'Pale blue dot' planets like Earth may make up only 1% of potentially habitable worlds

My observation. Statements like this in the article may make some folks uncomfortable I suspect and limit the potential for abiogenesis taking place in many different locations of the universe (abiogenesis that is not observed operating in nature presently).

"The land's erosion is part of a series of cycles that exchange water between the atmosphere and the interior. Our numerical models of how these cycles interact show that present-day Earth may be an exceptional planet."

I note reports on impact catastrophism in the solar system and how this could alter a postulated growing planet to become earthlike from a gas disc and end up with biological life flourishing on it. The two exoplanet sites now show 5199 confirmed http://exoplanet.eu/, and 5171 confirmed, https://exoplanetarchive.ipac.caltech.edu/index.html.

Apparently, none of the nearly 5200 exoplanets listed today are confirmed as truly earthlike and fit the model descriptions used for a habitable world or inhabited world. These place constraints based upon real world observation on reports that ET could be phoning home today :)

ref paper - Land/Ocean Surface Diversity on Earth-like (Exo)planets: Implications for Habitability, https://meetingorganizer.copernicus.org/EPSC2022/EPSC2022-506.html, 23-Sep-2022. “A balanced ratio of oceans to land is thought to be essential for the evolution of an Earth-like biosphere..."
 
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A couple of thoughts:

1. What is our ability to actually detect a planet the size and temperature of Earth in the habitable zones of the stars around us? How many stars are there where we could detect Earth in their habitable zones? And, have we looked at those stars with our latest capabilies, yet (e.g., the Webb Telescope)? It seems to me that is the real question, not how many of the planets that we have detected so far seem to be like Earth. Our current exoplanet detection techniques are highly biased towards large planets and planets close to their stars.

2. I am not so sure that we need land surface as well as ocean surface to get all types of life like we have in the oceans of Earth, today. But, clearly there would not be any life forms that evolved to walk on land, much less technologically developed life that uses fire, without some dry land on a planet. However, there is plenty of bio-electric activity in our oceans, today, so I am not going to rule out some sort of underwater technology being able to develop in enough billion years. Think "octopus" with a problem to solve looking at a volcanic vent and wondering what it might provide that might be useful.
 
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Unclear Engineer said in post #3, "However, there is plenty of bio-electric activity in our oceans, today, so I am not going to rule out some sort of underwater technology being able to develop in enough billion years. Think "octopus" with a problem to solve looking at a volcanic vent and wondering what it might provide that might be useful."

When I read this, I thought about the movie Abyss :)
 
My observation. Statements like this in the article may make some folks uncomfortable I suspect and limit the potential for abiogenesis taking place in many different locations of the universe (abiogenesis that is not observed operating in nature presently).
That’s likely, though abiogenesis might have begun at special ocean vents. Perhaps, however, land erosion is still important even in these regions.

Apparently, none of the nearly 5200 exoplanets listed today are confirmed as truly earthlike and fit the model descriptions used for a habitable world or inhabited world. These place constraints based upon real world observation on reports that ET could be phoning home today :)
We seem to have 10 exo candidates about the size of Earth (in other thread).

However, if the average is only ~ 1% of these that are Earth-like, then we will need about 45k more exoplanets to find the one we want. But, these first 5k are the easy-picking and not that ideal. So maybe only 15k are needed if we are getting a lot more G and F class star hosts.
 
"However, if the average is only ~ 1% of these that are Earth-like, then we will need about 45k more exoplanets to find the one we want. But, these first 5k are the easy-picking and not that ideal. So maybe only 15k are needed if we are getting a lot more G and F class star hosts."

Helio, I will be waiting :) When we reach 50k confirmed and no earths, I will ask again :) There are many thousands of candidates now pending so if nothing in that batch, wait some more apparently is the paradigm :)
 
Helio, I don't think it makes any sense to do probability math on the list of exoplants we have detected so far. That data is far from a random sample of everthing that is around us. It is highly skewed towards planets that are large and planets that are very close to their stars.

