Small M-dwarf stars, the most common type of star in the galaxy, are likely to be the primary target for our early investigations of habitable planets. The small size of these stars and the significant transit depth this allows when an Earth-mass planet crosses their surface as seen from Earth mean that atmospheric analysis by ground- and space-based telescopes should be feasible via transmission spectroscopy. Recent studies have shown that the James Webb Space Telescope has the precision to at least partially characterize the atmospheres of Earth-class planets around some M-dwarfs.
Soon-to-be commissioned ground-based extremely large telescopes will likewise play a role as we examine nearby transiting systems. But M-dwarfs make challenging homes for life, if indeed it can exist there. In addition to flare activity, we also have to reckon with the presence of water. Too much of it could suppress weathering in the geochemical carbon cycle, but too little does not allow for the development of a temperate climate. Thus new work on water content in such systems is welcome.
For purposes of reference, Earth’s seawater accounts for 0.023% of the planet’s total mass. According to Tadahiro Kimura, a doctoral student at the University of Tokyo, and Masahiro Ikoma (National Astronomical Observatory of Japan), a number of models suggest that terrestrial planets around M-dwarfs would have either too much water or no water at all. Are habitable planets around such stars, then, a celestial rarity?
In a new paper in Nature Astronomy, the authors argue that there is a mechanism beyond the infall of icy planetesimals that can produce water as a young planet accumulates its atmosphere. It involves interactions between the hydrogen-rich atmosphere, drawn from the protoplanetary disk, and the magma ocean that would be present from impacts during the early days of planet formation. Water is accumulated through the chemical reaction between atmospheric hydrogen and the oxides found in the surface magma – a magma ‘ocean’ – of the young planet. From the paper:
…water can be secondarily produced in a primordial atmosphere of nebular origin through reaction of atmospheric hydrogen with oxidising minerals from the magma ocean, which is formed because of the atmospheric blanketing effect[8], thereby enriching the primordial atmosphere with water. By assuming effective water production, we recently showed that nearly-Earthmass planets can acquire sufficient amounts of water for their atmospheric vapour to survive in harsh UV environments around pre-main-sequence M stars [9]. The results suggest that including this water production process significantly affects the predicted water amount distribution of exoplanets in the habitable zone around M dwarfs.
Image: Probability distribution of seawater mass fractions for planets of Earth-like mass (0.3-3 times Earth mass) located in the habitable zone around M-type stars (0.3 solar masses). Green is the result of calculations following the conventional model and considering only the acquisition of water-bearing rocks. Orange is the result when the model of the present study is used and the effect of water production in the primordial atmosphere is taken into account. The dotted line is the present-day seawater amount on the Earth. Credit: National Astronomical Observatory of Japan.
In this scenario, the amount of water present depends on how the planet forms. The authors have created a planetary population synthesis model that tracks the mass and orbital evolution of planets in formation, including among other things the structure of the protoplanetary disk, potential orbital migration, instabilities in multi-planet systems and the effects of water production in the primordial atmosphere. The model, which refines that presented in an earlier paper by the same researchers, allows the calculation of the amount of water that should be produced through the atmosphere/magma interaction.
The range of water outcomes is wide, but if we narrow it to planets with seawater mass fractions similar to Earth, most of this water is found to come through atmosphere/magma interaction rather than by incoming impacts by comets and other water-bearing objects. And it turns out that a few percent of planets with a radius between 0.7 and 1.3 times that of Earth produce the right amount of water to sustain temperate climates. Let me quote the paper on this – note that in the passage below, HZ-NEMP refers to nearly-Earth-mass planets in the habitable zone:
The HZ-NEMPs of 0.7–1.3 R?… have lost their hydrogen atmospheres completely, ending up with rocky planets covered with oceans. It turns out that those planets are diverse in water content and do include planets with Earth-like water content. Several climate studies argue the amounts of seawater appropriate for temperate climates, considering the effects of seafloor weathering, high-pressure ice, water cycling and heterogeneous surface water distribution… According to those studies, the appropriate seawater amount ranges from ?0.1 to 100 times that of the Earth.
