Among the lay public, probably one of the most misunderstood astronomical terms that has recently come into common use has to be “super-Earth”. All too often, the assumption is made by too many people that a super-Earth is just a larger size version of the Earth that is otherwise identical to our planet in all ways including its habitability. Nothing could be further from the truth.
Since the Kepler mission is able to determine the size of a planet based on the characteristics of its transit in front of its host star, the Kepler team developed a working nomenclature based only on the size of the planet detected. Planets with radii, RP, less than 1.25 times that of the Earth (or 1.25 RE) were classified as Earth-size. Those with radii in the 1.25 to 2.0 RE range were classified as super-Earths while those in the 2.0 to 4.0 RE range were classified as Neptune-size. The position of Earth-size or super-Earth-size planets in relation to their sun’s habitable zone (and hence their potential habitability) has absolutely no bearing on this size classification. In fact, the majority of Earth-size and super-Earth-size planets detected by the Kepler mission orbit far too close to their suns to be habitable mainly because Kepler is more likely to detect the transits of planets in smaller orbits.
Likewise, the terms Earth-size or super-Earth-size say absolutely nothing about the potential composition of a planet which is another important factor in determining a planet’s potential habitability. Looking at the size of several dozen planets discovered by Kepler with radii of less than 4 RE as well as the masses of those planets measured by precision radial velocity and transit timing variation techniques, it has been possible for astronomers to start to investigate the change in planets’ density (which gives an indication of their bulk composition) as a function of radius. Initial analyses published earlier this year by Marcy et al. as well as by Weiss and Marcy showed that while there was considerable variation from planet to planet in their samples, the density of super-Earths tended to increase with increasing radius as would be expected of rocky planets until a transition is reached somewhere around the 1.5 RE to 2.0 RE range. Larger planets then tended to become increasingly less dense.
Plot of planet density versus radius for 33 extrasolar planets and the planets in our solar system included in the analysis earlier this year by Marcy et al. (G. Marcy)
The interpretation of this result is that planets with radii greater than about 1.5 RE are increasingly likely to have substantial envelopes of various volatiles such as water (including high pressure forms of ice at high temperatures) and thick atmospheres rich in hydrogen and helium that decrease bulk density. As a result, they can no longer be considered terrestrial or rocky planets like the Earth but would be classified as mini-Neptunes or gas dwarfs depending on the exact ratios of rock, water and gas. The conclusion from this initial work was that there was a important transition in bulk composition that encompassed the upper end of the super-Earth size range from rocky planets to non-rocky planets.
This transition from rocky to non-rocky worlds is important for a number of reasons including assessing the potential habitability of these large worlds. As currently defined by scientists, a habitable world needs to be a rocky or terrestrial planet or moon. Mini-Neptunes or gas dwarfs are unlikely places for life as we know it to start or survive. As a result, the size threshold between rocky and non-rocky worlds is an important parameter in assessing the potential habitability of a planet just like its position relative to its sun’s habitable zone. But the precise value of radius or mass of this transition and its characteristics remained to be determined.
New Analysis
A new analysis of this transition from rocky to non-rocky planets was recently submitted for publication by Leslie Rogers who is currently a Hubble Fellow at the California Institute of Technology. In this paper, Rogers confined her analysis to transiting planets with radii less than 4 RE whose masses had been constrained by precision radial velocity measurements. This heterogeneous sample of 47 transiting sub-Neptune size bodies orbiting 27 stars included planets studied by Marcy et al. as part of a Keck HIRES radial velocity measurement campaign as well as five additional planets whose masses had been published earlier based on HIRES measurements. All but four of the planets in the sample had close-in orbits with periods less than 50 days with effective stellar fluxes ranging from 1.1 to 3,700 times that of the Earth. Planets with masses determined by Weiss and Marcy using the innovative transit timing variation technique were excluded from this analysis since this sample of planets may be affected by selection biases that favor low-density planets.
First, Rogers determined the probability that each of the planets in the Kepler-derived sample were rocky planets by comparing the properties of those planets and the associated measurement uncertainties to models of planets with various compositions. Next, she then applied a hierarchical Bayesian statistical approach to assess three different models for mass-radius distribution for the sample of planets. One model assumed an abrupt, step-wise transition from rocky to non-rocky planets while the other two models assumed different types of gradual transitions where some fraction of the population of planets of a given radius were rocky while the balance were non-rocky.
