A hidden 'super-Earth' exoplanet is dipping in and out of its habitable zone

[Admin]

With 10 times the mass of our planet, and spending only part of its orbit in the habitable zone, Kepler-725c is very different to Earth.

A hidden 'super-Earth' exoplanet is dipping in and out of its habitable zone : Read more

 

[Unclear Engineer]

A planet that large repeatedly going in and out of the calculated "habitable zone" with the frequency of its 207.5 earth-day "year" could very likely have enough water on its surface to smooth out the transient temperatures in an ocean and even on parts of any land areas. So, I see no reason to assume it would not be habitable, even by large, intelligent life forms.

But, with a thick atmosphere, life forms on such planets might not ever see a star from its surface. The evolution of scientific intelligence for a lifeform there might be entirely different from what we evolved from here on Earth.

 

[rod]

Space.com reported - "The newfound planet, called Kepler-725c, revealed itself only through "transit timing variations."..."Does this mean that Kepler-725c is only habitable for part of its 207.5-Earth-day year? What would happen to any life that might exist on the planet during the periods that it is outside of the habitable zone? Again, these are theoretical problems that scientists have been wrestling with, but now the existence of Kepler-725c suddenly makes them very real problems."

My note, using the exoplanet.eu (https://exoplanet.eu/home/) site we have 7490 confirmed now. TTV shows up in 35. Min mass is 1.0129 earth mass, max in this group is 22 Jupiter masses. Extrapolating abiogenesis from Charles Darwin warm little pond in his 1871 letter to exoplanets like Kepler-725 c or K2-18 b, needs much more work IMO. At somepoint, abiogenesis thoughts in astrobiology need to be confirmed science.]

 

[Helio]

Isn’t this more of a super Venus planet? I suspect it never reaches 1 AU at apsis. Also, the greater the ecc., the greater the luminosity on it.

 

[rod]

Helio et al. My spreadsheet using Keplerian orbit with e=0.44. Mass exoplanet = 10 earth masses. Mass of host star = 0.91 solar mass. e = 0.44. semi-major axis = 0.674 AU. Closest approach to star = 0.3774 AU, farthest orbit out to 0.97 AU. If this was in our solar system, orbit moves inside of Mercury and swings out to near where Earth is. If we lived on Earth with such an orbit, folks would have plenty to talk about when it comes to the weather and climate change I think

We have properties for Kepler-725 c listed here, https://exoplanet.eu/catalog/kepler_725_c--11052/

 

[Unclear Engineer]

The article says that the stellar heating of Kepler-725c is 1.4 times that on Earth. I think Venus is about 1.9, and rather constant. The 1.4 for Kepler-725c is an average over an orbit with a high eccentricity. The hottest parts of the orbit are the shortest, when the planet is at its closest to its star and moving at its fastest.

So, maybe somewhere between a hot-Earth and a Venus.

But, with its much larger size and potential for much different atmospheres, I would not write it off as an uninhabitable "super Venus" without some more data.

 

[Helio]

Helio et al. My spreadsheet using Keplerian orbit with e=0.44. Mass exoplanet = 10 earth masses. Mass of host star = 0.91 solar mass. e = 0.44. semi-major axis = 0.674 AU. Closest approach to star = 0.3774 AU, farthest orbit out to 0.97 AU. If this was in our solar system, orbit moves inside of Mercury and swings out to near where Earth is.

Thanks. That looks right.

Only the "optimistic HZ" equations allow planets 0.97 AU (solar equivalent apoapsis for Kepler-725).

If we lived on Earth with such an orbit, folks would have plenty to talk about when it comes to the weather and climate change I think

Indeed. But their visitor time would be short.

This is another planetary body where the porridge is a little too hot, likely more than a little, perhaps it's an "exotoast", but not burnt toast.

 

[Unclear Engineer]

I agree that humans would probably not find Kepler-725c "habitable". For one thing, its gravity is likely to be high, and its atmospheric pressure could be crushing (if it has an atmosphere).

But, it is hard to rule our some sort of life on a planet we know so little about - and we also know very little about how life begins.

Even places in our solar system, such as Titan, can be speculated to have some sort of life that is far different from the water-based version on Earth. See https://phys.org/news/2025-06-titan-liveliest-solar.html . Of course, that is the "cold version". It might be even easier to speculate about a high pressure + high temperature water version on a super-Earth.

 

[rod]

Interesting views on life out there in this discussion. 7494 are confirmed now at https://exoplanet.eu/home/

I did a review for all exoplanets found at 3 au or closer to their host stars using the site. 4426 exoplanets can be seen, Kepler-725 c, K2-18 b, and Trappist-1 system are found in the selection too. So far, none are confirmed with any form of life on them or if they are truly *earth like* or that abiogenesis could ever take place on any of them creating who knows what new life form. The mean distance is 0.373 au, so most are packed in close and closest is 0.0002468 au from its star. The mean mass is 5.048 Jupiter masses so big here. Min mass is 0.02 earth mass. IMO, astrobiology as a *science*, has a long way to go.

 

[Unclear Engineer]

There is still a lot of observation capability bias in our list of exoplanets. Heavy planets close to small stars are easiest to detect.

If we limit the list to stars similar to the Sun, how many planets between 0.75 and 1.5 AU have we detected around those? (Roughly Venus to Mars distances.)

 

[Classical Motion]

It appears that our system structure is the odd ball. The unusual.

It will be interesting to see what these new large scopes will find. Or perhaps help clear up.

 

[rod]

There is still a lot of observation capability bias in our list of exoplanets. Heavy planets close to small stars are easiest to detect.

If we limit the list to stars similar to the Sun, how many planets between 0.75 and 1.5 AU have we detected around those? (Roughly Venus to Mars distances.)