Clearly, target selection for exoplanet habitability would benefit from being able to exclude planets that are unlikely to be habitable, which according to this paper would include habitable zone worlds with radii > 1.3R? that have deep oceans with high-pressure ice, and planets with ocean mass fractions greater than 100 times that of Earth. The authors believe that we should be able to identify such worlds if planetary mass and radius can be measured within ? 20% and 5% accuracy respectively. Having eliminated these, we turn to planets in the 0.7 to 1.3R? range. The authors refer to them as ‘water-poor,’ in comparison to their larger cousins, but they still can have seawater fractions similar to that of Earth:
…the HZ-NEMPs with appropriate amounts of seawater for habitability are estimated to account for ?5% of the “water-poor rocky planets” orbiting 0.3M M dwarfs. This frequency becomes higher for larger stellar mass, and around 0.5M stars, for example, more than 10% of the water-poor rocky planets are expected to have the appropriate amounts of seawater.
So 5% to 10% of the M-dwarf exoplanets in the appropriate size range (< 1.3R?) have the fraction of water needed for habitability. The paper makes this prediction: Survey missions like TESS and the upcoming PLATO should detect approximately 100 Earth-sized planets in the habitable zone around M-dwarfs. 5 to 10 of these, according to this model, are likely to be planets with oceans and temperate climates, a sharp contrast to earlier studies indicating such worlds should not exist.
The paper is Kimura & Ikoma, “Predicted diversity in water content of terrestrial exoplanets orbiting M dwarfs,” Nature Astronomy 29 September 2022 (abstract / preprint). The authors’ earlier paper on water enrichment is Kimura & Ikoma, “Formation of aqua planets with water of nebular origin: effects of water enrichment on the structure and mass of captured atmospheres of terrestrial planets,” Monthly Notices of the Royal Astronomical Society 496, 3755 (2020) (abstract).
Huh!
Of course, only long-term statistical analysis will prove or give lie to this theory, but exciting times that we have the tools to do such analysis now!
In the last couple of years I have seen similar Solar System based arguments for water surface levels over time for terrestrial planets.
Not based on solar cycles, but the internal heating of the planets.
To summarize the argument simply: Venus, Earth and Mars have internal sources of heat from formation – and even some from decaying heavyelement isotopes. But as time goes by the overall core heat reduces even though there are molten regions in the core. And the overall effect is that internal hydration in regions near the surface INCREASES. If we ran time backward high speed like a VCR tape, the resulting movie over geologic time would have ocean water gushing out of the interior as though it were steaming out of hot charcoals. But what we are experiencing with time’s arrow is the opposite. The crust has been able to absorb more and more water with time.
Clearly, with the cases of Mars and Venus, the atmospheric loss and water vapor into space has been considerable in comparison to the Earth, but it tends to distract attention from the terrestrial planet cooling phenomenon as well. For one thing, if a billion or so years ago the Earth could have been frozen over entirely, it suggests that there was more surface water during that epoch. But it did not necessarily evaporate into space.
Consequently, looking at M dwarf systems with planets, and especially thjose like Trappist-1, one could suspect that there would be epochs for each of the planets when the circumstances are most conducive to life: a combination of distance from the stellar prime, its original volatile budget, magnetosphere and rotation, loss of volatiles into space – and re-absorption into the crust.
Pollyanna analysis, maybe, but I subscribe.
Thank you for covering the results of this intriguing study.