The analysis using all three models had their most likely transition mid-points (i.e. where the abrupt transition takes place in the step-wise model or a 50-50 split in rocky and non-rocky planets exists in the gradual transition models) at about 1.5 RE. Rogers’ analysis mildly favored a step-wise transition over the gradual transition models but she readily admits that a larger sample of planets with radii less than 2 RE with known masses will be required to definitively determine which model more accurately reflects the true mass-radius function of planets. Likewise, Rogers’ found no clear evidence of any dependence on the transition value as a function of stellar flux because of the limited size of her sample. In an effort to assess the sensitivity of her results to the chosen sample, Rogers excluded ten planets with the most uncertain masses and found little change in the results.
Based on a statistical analysis of the sample of planets Rogers chose, the transition from rocky to non-rocky planets takes place at no greater than about 1.6 RE at a 95% confidence level. Assuming a simple linear transition in the proportions of rocky and non-rocky planets, no more than 5% of planets with a radii of about 2.46 RE will have densities compatible with a rocky composition to a 95% confidence level. Adopting the 1.6 RE radius threshold as the limit beyond which planets are increasingly unlikely to have rocky compositions is equivalent to a mass threshold of about 6 ME assuming an Earth-like composition. Rogers notes that many recent models for planet formation indicate that planetary embryos with masses of 6 ME or larger can readily accrete gas directly from the circumstellar disk that surround young stars. Rogers’ result that indicates larger planets are much more likely to be non-rocky seems to provide evidence for this.
An obvious potential counterexample to this maximum rocky-planet size threshold is the case of Kepler 10c which made the news less than two months ago. Dumusque et al. used precision radial velocity measurements from HARPS-N to determine that the mass of this planet is 17 ME or about the same as Neptune’s mass in our Solar System. Combined with the radius of 2.35 RE determined by Kepler measurements, the density of Kepler 10c comes out to be 7.1±1.0 g/cm3 leading Dumusque et al. to claim that it is strong evidence for the existence of massive solid planets. While at first blush this density, which is greater than Earth’s, might lead to the conclusion that Kepler 10c is a rocky planet, Rogers counters that its density is in fact inconsistent with a rocky composition by more than 1σ and that there is only about a 10% probability that Kepler 10c is in fact predominantly rocky in composition. It is much more likely that it possesses a substantial volatile envelope albeit smaller than Neptune’s given its higher density.
Impact on Potential Habitable Planets
This new analysis starts to set quantitative limits on the maximum size of not only rocky planets but the maximum size of potentially habitable planets since non-rocky worlds like gas dwarfs and mini-Neptunes are generally considered unlikely sites for life to develop. Rogers specifically mentions the impact of her work on one illustrative case in her paper: Kepler 22b which was the first planet discovered in the habitable zone of another star with an accurately measured radius.
Kepler 22b has a radius of about 2.4 RE but radial velocity measurements to date have been unable to detect it. As a result, only very high upper limits have been set on its mass leaving much ambiguity about its actual properties. But based on Rogers’ analysis of the mass-radius relationship of currently known Earth to super-Earth size planets, the fraction of planets with a radius of 2.4 RE that are rocky is less than 2% to the 95% confidence level. It is much more likely that Kepler 22b has a volatile envelope that contributes significantly to its volume. As a result, it is highly unlikely that Kepler 22b is a terrestrial planet and, assuming that not all terrestrial planets in the habitable zone are necessarily habitable, even less likely that it is habitable. Barring additional scientific evidence to the contrary, it makes little sense to consider Kepler 22b a potentially habitable planet.
Diagram illustrating the Planetary Habitability Laboratory’s current (as of July 3, 2014) list of potentially habitable exoplanets. According to the latest analysis of Kepler data, all but two of these planets are unlikely to be terrestrial planets never mind potentially habitable planets. Click on image to enlarge (PHL)
To gauge the impact of Rogers’ analysis on other worlds considered by some to be potentially habitable, I have applied her rocky planet size limit criterion to one of the more widely publicized lists of potentially habitable worlds: The Habitable Exoplanet Catalog maintained by the Planetary Habitability Laboratory (PHL) at the University of Puerto Rico at Arecibo. Table 1 below lists the planets from their “main database” for which only the radii are known listed in descending order of their calculated Earth Similarity Index (ESI) value. All data in Table 1 come from the PHL catalog.
