Astronomer Abel Méndez from the Planetary Habitability Laboratory at the University of Puerto Rico has put together a Periodic Table of Exoplanets, where each of the 3700 confirmed exoplanets is slotted into its own discrete category - including planets that could harbour life.
Image: Planetary Habitability Laboratory/University of Puerto Rico at Aricebo
Since the discovery of the first exoplanet in 1992, astronomers have catalogued thousands of distant worlds. Like snowflakes, no two planets are the same, ranging in size, composition, and distance from their respective host stars. Some of these planets resemble the ones found in our own Solar System, while others are completely alien, exhibiting features typically found in tawdry, LSD-inspired science fiction novels.
To make sense of it all, Méndez has created a chart that neatly pigeon-holes each known exoplanet into one of 18 categories. Which makes sense. We're starting to see some patterns emerging in the data such that we can brand these exoplanets as belonging to certain types, such as "warm subterrans", "cold Neptunes" and "Hot Jovians".
At the broadest scale, Méndez distinguishes planets according to their temperature, or distance from the host star, placing worlds in one of three zones: the Hot Zone, Warm Zone or the Cold Zone. For astrobiologists, the Warm Zone is the most significant, as it represents the swath of space in a stellar system where life is likely to emerge. That said, the concept of the "habitable zone" may be a bit of a misnomer, as life may be able to emerge outside of this area, such as on moons in orbit around gas giants.
The table also organises the planets according to size, including various forms of terrestrial, rocky planets (which Méndez refers to as "terrans"), and gassy exoplanets resembling Neptune and Jupiter. Each planet has a number assigned to it, representing the number of known worlds of that type known to astronomers. So for example, we know of 599 "Hot Terrans" and 285 "Cold Jovians". A percentage value is placed in the bottom-left corner of each box to signify the proportion of each.
Artist's depiction of a hot Neptune-like planet in orbit around its host star. (Image: NASA/JPL-Caltech)
"We know of over 3700 planets around other stars. They are very diverse," Méndez told Gizmodo. "We can roughly classify them by their size and temperature. Only warm planets with the right size, similar to Earth, might provide some of the conditions for extraterrestrial life."
Looking at the chart, 53 planets (31 Warm Superterrans, 21 Warm Terrans and one Warm Subterran) have the right size and temperature to potentially foster life, and exhibit features such as liquid surface on the water and a stable atmosphere. "Unfortunately, we don't know yet if they also have the right amount of water (e.g. oceans) or the right atmosphere for life too," said Méndez.
He explained that most are hot planets, but that isn't because they're necessarily the most common, but because they're easier to detect. Once we improve the size and sensitivity of telescopes, Méndez expects astronomers to find more cold planets.
Small planets appear to be the most common, especially super-Earths and Mini-Neptunes, that is, exoplanets slightly larger than Earth, but smaller than Neptune or Uranus. Most known stellar systems have at least one planet, but Méndez says this might be an observational bias as planets are hard to detect. We know of at least three systems with seven planets, for instance, and there are hints of many more.
To that point, and at the top right corner of the chart, Méndez lists the number of known stellar systems according to the number of exoplanets they contain (for example, we've discovered 46 star systems with four planets). Importantly, these systems could contain more planets still waiting to be discovered.
"I'm overwhelmed by the number and diversity of planets in the stars around us. So many places to explore in our own Solar System, but much more is waiting for us beyond," Méndez told Gizmodo. "I won't be very surprised by another planet with life, Earth is the example that this is possible. I will be more surprised by something we haven't seen before."