I asked in reply #3 what stars, around which we have looked for planets, would we have been able to detect an Earth size planet in those star's habability zones. I suspect that is a pretty small number of actual opportunities to find a planet that is much like Earth in terms of both its size and its degree of heating by its star.
 
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Helio, I don't think it makes any sense to do probability math on the list of exoplants we have detected so far. That data is far from a random sample of everthing that is around us. It is highly skewed towards planets that are large and planets that are very close to their stars.
Right. I alluded to this but you had already covered this point earlier. Observational bias is almost always a big issue for astronomers. Often they can adjust for these to a fair degree. But exoplanets have too many unknown variables to be very definitive, which may explain why my efforts aren’t that inferior, I suspect. At some point, I hope, these unknowns will be better known and better tables will emerge. 30 meter scopes, dedicated space scopes, JWST, etc. may be adequate in several years for us to see this.

I asked in reply #3 what stars, around which we have looked for planets, would we have been able to detect an Earth size planet in those star's habability zones. I suspect that is a pretty small number of actual opportunities to find a planet that is much like Earth in terms of both its size and its degree of heating by its star.
Well, we seem to have 10 candidates.

That may seem small for ~5200 planets, but consider that we now have star catalogs that have, for one, 2 billion stars.

So, using a 1% number of HZ Earth-sized exos, gives us over 38k candidates. But these must be discounted for those near the central zone of the galaxy.
 
"Well, we seem to have 10 candidates."

Helio, I have seen this HZ figure from you more than once for exoplanets in the confirmed lists. Do these 10 show up in the atmosphere and temperature list? Exoplanet atmospheres (iac.es)

120 are currently listed so atmosphere molecules and temperatures are better determined in this list.
 
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"Well, we seem to have 10 candidates."

Helio, I have seen this HZ figure from you more than once for exoplanets in the confirmed lists. Do these 10 show up in the atmosphere and temperature list? Exoplanet atmospheres (iac.es)

120 are currently listed so atmosphere molecules and temperatures are better determined in this list.
Thanks for that link. I hadn't seen it before. Unfortunately, their CSV download doesn't work properly. It puts their entire row into a single Excel cell. Or am I doing something wrong? Is there a fix for this?

Only Trappist-1 e is in both lists, which is restricted to the Earth-sized exos in my list. Their listings are mainly Jupiter-class and many are hot Jupiters.

[I did not check to see if the others in their HZ, but given their size, should I? ]
 
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Thanks for that link. I hadn't seen it before. Unfortunately, their CSV download doesn't work properly. It puts their entire row into a single Excel cell. Or am I doing something wrong? Is there a fix for this?

Only Trappist-1 e is in both lists, which is restricted to the Earth-sized exos in my list. Their listings are mainly Jupiter-class and many are hot Jupiters.

[I did not check to see if the others in their HZ, but given their size, should I? ]
Helio, I had the same issue importing into MS Excel. Using MS ACCESS with .txt file and delimited, I had no trouble and created my own SQL queries and reports. The TRAPPIST-1 exoplanets, not all are represented and temperatures range below 273 K to 400 K, some may have He present, others featureless spectrums found. Most with atmosphere molecules measured are hot jupiters on the list along with ultra-hot exoplanets.
 
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Here is a report on HZ exoplanets, numerous listed in Table 1 and stellar properties in Table 2.

