I have just scanned my 100th Earthlike World. I have been spreadsheeting the various vital statistics for these worlds I've scanned, seeking planets that are as close as possible to "perfect matches" for Earth. But here are some statistical findings. First, the data set: all of the planets are in procedurally-generated systems, except for two: TRAPPIST-1 and HIP 19789, so they should represent a good "random sampling" of randomly generated planets. The stars they orbit are, admittedly, biased as I usually filter out K and M class stars. So, what do the stats look like?
Mass: the difference between the median and the mean is striking. It means that there are an awful lot of tiny planets, balanced out by a small number of super-large planets. Still, both averages are below 1.00, meaning that smaller-than-Earth Earth-likes are far more common than larger ones. I only found nine planets that were close matches to Earth itself (which I defined as being between 0.9 and 1.1 Earth-masses). The data ranged from 0.2796 to 2.3250. I believe there are no "hard limits", as it is surface gravity rather than planet mass that is the critical criterion.
Gravity: the surprise here is that, given the widely disparaging mass statistics, the two gravity statistics are much closer together - meaning that the tiny planets must, on average, be much denser than Earth in order to generate the near-match-to-Earth statistics. 22 planets had Earth-matching gravity (between 0.90 and 1.10) and of those 22, only six of them also had Earth-matching mass. My data ranged from 0.67 to 1.56; I believe the hard limits for ELW qualification are 0.4 to 2.0.
Temperature: the mean and median are again close together, but the thing to note here is that "Earth-like" planets are, on average, several degrees colder than Earth itself is. I found only nine planets with Earth-matching temperature (which I defined as being between 286 K to 290 K) and only two of those had matching mass or gravity. Observed temperatures ranged from 261 to 317; I believe the hard limits to qualify as an ELW are 260 and 320.
Atmospheric pressure: here's an odd statistic. The median is below Earth-normal, while the average is above Earth-normal. So once again, a whole bunch of thin-aired planets push the median down, while a small number of planets with super-thick atmospheres push the mean up. I assume there are hard limits on the atmosphere to qualify as Earth-like, but I'm not quite sure where they are: I have no planets below 0.40 atmospheres, and only two above 4.00 atmospheres. Only six planets had "Earth-matching" pressures in the 0.90 to 1.10 range; this was the hardest of any of the parameters to find an Earth-match for.
Day length: an excellent example of why it's important to look at both median and mean when talking about "averages". Here, a tiny number of tidally-locked planets with super-long days (I found eight worlds with days over five months long!) have pushed the mean to nearly a month long, when the vast majority of worlds have far more Earth-like day lengths. Only 13 planets had an Earth-matching day (defined as 0.9 to 1.1 days), with observed measurements ranging from a dizzying 0.4 days to a ponderous 353.4 days. There doesn't seem to be any limit, though one can argue exactly how "Earth-like" a planet tidally locked to it's star is actually going to be.
Other observations that did not lend themselves easily to statistical averaging:
- 22 worlds had "Earth-like volcanism" (silicate magma). Volcanism tended to follow planet mass, with much-smaller-than-Earth usually having "None", and larger-than-earth having "iron magma".
- 12 of the worlds had an "Earth-like night sky" in terms of having only a single moon and no rings. I didn't find any ringed ELWs or worlds with more than one moon, but having either of those in the sky would, in my mind, definitely render a planet as being "not a parallel Earth".
Finally, my goal of finding an "exact match for Earth". Of the seven parameters I tracked as outlined above, no planet matched Earth in all seven categories. The best I found were three of the planets which matched in four of the categories:
- WREGOE AK-C C27-7 1 has matching gravity, volcanism, day length and moon
- OUTORST WV-D d12-7 B 3 has matching mass, gravity, volcanism and day length
- OUTORST ZZ-G d10-1 A 5 has matching mass, gravity, volcanism and moon
Of these, the first one probably qualifies as being the "most Earth-like" I have found to date, though at 276 K it's a bit chilly, and the 3.98 atmospheres pressure might be (literally!) a little hard to swallow.
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Mass: the difference between the median and the mean is striking. It means that there are an awful lot of tiny planets, balanced out by a small number of super-large planets. Still, both averages are below 1.00, meaning that smaller-than-Earth Earth-likes are far more common than larger ones. I only found nine planets that were close matches to Earth itself (which I defined as being between 0.9 and 1.1 Earth-masses). The data ranged from 0.2796 to 2.3250. I believe there are no "hard limits", as it is surface gravity rather than planet mass that is the critical criterion.
Gravity: the surprise here is that, given the widely disparaging mass statistics, the two gravity statistics are much closer together - meaning that the tiny planets must, on average, be much denser than Earth in order to generate the near-match-to-Earth statistics. 22 planets had Earth-matching gravity (between 0.90 and 1.10) and of those 22, only six of them also had Earth-matching mass. My data ranged from 0.67 to 1.56; I believe the hard limits for ELW qualification are 0.4 to 2.0.
Temperature: the mean and median are again close together, but the thing to note here is that "Earth-like" planets are, on average, several degrees colder than Earth itself is. I found only nine planets with Earth-matching temperature (which I defined as being between 286 K to 290 K) and only two of those had matching mass or gravity. Observed temperatures ranged from 261 to 317; I believe the hard limits to qualify as an ELW are 260 and 320.
Atmospheric pressure: here's an odd statistic. The median is below Earth-normal, while the average is above Earth-normal. So once again, a whole bunch of thin-aired planets push the median down, while a small number of planets with super-thick atmospheres push the mean up. I assume there are hard limits on the atmosphere to qualify as Earth-like, but I'm not quite sure where they are: I have no planets below 0.40 atmospheres, and only two above 4.00 atmospheres. Only six planets had "Earth-matching" pressures in the 0.90 to 1.10 range; this was the hardest of any of the parameters to find an Earth-match for.
Day length: an excellent example of why it's important to look at both median and mean when talking about "averages". Here, a tiny number of tidally-locked planets with super-long days (I found eight worlds with days over five months long!) have pushed the mean to nearly a month long, when the vast majority of worlds have far more Earth-like day lengths. Only 13 planets had an Earth-matching day (defined as 0.9 to 1.1 days), with observed measurements ranging from a dizzying 0.4 days to a ponderous 353.4 days. There doesn't seem to be any limit, though one can argue exactly how "Earth-like" a planet tidally locked to it's star is actually going to be.
Other observations that did not lend themselves easily to statistical averaging:
- 22 worlds had "Earth-like volcanism" (silicate magma). Volcanism tended to follow planet mass, with much-smaller-than-Earth usually having "None", and larger-than-earth having "iron magma".
- 12 of the worlds had an "Earth-like night sky" in terms of having only a single moon and no rings. I didn't find any ringed ELWs or worlds with more than one moon, but having either of those in the sky would, in my mind, definitely render a planet as being "not a parallel Earth".
Finally, my goal of finding an "exact match for Earth". Of the seven parameters I tracked as outlined above, no planet matched Earth in all seven categories. The best I found were three of the planets which matched in four of the categories:
- WREGOE AK-C C27-7 1 has matching gravity, volcanism, day length and moon
- OUTORST WV-D d12-7 B 3 has matching mass, gravity, volcanism and day length
- OUTORST ZZ-G d10-1 A 5 has matching mass, gravity, volcanism and moon
Of these, the first one probably qualifies as being the "most Earth-like" I have found to date, though at 276 K it's a bit chilly, and the 3.98 atmospheres pressure might be (literally!) a little hard to swallow.
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