A couple of questions:
How likely is it that habitable planets, including the ones described in this study, will hold onto their oceans without life over long timeframes (e.g. 1+ billion years)? In other words, is the presence of life a necessary condition for the long term presence of surface liquid water on a habitable zone planet? The answer to this question has implications regarding the ethics of future interstellar colonization—if there are many perfectly habitable planets with liquid water but no native life, then this might help alleviate valid concerns regarding how our colonization might affect the native life forms…
Did Luna form in a matter of hours? What does this say about the formation of other moons? And planets?
https://www.nasa.gov/feature/ames/lunar-origins-simulations
Most moons and planets are created by accretion, like the moons of Jupiter, a slow process of rings slowly coming together. so the earth moon model has no impact on those models as it’s a result of a very specific instance of a side-on collision where Theia was already an object, and part of Theia was flung as an almost complete moon into orbit around the earth.
I am biased against the idea that an Earth sized world will be a water world without any land. The size of the star also matters because the protoplanetary cloud of an M dwarf has less gas and dust than a G star, so therefore it might have less water. Direct spectra and imaging will certainly be the ideal since if there was a water world, we should be able to tell if it has more water than land by looking at the light from the phase angle from ninety degrees to zero degrees.
I have some doubts about this paper. The charts show that the H2 atmosphere could create as much water by planet mass as Earth has. Earth has 2.4E-4 H2)/planet Mass.
Nasa has a PPT presentation concerning O2 extraction from the lunar regolith. A Quick Lunar Material-Mineral Primer for Oxygen Extraction that suggests the best case extraction of O2 from the lunar regolith is much less than 1%, around 0.04% O2, { 4E-4).
Therefore to get to terrestrial H2) content, fully 1/2 the planet’s mass must have the O2 removed to form water. This implies a very large H2 atmosphere comparable to the planet’s rocky mass, and that all the rock can be so processed as if in a furnace, and then make its way to the surface (and also stay in the mantle) to form oceans.
In addition, what happens to the processed rock? Does the structure sufficiently change to make it react differently to geologic processes, especially weathering to provide the needed feedback to stabilize temperature?
If the M_Dwarfs have a propensity to be either too wet or too dry (although their models suggest only wetter), does this extra water enrichment even help, or does it just make wet planets even wetter? Figure 2 is hard to interpret in this regard, but it does seem to show an increase in wetter worlds with just a small probability of around 1% having Earth-like water composition.
Lastly, IIRC, M_Dwarfs have a very intense luminosity increase for the young star, compared to warmer stars, F,G,K. This would imply that conditions my not remain stable enough for the planet to remain habitable. If the authors’ model is correct, wouldn’t most of the water be created during the early planetesimal aggregation stage, which would result in losses during the high luminosity stage of the primary before “settling down” for its long period of mainstream life?
Hard data from telescopes to determine the surface of M_Dwarf rocky worlds in the HZ will help resolve the reality from the speculative models.
@Alex:
How likely is it that habitable planets, including the ones described in this study, will hold onto their oceans without life over long timeframes (e.g. 1+ billion years)? In other words, is the presence of life a necessary condition for the long term presence of surface liquid water on a habitable zone planet? Is it likely or unlikely that there could be habitable ocean planets with surface water existing for billions of years that are nevertheless sterile? The answer to this question has implications regarding the ethics of future interstellar colonization—if there are many perfectly habitable planets with liquid water but no native life, then this might help alleviate valid concerns regarding how our colonization might affect the native life forms…
Interesting question, but I am not the one to ask. Life does have a Gaian effect, but other influences are likely greater. Cosmic impacts, continental arrangements, orbital eccentricity, the star’s stability, and close supernovas, will all affect whether the planet can stably retain its oceans.
While a theoretical simulation, Toby Tyrrell’s idea about chance on long-term planetary habitability gives one a sense of possible instability due to events that cause positive forcings away from habitability.
Tyrrell, Toby. “Chance Played a Role in Determining Whether Earth Stayed Habitable.” Communications Earth & Environment, vol. 1, no. 1, (2020), doi:10.1038/s43247-020-00057-8.
My CD post on his paper is here: Are Planets with Continuous Surface Habitability Rare?
Spoiler alert: the answer is the probability of long-term habitability is low.