A Catalog of Habitable Zone Exoplanets, https://arxiv.org/abs/2210.02484

"The search for habitable planets has revealed many planets that can vary greatly from an Earth analog environment. These include highly eccentric orbits, giant planets, different bulk densities, relatively active stars, and evolved stars. This work catalogs all planets found to reside in the HZ and provides HZ boundaries, orbit characterization, and the potential for spectroscopic follow-up observations. Demographics of the HZ planets are compared with a full catalog of exoplanets. Extreme planets within the HZ are highlighted, and how their unique properties may affect their potential habitability. Kepler-296 f is the most eccentric <2 R⊕ planet that spends 100% of its orbit in the HZ. HD 106270 b and HD 38529 c are the most massive planets (<13 MJ) that orbit within the HZ, and are ideal targets for determining the properties of potential hosts of HZ exomoons. These planets, along with the others highlighted, will serve as special edge-cases to the Earth-based scenario and observations of these targets will help test the resilience of habitability outside the standard model. The most promising observational targets are HD 102365 b and 55 Cnc f, and the best candidates that are <2 R⊕ are GJ 667 C c, Wolf 1061 c, Teegarden's Star b, and Proxima Cen b."

ref paper, 33-pages, https://arxiv.org/pdf/2210.02484.pdf, draft 07-Oct-2022.

From the PDF report, “1. INTRODUCTION Exoplanet discoveries over the past several decades have revealed a vast diversity of planetary architectures (Ford 2014; Winn & Fabrycky 2015), and shown that terrestrial planets are far more common than their giant planet counterparts (Borucki 2016). In these ongoing exoplanet searches, discovering those planets that may harbor life has been a primary objective for the astrobiology community (Fujii et al. 2018; Schwieterman et al. 2018; Glaser et al. 2020; Lisse et al. 2020). A potential pathway toward the identification of such worlds is to constrain the stellar and planetary parameter space that may allow for the presence of surface liquid water. Such is the premise of the habitable zone (HZ), defined as the region around a star where water can exist in a liquid state on the surface of a planet with sufficient atmospheric pressure (Kasting et al. 1993; Kopparapu et al. 2013, 2014; Kane et al. 2016). The HZ broadly consists of two main regions; the conservative habitable zone (CHZ) and the optimistic habitable zone (OHZ), shown in Figure 1 as the light green and dark green regions, respectively. The CHZ inner boundary is the runaway greenhouse limit, during which water loss can occur through photodissociation of water molecules in the upper atmosphere. The CHZ outer boundary is the maximum greenhouse, where the planetary temperature conditions allow condensation of substantial atmospheric CO2 on the surface (Kopparapu et al. 2013). The OHZ inner boundary, the Recent Venus limit, is based on the empirical observation that the surface of Venus has been dry for at least a billion years, but may have had conditions suitable for surface liquid water prior (Kane et al. 2014; Way et al. 2016). The outer edge of the OHZ, the Early Mars limit, is based on evidence that Mars appears to have harbored surface liquid water ~ 3.8 Gya (Kopparapu et al. 2013).”

My note. “Table 1. Habitable Zone Planet Properties.”, and “Table 2. Stellar Properties.”, lists many dozens in the tables.

"5. CONCLUSIONS In this paper, we present a complete catalog (at time of writing) of planets that orbit within or through the HZ, including the HZ boundaries and the percentage of the orbital period that each planet spends in their star's HZ. Observational metrics for each planet, such as TSM values and RV amplitude, are included to facilitate selection for future follow-up observations."

Some may be interested in reading this report.
 
Thanks, Rod. That is quite a list.

The listing uses the formulation from Kopparapu, et. al., from 2014. But their work only gave values for three mass sizes, and nothing over 5 Earth masses. So, how did they extrapolate to over 1500 masses? Perhaps I just need to read it more carefully.

Also, they present two HZs: A conservative HZ and an optimistic HZ. The latter being larger to include a recent Venus orbit and a Mars orbit since it is assumed they once had liquid water. This certainly increases the listing, though they show these differences in the table.

Their program also requires both exo mass and radius, unlike some other methods. Only 22% of the exos have both a known mass and radius. But they are using a method to calculate for both values when only one is known, adding ~ 500 exos to be calculated to produce their list. [I might add this feature as well.]
 
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Helio, yes on the conservative HZ and optimistic HZ. I studied the 33-page report and noted from it.