Table 1: List of PHL Potential Habitable Planets with Known Radii
|
Name |
ESI |
RP (Earth = 1) |
RP < 1.6 RE |
|
Kepler 62e |
0.83 |
1.61 |
NO |
|
Kepler 283c |
0.79 |
1.81 |
NO |
|
Kepler 296f |
0.78 |
1.79 |
NO |
|
Kepler 61b |
0.73 |
2.15 |
NO |
|
Kepler 22b |
0.71 |
2.35 |
NO |
|
Kepler 298d |
0.68 |
2.50 |
NO |
|
Kepler 62f |
0.67 |
1.41 |
YES |
|
Kepler 186f |
0.64 |
1.11 |
YES |
|
Kepler 174d |
0.61 |
2.18 |
NO |
As can be seen, seven out of the nine planets on PHL’s list of potentially habitable planets with known radii exceed the 1.6 RE radius limit and are therefore unlikely to be rocky planets. It is more probable that these seven planets are mini-Neptunes or gas dwarfs that are unlikely to support life as we know it. Only two of the planets, Kepler 62f and Kepler 186f are small enough to be likely rocky planets and, at least on the basis of probable composition, should be considered potentially habitable (for a fuller discussion of Kepler 186f, see “Habitable Planet Reality Check: Kepler 186f“).
It should be noted that the case for Kepler 296f is more complicated than earlier assumed and that the data listed in the PHL catalog are incorrect. It turns out that Kepler 296 is actually a binary star that was unresolved by Kepler which affects the properties of the detected planets that had been derived under the assumption that the star was single. An analysis of Hubble Space Telescope images of Kepler 296 and other stars recently submitted for publication by Star et al. shows that Kepler 296 actually consists of M0V and M3V red dwarf stars 1,200 light years away with a projected separation of 80 AU. Since it is not possible to determine which star the detected planets orbit from the Kepler data alone, there are even odds that Kepler 296f orbits one of these components or the other. If Kepler 296f orbits the brighter A component, Star et al. calculate that it has a radius of 2.4 RE and orbits closer than the inner edge of this star’s habitable zone. Given its size and position, it is unlikely to be a rocky planet and even less likely to be habitable. If Kepler 296f orbits the B component, its orbit would be comfortably inside the habitable zone of this star but it would have a radius of 3.4 RE making it far too large to be a rocky planet. In either case, Kepler 296f is very unlikely to be a habitable planet.
Hubble Space Telescope image of the Kepler 296 showing it to be a close binary with the B component on the left and A on the right. It is not currently known which star hosts the five planets detected by Kepler. (Adapted from Star et al./STScI/NASA)
The remaining dozen planets on PHL’s list of potentially habitable planets were discovered by precision radial velocity measurements and do not have measured radii. Still, the potential nature of these planets can be evaluated using the optimistic upper mass limit for rocky planets of 6 ME assuming a planet with a 1.6 RE radius and an Earth-like composition. It must be remembered, however, that precision radial velocity measurements only provide a minimum mass or MPsini of a planet since the inclination of the planet’s orbit to our line of sight, i, is unknown. The inclination of the orbit or the actual planet mass must be determined by other methods.
Table 2 below lists the remaining 12 planets in PHL’s catalog that were discovered using the radial velocity technique. The data in this table were taken directly from the main database for PHL’s Habitable Exoplanets Catalog. These planets are once again listed in descending order of their ESI values which were presumably calculated under the overly optimistic assumption that their actual mass equals their measured minimum mass. Listed in this table is the probability that a randomly oriented orbit for each planet produces an actual planet mass that is less than Rogers’ 6 ME threshold for a rocky planet. The last column gives a qualitative assessment of whether or not each planet is a terrestrial planet keeping in mind that the 6 ME threshold represents the maximum likely point where there is a 50-50 chance of a planet being a terrestrial planet.
