to assume that what we can currently detect is all that the Milky Way has to offer. If there's anything out there like our own Solar System, it's very possibly beyond our detection capabilities.
" Things like the Solar System are very hard for us to find, they're a bit beyond us technologically at the minute," Horne says.
" The terrestrial planets would be very unlikely to be picked up from any of the surveys we've done so far. You're very unlikely to be able to find a Mercury, Venus, Earth and Mars around a star like the Sun."
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