My note. “Table 1. Habitable Zone Planet Properties.”, and “Table 2. Stellar Properties.”, lists many
dozens in the tables.

"5. CONCLUSIONS In this paper, we present a complete catalog (at time of writing) of planets that orbit
within or through the HZ, including the HZ boundaries and the percentage of the orbital period that
each planet spends in their star's HZ. Observational metrics for each planet, such as TSM values and RV
amplitude, are included to facilitate selection for future follow-up observations."

My note. From the 33-page PDF report, Table 1 lists many exoplanets in their potential HZ with some
properties. The smallest earth radii listed is TRAPPIST-1 d at 0.77 earth radii size. The largest earth radii
size exoplanet listed in Table 1. is Kepler-315 c at 14.53 earth radii. Jupiter in our solar system is 11.2
earth radii size. This is a nice list for the astrobiology department to chew on, as well as others like me. :)
 
I was interested in the Chen paper (2017) that they used to determine mass and radii when only one value was known. Given we have so many more exoplanets, I did my own plots, ignoring Jovian worlds.

Notice how nice the fit is in the Terran Worlds plot, but not in the Neptunian Worlds fit. [Orange is the paper's exp. value of 0.59. The green line is mine and at 0.45 exp.] The second shows that this variation might be an issue for this 2022 report and listing.

View: https://imgur.com/eDT9Pjv



View: https://imgur.com/bnJ2Cth
 
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Helio, the 33-page report describes how they developed the list using CHZ and OHZ. Large jovian type exoplanets are included because astrobiology is looking habitable exomoons too. Because this abstract and paper is now published at the arxiv.org site, I will use this as the SOR or System of Record for current exoplanets defined within their CHZ or OHZ zones. Nice job on the Excel charts.
 
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When I have time, I will tweak their list, using the improved exponent, and add the other three methods for comparison. I feel they are a big too optimistic, but I hope they are right.
 
FYI. Here is the NASA ADS Abstract on the October 2022 catalog defining the HZ. https://ui.adsabs.harvard.edu/abs/2022arXiv221002484H/abstract

I went back and reviewed Wolf 1061 c and GJ 667 C c as likely good targets for a habitable exoplanet referenced in the abstract.

The October 2022 33-page PDF, "Table 1. Habitable Zone Planet Properties." list GJ 667 C c as 3.81 earth mass with 1.73 earth radii. Mean density ~ 4 g cm^-3. The PDF report list Wolf 1061 c with 3.41 earth mass and 1.6 earth radii. Mean density ~ 4.5760E+00 g cm^-3. P = 17.87 days, e=0.11.

An October 2013 survey reported on the empirical HZ or EHZ. This NASA ADS Abstract is earlier and reports on surveys conducted using closer stars to the Sun. GJ 667 C c was part of the report. 'The Solar Neighborhood XXIX: The Habitable Real Estate of Our Nearest Stellar Neighbors', https://ui.adsabs.harvard.edu/abs/2013AJ....146...99C/abstract, October 2013. Abstract: "We use the sample of known stars and brown dwarfs within 5 pc of the Sun, supplemented with AFGK stars within 10 pc, to determine which stellar spectral types provide the most habitable real estate—defined as locations where liquid water could be present on Earth-like planets. Stellar temperatures and radii are determined by fitting model spectra to spatially resolved broadband photometric energy distributions for stars in the sample. Using these values, the locations of the habitable zones are calculated using an empirical formula for planetary surface temperature and assuming the condition of liquid water, called here the empirical habitable zone (EHZ). "

Plenty of published reports now discussing the EHZ, CHZ, and OHZ. I will use those reports as the SOR when discussing exoplanets in the HZ. We also have space.com earlier report on the top 10 list for possible earth-like exoplanets. The 10 most Earth-like exoplanets, https://www.space.com/30172-six-most-earth-like-alien-planets.html

Gliese 667Cc is listed here too. Interesting, none of these show up as atmospheres determined yet and what molecules in their atmospheres could be present. Exoplanet atmospheres (iac.es)
 
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I think the Rare Earth hypothesis should be promoted from hypothesis to default assumption, barring emergence of surprising discoveries.