Table 2: List of PHL Potential Habitable Planets with Known MPsini
|
Name |
ESI |
MPsini/ME |
Probability MP < 6 ME |
Terrestrial Planet? |
|
GJ 667Cc |
0.84 |
3.80 |
77% |
Possibly |
|
GJ 832c |
0.81 |
5.40 |
44% |
Probably Not |
|
τ Ceti e |
0.78 |
4.29 |
70% |
Possibly |
|
GJ 180c |
0.77 |
6.40 |
0% |
Highly Improbable |
|
GJ 667Cf |
0.77 |
2.70 |
89% |
Probably |
|
GJ 180b |
0.75 |
8.30 |
0% |
Highly Improbable |
|
GJ 163c |
0.75 |
7.26 |
0% |
Highly Improbable |
|
HD 40307g |
0.74 |
7.09 |
0% |
Highly Improbable |
|
GJ 422b |
0.71 |
9.90 |
0% |
Highly Improbable |
|
Kapteyn b |
0.67 |
4.80 |
60% |
Possibly |
|
GJ 667Ce |
0.60 |
2.70 |
89% |
Probably |
|
GJ 682c |
0.59 |
8.70 |
0% |
Highly Improbable |
Right off, half of the dozen planets on this list have minimum masses that already exceed the 6 ME mass limit and are unlikely to be rocky planets. Folding in the uncertainty of their actual masses, it is much more likely that these planets are gas dwarfs or mini-Neptunes or even larger – in some cases maybe much larger. Of the remaining six planets with non-zero probabilities of having actual masses less than the 6 ME mass limit, Kapteyn b and GJ 832c still have uncomfortably high probabilities of being volatile rich (for a fuller discussion about these planets, see “Habitable Planet Reality Check: Kaptyen b” and “GJ 832c: Habitable Super-Earth or Super-Venus?“). The situation with the three planets on this list orbiting GJ 667C are not as rosy as the probabilities that they do no exceed the 6 ME mass limit would suggest. While there seems to be a fair chance that GJ 667Cc is in fact a terrestrial planet, a new analysis of the data used to discover the even more promising GJ667e and f now shows that they most likely do not exist and that subtle stellar activity was mistaken for planets (for a full discussion, see “Habitable Planet Reality Check: GJ 667C“). Likewise, τ Ceti e, which has a fair possibility of being a rocky planet, remains unconfirmed and is also strongly suspected that its identification is the result of stellar activity masquerading as a planet (see “A Review of the Best Habitable Planet Candidates“).
What is immediately obvious from the application of Rogers’ analysis of Kepler results is that most planets that have been optimistically considered “potentially habitable” by some are probably not habitable at all. Instead it is much more likely that most of the planets in the main database of PHL’s Habitable Exoplanet Catalog are instead gas dwarfs, mini-Neptunes or larger planets with extensive volatile envelopes. In addition, about three-quarters of the planets on PHL’s list of potentially habitable planets among Kepler’s list of planet candidates are also not likely to be rocky planets because of their large radii. For PHL’s ESI to be of any meaningful scientific value in the future, they need to reevaluate how the ESI is calculated and take into account the probability that a given planet is even a terrestrial planet. In addition, they need to stop assuming that the minimum planet mass derived from precision radial velocity surveys is the equivalent of the actual mass.
The Future
While the analysis of this initial sample of planets discovered by Kepler with radii less than 4 RE has set some limits on the maximum size of rocky planets (and hence potentially habitable planets), the sample involved in the work by Rogers, Marcy et al. as well as Weiss and Marcy were rather limited in size. Fortunately, many more planets in this size range remain to be discovered during the ongoing analysis of the database from Kepler’s primary mission and the follow up observations by the Keck HIRES team and others. Yet more planets are expected to be discovered during Kepler’s “K2” extended mission currently underway. NASA’s upcoming TESS (Transiting Exoplanet Survey Satellite) mission should discover still more planets in this size range and will be especially important since this mission will concentrate on brighter stars that can yield more accurate masses of smaller planets via precision radial velocity measurements. With the larger sample the future promises, it should prove possible to not only better define the transition from rocky to non-rocky planets but also discern its dependence on a host of factors allowing scientists to better understand the limits of planetary habitability.