Without life, particularly oxygenic photosynthesis, the atmosphere would not have a breathable oxygen composition. How useful would that be for colonization? Exploitation, yes, but colonization? It would be like colonizing Mars or the Moon, people would be restricted to living in “containers” with breathable air. The only advantage might be that the containers would not have a pressure differential between the inside and outside, making structures easier to manufacture.
If the atmosphere was N2/CO2, with H2O in the air and oceans, then it would be a ready source of CHON as major elements of life.
Red dwarfs are born much more luminous than they are later in their lifes – that’s the main problem for water. TRAPPIST-1 had hundred times it’s current luminosity at the start, 10x at the age of 10 million years, and still around 2x at 100 MYr. Almost the same for Proxima.
We have an example in our own Solar System. Jupiter had considerable luminosity for several million years, and Io is completely dessicated, Europa has comparatively thin layer of water, and outer satellites retained all of their water but no atmospheres.
If TRAPPIST-1 planets formed quickly, all of them were hotter than Mercury initially – they’re likely seven Ios, and the refined densities converge to the same bone-dry compositions. There still could be atmospheres outgassed by tidally-induced volcanic activity, or maybe outer planets indeed managed to capture some volatiles while the disk was still optically thick, but most planets in HZs of red dwarfs are likely completely barren.
From what it sounds like, the paper takes that into account – the Earth-water-comparable planets are those that survive the Pre-Main Sequence roasting.
I thought that water worlds had more water because they had to be super Earth’s with a larger gravity than Earth so they got more water from the protoplanetary disk cloud. This makes sense, but in theory it does not limit a water world to super Earths considering water might come from meteors, comets, etc.
Here is a basic, if not stupid, question. How does water form in interstellar or interplanetary space? Does H2 and O2 somehow chemically bond on dust particles? Does H2 and O2 directly bond in space? Where did the water originate for icy comets? Just wondering. Thanks.
Water isn’t the only compound found in the interstellar medium. There are many others, including all sorts of organic compounds, like methane, alcohol, formaldehyde, ammonia, etc. I think the list is close to a 100 by now.
I do know the surfaces of dust particles plays a role in some of them.
Good question!
https://en.wikipedia.org/wiki/Astrochemistry
We need more subdivisions of what we term “habitable”‘
– for aquatic organisms – micro to macro
– for complex land life and aquatic organisms
– for intelligent (sapient) complex life able to leave its planet and return
I suggest that unless there is strong evidence against it, the lower radius limit should be 0.6, as Mars still has ice and atmosphere at 0.5. Some planets will also have more massive cores. The upper limit to truly Earthlike planets seems to be about 1.2 radii, as above this number, the density rapidly decreases, suggesting too many volatiles to have a rocky surface.
These limits are not directly linked to the evolution of land/water proportion as the article covers, but a slightly narrower focus of attention would give better choices for future investigations.
John Strickland
Author of “Developing Space” and “Settling Space”
Earth’s seeming uniqueness could be attributable to an earlier massive collision: moon, oceans, continents, magnetosphere, atmosphere all shaped dramatically.
Life found on other plants ÷ Other planets found. So far zero. At what point do we conclude the universe isn’t fine-tuned for life?
Outside of the solar system, we haven’t yet looked for life, just planets in the HZ that could be potentially habitable. Whether they are inhabited or not, we just don’t know. But perhaps within the decade, we will start to get data, and that will indicate:
1. Whether worlds in the HZ are truly habitable with surface liquid water, and
2. Whether life on these habitable worlds is very rare, ubiquitous, or something in between.
Even if the universe is “fine-tuned for life”, that only implies the universe is sufficiently ready for life, not that life has readily appeared. In extremis one could argue that we are the life that emerged and that the universe is ready for us to green it.
I think we have a long way to go Afterthought. We know almost nothing about life within our own solar system outside of Earth. And we really know nothing at all about life in other solar systems.