There are countless ways for rocky planets never to develop life, or no lifeforms above unicellular life, or not to remain stably habitable long enough for multicellular life to evolve into tool-using intelligent species that eventually build a technological civilization. And comparatively a vanishingly small number of narrow paths for these to happen.

I am reminded of what Tolstoy said about happy families, that they are all alike, but every unhappy family is unhappy in its own way.

If the universe is an experiment for a science fair conducted by a teenage god, then perhaps every galaxy is a petri dish designed to bring forth at most one (1) spacefaring civilization ... competing to figure out the answer to the ultimate question :)
 
I don't agree that we are anywhere near the point of justifiably assuming that life does not develop or evolve in a lot of different types of planets' environments. Let's see what we can find on Mars and some of the icy moons of Jupiter and Saturn before jumping to that conclusion.

And, when it comes to assumptions about technological species ever developing from those beginnings, we need to remember that our own technological society has been capable of emitting detectable signals for only about 100 years, which is only about 0.00000002 of the time that our planet has existed. If we are destined to crash our society in the near future, rather than continue to develop it into levels that would amaze us today, then even if every star in our local galactic neighborhood has one planet that sooner or later develops a short-lived technological civilization similar to ours today, the probability of overlapping in time is tiny.

So, I don't agree that our sample size can justify any assumption that we are on the only planet that could develop life or a technological life form. On the other hand, I also do not think that other life forms on other planets need to be very similar to us, nor do I think their technologies need to be very similar to ours. So, if you want to assume that there is no other planet with a technological life form exactly like Earth and us, then I agree that is likely, but not really relevant to the more general question.
 
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Can we perhaps agree that an astronomically large number of individually extremely improbable favorable events (and conversely, absence of fatal events) had to happen to get us where we are now? There is nothing at all inevitable in our existence. Otherwise it would not have taken 99.99999998% of the time that our planet has existed for "us" (or someones like us) to come into being. Sample size of only one, yes, but this sample contains a lot of meaningful data when held up against what we already know about the rest of the universe.
 
Murgatroyd, there are several problems with your interpretation of the one known data point of life evolving to technological capabilities.

First, it is not clear how many of the events you are considering are actually necessary for life to originate and evolve to that state. in fact, it seems that life survived and evolved towards our current condition despite some extreme events that killed most of the species on our planet several times over its history.

Second, you are not considering any other paths for life to evolve to a technologically capable state. It is certainly not clear that there are no other ways that life could progress to technological capability.

There are mathematical techniques to probabilistically combine multiple paths to an end point, but we do not have the data to produce reliable input probabilities to make useful calculations at this point in our understanding of the universe, or even the stars near us in our galaxy.

And, finally, even with a probability of 2 x 10 ^-8 per year, that gives an expectation value of 1 for any other planet that is just like Eartha t 4.6 billion years old. So, how many other planets are in our galaxy that are close enough to Earth's conditions? Even one planet in a billion of the many planets orbiting the hundreds of billions of other stars estimated to be in our galaxy being like Earth would still produce an expectation value of something like 2 thousand possibilities for a technological species similar to us occurring somewhere in the galaxy. Furthermore, many stars in the galaxy are older than 4.6 billion years, and the galaxy will probably create many more stars and planets before it somehow "ends". So, it is very hard to say that no other planets anywhere in our galaxy will probably have any technological species ever evolve during the life of the Milky Way.

Yes, that above math was sloppy. To do something rational, we need to look at the natures of the stars and their planets that are in our local vicinity, and ask whether they might give rise to life, and if it is likely to have been in existence long enough to have evolved technological capabilities. We will never know if there is some other planet just like Earth with a species just like us if it is on the other side of our own galaxy, much less if it is in a galaxy a billion light years from here.
 

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