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Related Reading
“The Transition from Super-Earth to Mini-Neptune”, Drew Ex Machina, March 29, 2014 [Post]
“Habitable Planet Reality Check: Kepler 186f”, Drew Ex Machina, April 20, 2014 [Post]
“Habitable Planet Reality Check: Kapteyn b”, Drew Ex Machina, June 6, 2014 [Post]
“GJ 832c: Habitable Super-Earth or Super Venus?”, Drew Ex Machina, June 27, 2014 [Post]
“Habitable Planet Reality Check: GJ 667C”, Drew Ex Machina, September 7, 2014 [Post]
“A review of the Best Habitable Planet Candidates”, Centauri Dreams, January 30, 2015 [Post]
“Abundance of Earth Analogs”, Drew Ex Machina. June 25, 2014 [Post]
“Habitable Planet Reality Check: 55 Cancri f”, Drew Ex Machina, May 7, 2014 [Post]
“The Extremes of Habitability”, SETIQuest, Volume 4, Number 2, pp. 1-8, Second Quarter 1998 [Article]
General References
Xavier Dumusque et al., “The Kepler-10 Planetary System Revisited by HARPS-N: A Hot Rocky World and a Solid Neptune-Mass Planet”, The Astrophysical Journal, Vol. 789, No. 2, Article id. 154, July 2014
Geoffrey W. Marcy et al., “Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets”, The Astrophysical Journal Supplement, Vol. 210, No. 2, Article id. 20, February 2014
Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, arVix 1407.4457 (submitted to The Astrophysical Journal), July 16, 2014 [Preprint]
Kimberly M. Star et al., “Revision of Earth-Sized Kepler Planet Candidate Properties with High Resolution Imaging by Hubble Space Telescope”, Submitted to The Astrophysical Journal, July 3, 2014 [Preprint]
Lauren M. Weiss and Geoffrey W. Marcy, “The Mass-Radius Relation for 65 Exoplanets Smaller than 4 Earth Radii”, The Astrophysical Journal Letters, Vol. 783, No. 1, Article id. L6, March 2014
Habitable Exoplanet Catalog: Data of Potentially Habitable Worlds, Planetary Habitability Laboratory Web Site [Link]
Great article! Lots of information to chew on above ordinary press releases, which is hard to find. Thanks for your work!
Hi Drew
Are there any Potentially Habitable Planets that a robust case can be made for?
Hey Adam,
Robust case? No. The best bets so far are Kepler 62f and Kepler 186f. But aside from their radii and orbital characteristics, we still do not know much about these planets. Some knowledge about their masses would be a good start in order to start pinning down their potential bulk compositions at least (especially for Kepler 62f which is large enough to have a couple tens of percent chance of not being a terrestrial planet). We are far too early in this process (both in finding planets and knowing how to recognize a habitable planet) to start making a “robust case” for the habitability of any planet yet.
The RV list also contains several entries that suffer from the issue that the planet in question may not actually exist. Recent results taking into account red noise have basically reduced Gliese 667C to a two-planet system, the Tau Ceti claims are somewhat problematic (particularly for the outermost planets) and HD 40307g seems not to have been confirmed according to a recent HARPS presentation.
Regarding the probability of the true mass being below 6 Earth masses for a given m sin i, is the random orientation assumption actually reasonable? I wonder how much the estimates would change if you add a more realistic mass distribution.
I definitely agree that the cases for the planets found via RV measurements for GL 667C and Tau Ceti are hardly on the firmest footings to begin with (which will be the topic of future posts). The situation with GJ 581, where the RV signatures of two potentially habitable planets ended up being the result of subtle magnetic activity on the star modulated by its rotation, is a perfect example of what might be going on with GL 667C and Tau Ceti.
As for the assumption about the orbital inclination of the planets being random, without any a priori knowledge to the contrary (e.g. a planet mass distribution function that has yet to be determined with any confidence given existing data) it is a perfectly reasonable assumption to make. In fact, it is the assumption that is usually made in cases like this (except by PHL in the calculation of the ESI values).
Thanks for the comment!!!
When the Moon was formed, it was due to a massive collision with an object thought to be as large as Mars. Wondering if Earth gained a large amount of mass at that time and thus this could prove a very rare, yet very critical reason that we have life on Earth. Wondering also if this early collision brought with it important elements like water, or chemicals that Earth normally would not have had. Also could have changed the spin and axis as well as magnetic fields we have due to chemical makeup from that early/massive collision. I think percentage wise our Moon is so large it would normally be considered a dwarf planet and otherwise impossible to have a moon that large by percentage and thus critical to create tides and mix the life soup and thus another critical item for life to start or evolve as fast as it did on Earth. With future mapping of our Milky Way Galaxy it will be interesting if they can find a real earth twin (matching Sun within 5%, and matching Moon within 5% and also all the other critical details that match to Earth with orbit, mass, percentage of surface liquid water, etc. In all the billions of planets in this galaxy, is there one earth twin that formed like the Earth did and would be considered a true 5% twin….? I bet not and maybe one reason for Fermi’s Paradox and why it is so quiet out there.