To alleviate some of the weekly load of refreshing the site:
Of late, when we discuss the problem of uniqueness or pervasiveness of life in a host of environmental conditions, I see some background issues:
1. Does life develop in situ, if at all, and completely independent from elsewhere?
2. If there is an “observer” issue in quantum mechanics that influences physical process outcomes, what if earth is the only example of life over vast stretches of space and time? And if this idea really applies what threshold does it require to be of any consequence: biological existence, self-awareness, observation of the phenomenon?
3.Could we be perhaps at the point where there are two schools like there were about cosmology; i.e., the Fred Hoyle steady state proponents and the big bang group with George Gamow?
4.If life should develop on a planet, does it have to adhere to all the regulations we associate with life on Earth such as its early formation stages?
Examples:
a.)having a collision providing an enormous moon and then waiting around for 2 or 3 billion years to develop mitochondrial, multi-cellular organisms ?
b.) For intelligent life, discovering papyrus and other communications media…
These issue do relate to water on M dwarf planets in one way or another. Whether HZs are truly habitable or if they exist could life be there or sustain itself.
And I would maintain that initial conditions do matter. But in terms of predicting habitability on particular planets orbiting at particular distances from particular stars, we are investigating a multi-dimensional envelope in space and time with differing initial and boundary conditions.
For stars, taken by themselves, once nuclear fusion processes were identified in their interiors, detailed models for their evolution with time based on initial mass before ignition were worked out in the 1950s. A Dover paperback early guide (1958) is Martin Schwartzchild’s book on “The Structure and Evolution of the Stars”. Astrophysics textbooks since have been variations or elaborations. But still, the outcome of stars with various masses differ so much that various diagrams are needed to describe them. They result in elaborate overlays of curves, to say the least. Usually on log scales.
And then we add planets forming at varying distances, masses and initial stocks of volatiles. In cases where there are several terrestrial planets in a system experiencing stages of stellar evolution, what then?
Narrowing to F, G, K, M main sequence dwarfs, the HZ space over time will vary in location with the flux energy provided by the star. For Earth at 1 AU it’s about 400 K effective stellar temperature from a body with 5800 K surface temperature. This simplifies the search space for a start. Other bodies’ “mileage” will surely vary with stellar and geological history, spectral peaks blackbody or otherwise, reflectivity, rotation rate, greenhouse, tidal perturbation…
And that last one comes up often. Now that we discovered the turbulent history of the Earth-Moon system, we are tempted to apply it to every HZ exoplanet. And we suspect that the captured situation is highly unlikely. I agree. But with a system like the Galilean Moons or Trappist 1, there should be a lot of resonances to make up for that.
Clearly the 4 jovian satellites offer very different surface histories and internal heating. It is not clear to me that a system like Trappist 1 would have volatiles completely removed in formation than it would for the Galilean four satellites. And if the surface volatiles of the four moons or the other solar terrestrials seem to have been lost, there is more and more evidence that crusts of the planets and moons still maintain large water reservoirs. Mercury and Io likely exceptions.
But it is not clear to me that M dwarf exoplanets would resemble them more than other possibilities.
The infall process from protostar to star varies across the HZ diagram. It occurs faster with more massive stars. And consequently, one would infer that the circumstellar processes for mass variation would be drawn out as well. Ice infall from the outer regions would take longer, and differentiation of the volatiles in the crust might take longer too.
But this can be resolved as soon as we get some conclusive Trappist data.
Now, assuming there are volatiles enough to allow for organic life, what in physical science or cosmos structure would initiate a widespread life propagation.? Choosing my words here so as not to invoke successive miraculous events. Just that there could be basics enough spread galaxy wide which would increase the probable success; akin, if you will, to the parable of the farmer with seeds being sown.
I do not know for certain what that would be. There is only hint of it from our New Horizons explorations so far into the Kuiper Belt and perhas what could lie in the Oort Cloud. If its hydrocarbons are organic or even provided with viruses, I bet our solar system is not the only one with such a cloud. And if ours is on order of 100,000 AU diameter, then it is likely that stars in relative motion interact enough to make this source material shared widely and disturbed from time to time.