Using the .eu site, I found 331 exoplanets. All are found in the other group from 3 au or closer. The min mass is 1.43 earth masses, the max is 69 Jupiter masses. Looking at exoplanets from 3 au or closer indicates that most orbit around stars where there is much more mass than found in our solar system from 3 au or closer to the Sun. Our solar system is clearly not the norm configuration.

 

[Unclear Engineer]

I don't think we can say "Our solar system is clearly not the norm configuration." We have methods of detection that favor finding large planets orbiting close to small stars.

That is why in my previous post that I asked you to sort first for Sun-like stars, then sort for planets that are between 0.75 AU and 1.5 AU from those stars. We would still be most likely to find the largest planets that fit those criteria, not all planets that fit. But, it would give us a better idea of how many might be available for more detailed study in what we think is the "habitable zone" of stars like ours.

 

[Helio]

Here is a current listing of all exoplanets by stellar type that are in the extended HZ (luminosity bump -- "lumidge"). This matches the Equilibrium Temperature method, which allows exos roughly midway between Venus and Earth, and an outer limit of about 15% farther than Mars.

These are equivalent distances, not actual, which uses luminosity to determine the HZ for exoplanets for each respective star.

I had not considered doing this summary, but have presented the long lists. I kinda like this approach, so I will incorporate in the program. So, if you would like to see a different range (eq. dist.), let me know.

 

[Unclear Engineer]

So, for the 99 planets orbiting the type G stars within their habitable zones, what is the planets' size distribution?

 

[George²]

It will be interesting to see what these new large scopes

What do you mean with this? ↑
Only WEBB telescope is new, big and on the work. No one of the long-awaited large ground-based telescopes are not ready for operation, and several of them, while not canceled, are not even under construction. Only the ELT is under construction and could be operational within a few years.

 

[Helio]

So, for the 99 planets orbiting the type G stars within their habitable zones, what is the planets' size distribution?

We still lack a lot of useful data for much of a distribution. I can estimate radii and mass if one of these is known, but I'll need to tweak a prior tweak (because it was working).

So here's the entire list for G Type exoplanets in their HZ, but only exos with a radii < 25x (if known) that of Earth:

 

[Classical Motion]

I had to wait thru delays and overruns for WEBB. I had to double wait for Hubble.

I will have to wait for new larger scopes too. That’s all I meant.

 

[Helio]

Only the ELT is under construction and could be operational within a few years.

The GMT should see first light in 2029.

 

[Helio]

Here's a summary that shows the more Earth-like exoplanets by known size...

 

[rod]

Helio et al. There needs to be an authoritative system of record that shows all *earth-like* exoplanets claimed now IMO. I read various reports and see different definitions in use. IMO, none are likely *earth-like* when I look around on our planet.

Team finds younger exoplanets are better candidates when looking for other Earths, https://phys.org/news/2022-05-team-younger-exoplanets-candidates-earths.html, exoplanet is K2-36 b

The 10 most Earth-like exoplanets, https://www.space.com/30172-six-most-earth-like-alien-planets.html

The discovery of a new Earth-like planet could shed further light on what makes a planet habitable, https://phys.org/news/2024-07-discovery-earth-planet-habitable.html, "In an exciting breakthrough for astronomy and the search for extraterrestrial life, a team of international scientists has announced the discovery of Gliese 12 b, a temperate, Earth-sized exoplanet just 40 light-years away—a relatively neighborly 378 trillion kilometers from earth."

Even the use of temperate varies as Helio previously mentioned when we look at atmospheres documented now.

Exoplanet atmospheres

research.iac.es

 

[Helio]

Helio et al. There needs to be an authoritative system of record that shows all *earth-like* exoplanets claimed now IMO. I read various reports and see different definitions in use. IMO, none are likely *earth-like* when I look around on our planet.

Yeah. “Earth-like” is a stretched term to represent the likely orbital environment for an exo about the size of Earth.

The advance in atmospheric data will greatly help enhance this term.

The bigger scopes will likely take us there, but by the time we get, if ever, a telescope large enough to observe a McDonalds on an exo it will be because someone already travelled there in their McRocket.

Even the use of temperate varies as Helio previously mentioned when we look at atmospheres documented now.

Yes. Each HZ is broadened to take in the advantages of exo atmospheres.

There is, however, an atmospheric model that uses atmospheric calculations, tweaking the HZ. Their results are different for many exo candidates. [I’ve presented in the past their results in the exo stat thread.]

 

[Unclear Engineer]

I am still looking at the detection bias issue. Helio's table isn't much help, because so few of the planets around type G stars have diameters listed.

But, from what is there, it still looks like we are much better at detecting large planets than small ones. I would not be confident in extending that conclusion to say that there are many more large planets than small planets in the habitable zones of type G stars.

So, exactly how "unusual" or own planetary system is will not be evident until we have good probability of detecting any planet the size of Mars or larger around these type G stars. So far, we seem to be detecting only things larger than Earth.

 

[rod]

Okay, looks like there is much to study in exoplanets and astrobiology Just defining and measuring specific molecules and types of atmospheres remains largely unknown for the vast majority as the iac site shows for exoplanets. When using a model atmosphere to define HZ, good to show this. The .eu site today has 7498 confirmed. Using the site filters, 228 are small. A number smaller than earth, some smaller than mercury size. I also look at stars with multi planet systems. Kepler-90 system has 8 exoplanets orbiting around a 1.2 solar mass star. TRAPPIST-1 system has 7 around 0.08 solar mass star. HD 219134 shows 7 exoplanets around 0.794 solar mass star. Barnard's star shows 4 around a 0.162 solar mass star.

Efforts to show a true *earth-like* exoplanet remains to be shown IMO. This includes life on it.