Thanks for sharing your thoughts on this important issue. To address some of the points you made in your comment, yes, the Moon is generally believed to have formed as a result of a collision with a Mars-size object called Theia about 100 million years after the Earth formed. But the mass added to the Earth by this collision was probably only on the order of 10%. That is insignificant compared to the total mass range of potentially habitable planets which runs from 0.1 to 0.25 ME on the low side to as high as 4 to 6 ME on the high side. The Earth would have been large enough to be habitable even without this last major collision early in its history.
While the formation of the Moon certainly had an effect on the Earth, it likely did not substantially change the bulk composition of the Earth nor substantially affect its inventory of volatiles (e.g. water most of which came from asteroids and comets over the hundreds of millions of year after the formation of the Moon).
The argument made by many who adhere to the “rare Earth” belief of planetary habitability (an extreme form of which you seem to embrace) that a large Moon is required to maintain habitable conditions on Earth has been proven to be false. While it is true that the Moon helps stabilize the tilt of the Earth today and if it were to be removed Earth’s tilt would change dramatically over time, that is not the equivalent of the situation of the Moon never existing in the first place. If the Earth never had the Moon, its rotation rate would be much faster today than it currently is and its tilt would be much more stable as a result even without a Moon. Even if the tilt of a potentially habitable planet were to vary widely, numerous studies published in peer-reviewed literature over the last couple of decades have already shown it would have a minimal impact on planetary habitability especially for planets in the outer parts of the HZ which would naturally have denser atmospheres as a natural consequence of the carbonate-silicate cycle which controls planetary temperatures. The presence of oceans would help to further stabilize the climate.
As for any role of tides might have played in the origin of life or its subsequent evolution, a large moon is not required. In the case of the Earth, the Sun generates tides that have about half the current amplitude of the lunar tides. As a result, a Moonless Earth would still experience significant ocean tides.
Thanks so much for your comments and your interest in this topic. I hope my explanations have helped clear up some issue for you.
Drew LePage
Hi, thanks for your reply.
I would think that any collision of a Mars sized object would/could affect the orbit and spin of the Earth. It would also affect the tilt and thus affect seasons as we know it as well as the length of the day and other important things related to tilt or spin. Not sure how much of that impact could also affect plate tectonics, or actual makeup of critical minerals or chemicals that later lead to life on earth. I have heard the theory that asteroids during the bombardment period is to give credit to the water ratios, but would likely be hard to prove it did not come from an early Mars like collision. I guess for me, thinking about the moon and its role with life or speed of evolution is a bit like understanding compound interest over long periods. Of course anyone with a 401K that has looked at what difference you get over 40 years on how much your broker makes… that last 2%. so it is fun to play a bit with math and realize what the difference of 5%, vs. 7% over 40 years is… it is just amazing. Now for earth take that out to 4.5 billion years…. and since every planet like Earth that rotates a Sun like ours (that heats up to boil off your oceans in about 6-7 billions years or less, thus everyone on a ticking time bomb to total life elimination. ..so maybe that last little detail or series of details we think is a minor issue, turns out later to be that 2% over 4.5 billion years and thus makes all the difference. Details like our magnetic shielding seem very critical. Hard to think really that trees were not even invented by mother nature until about 350 million years ago.. heck sharks may prove older by 50 million years… so oceans seem to be where the life cake starts and mixes and what exactly was going on for the first 4 billion years..(high tide + low tide swing of moon + sun with Moon being more powerful for tides than just sun) . more complex animal life on earth is really just showing up in the last 500 million years and during that time we know of about 5 mass extinctions (that we have records on). Anyhow, I guess my point is we don’t know for sure what details are critical for life to start and then also to evolve and at what speed. Life seems to have critical levels or layers of complexity that happens in a special order or can’t happen out of order, so some of these things like how strong the high tides or low tides are over 4.5 billion years may seem small or like that last 2% you pay your broker and don’t worry about, but 4.5 billion years later its all the difference. Just my humble opinion of course. As we get better about mapping our own back yard (milky way) it will be fun to see if they can find an Earth Twin. My gut says you will NEVER find life that starts on a planet that orbits a red dwarf (anymore than you can bake a cake with a match at 500 yards away) regardless of what they calculate the habitable zone at. I think we continue to learn a lot by modeling on computers and hope these new super computers can teach us a lot more than we know now about what could be possible and how to better guess where life could develop. I think we need to focus our efforts to find a sun like twin, then earth like twin, then Moon like twin and those 3 items will prove critical to finding any life creating planet that has the chance to be close enough to ever matter. Unless we can decode our own dna… live forever… then hitch a free ride to the great attractor, survive the minor bump with Andromeda and a few others… get off this rock before our sun heats up or swallows earth up… Anyhow, hope to be wrong on this rare earth theory and the universe is full of life, both simple and complex, but I have a feeling Earth is not alone… just first by a few billion years or more.
…. ps… on the idea Earth may be first by a few billion years…..
What drives me crazy is that we think, what if another alien civilization is just a few million years ahead of us…. their technology would be so far advanced we likely could not recognize them, thus might be proof of why we can’t find anything yet (seti). We only look at the math from one side, the side of our clock starting at the Big Bang and then us showing up just recently (they found stone tools dating back to 250,000 years in turkey recently..wow). We know life is built with layers of complexity and thus could not have happened too early (example we likely need a 2nd or 3rd generation sun to have the heavier elements needed) or like understanding you can’t invent a Ct Scan device before you first figure out stone tools or the wheel, again everything has layers of complexity and that requires an order and most of all time/oppourtunity. So for example the closest star to earth is Proxima Centauri and is a dim red dwarf star that depending on what math you use is expected to last 4 – 8 trillion years (the math varies a bit, but they show some red dwarfs that can live 10 trillion years). So lets pretend just for a quick example that the life of the universe would last at least the life of one small red dwarf or 10 trillion years. Put that back into perspective as 13.7B/10Trillion as where we are right now to where the universe lifespan is (relative to a star designed to live) and that = 0.00137. The rates of white males living in USA today is 77.76 years on average so just for fun to compare a human lifespan average vs. Earth so far, that equals about 38 days. Imagine being born and in the first 38 days of that lifespan we expect intelligent life to be popping up all over the place throughout the universe. Of course the universe could easily live 10x or more longer, so maybe I have this math off by a decimal place, but again, I think our viewpoint of the math is from only one side and that is the assumption we are late to the life party and my point is when we do the math from the other side of the extreme then it looks different and you can start to wonder why we got life here on earth so early and then, what if … that process that made the earth is very, very specialized (luck of course, we are no special, just lucky) and what if every single thing turns out to be important (not by itself, but combined together, like that 1% + 1%, + and so on to add up to be a huge difference in total and over billions of years. What if life that develops on other planets runs into the great filter of time… what if intelligent life develops at about 10 billion years normally, but your oceans boil off at about 7 billion, so you never get there (or invent spaceships to leave the planet in time to survive, or thus all your knowledge is lost forever) …thus everyone is all in.. but too late. Anyhow the great filter is the 4th dimension. Btw.. have they found any sun in our galaxy that is an exact match to our own in every way? Just within 5% match on every characteristic (I think our sun is more rare than we have been told, or I think the details are more important than we thought). Just my opinion of course and I hope to be wrong and they find life on mars soon vs. just that the oven on mars was so wonderful to start life, but somehow it never did. btw..how crazy is it that we have never found a meteorite from beyond our own solar system. ever. nothing older than 4.7 billion years old… ever. I think that could prove life started here vs. transported somehow.
Thank you for this excellent piece. Your blog is a great resource and i’ll come back
I still think a conservative approach towards planet sizes is ill-advised right now considering almost every time we send a probe to planets or moons in our own solar system, predictions are smashed.
Also, Im of the belief that we will have a better chance finding a habitable planet in the form of a moon. Whatever the number of planets are out there, moons will probably be 10x that many. All planets in the habitable zone (and maybe even those just outside of it), regardless of size, need to be studied in greater depth.