Likely Sholz’s Star had an Oort Cloud too which interacted with ours in its 70 k years ago near passage.
Reviewing the above, I think I have touched on much of those four questions – save the observer’s effect. To quote:
“Schrödinger’s cat is a thought experiment illustrating a paradox of quantum superposition. In the thought experiment, a hypothetical cat may be considered simultaneously both alive and dead as a result of its fate being linked to a random subatomic event that may or may not occur.” And then an observer intervenes. Right?
So if this is really valid, our presence in the universe – or absence – has impact we are not fully aware of – but it is.
The contrary argument is given by John Bell, the pioneer of quantum entanglement:
“Was the wave function waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer for some highly qualified measurer—with a PhD?”
I don’t really know how he answered his question, and especially considering his area of expertise, have to wonder.
Speculation can be fun, but we really need data to answer these questions – data we won’t have in our lifetimes.
If we found the biology of life in different star systems throughout the galaxy was near identical, then that would be a strong argument for a common origin. Conversely, different biologies would indicate unique abiogeneses in each system.
If we can get that data, we will also be able to get the data on planetary evolution which in turn will answer the question of planetary conditions needed for abiogenesis. For life with a common origin, planetary evolution may not be a relevant question. How the common origin was dispersed would be an interesting question, and it might indicate whether the dispersal was natural or artificial.
If we look at the history of speculation about life’s origins and presence in other bodies, the speculations offered nothing of value, even if interpreted as hypotheses. Divine creation in days, no evolution, intelligent life on the other planets of our system – all wrong. Extending modern versions into the galaxy and beyond just seems like more of the same.
IDK what instrumentation and techniques might be available in the future, but as of now, it seems to me we need probes to reach those planets and to take samples for analysis. Current physics suggests that will be limited by c and hence take a long time, perhaps longer than humans will be around. That rules out early answers unless the samples are delivered to us gratis – a celestial object from another system enters our system and we find life in it, or an artificial object or signal arrives in classic SciFi terms and we can get the answers directly. In the shorter term, all we can do is explore the worlds in our system and devise ever better experiments to determine the most parsimonious way abiogenesis can occur. Personally, I can live without knowing the answers, and get my speculative dopamine shots from reading good SciFi.
YMMV, as the history of religions and the stories they tell indicates.
A.T.
Thanks for your feedback.
Amid the discussion, I neglected to mention the transforming effect of life introduced to a planet. Whether observed here or theoretically “there”.
There in its absence, if it survives stellar environment long term, the atmosphere originally we would expect to be inerts such as CO2 and N2. If some form of life is introduced, we do not necessarily expect a someday 78 to 21 breakdown of N2 and O2, but some sort of substantial readjust.
Now that does not settle the question of whether “abiogenesis” or not if we are stuck for a long time with one example. If the atmospheric signals are similar and frequent, however, we might have a clue that the formula has a common origin or root. For a number of reasons I hope for this answer. The alternative would be as you say. We could search the whole galaxy for generations and be unable to resolve on account of likelihood issues.
Still, I have to wonder whether there are additional laws of conservation beside energy and momentum or mass and radiation through a control volume. One way we could define life and sentience better is to detect any conservation of it in similar frames.
RE:
“If we found the biology of life in different star systems throughout the galaxy was near identical, then that would be a strong argument for a common origin. Conversely, different biologies would indicate unique abiogeneses in each system.”
Should we get a chance to examine that, i suspect that the data would indicate an intermediate answer; i.e., that there are some sort of seeding fundamentals but diversification based on environment and circumstance. Enormous possibilities of variation as our own natural history and present day indicate.
On the other hand, one of my college instructors when he got to the subject of statistical mechanics, he would say that there was a possibility that all the molecules in the room could be stacked up in a corner of the room when he entered it. In this application with abiogenesis between virus and communicating on this medium, or aiming a SETI communications transmitter, If we found the biology of life in different star systems throughout the galaxy was near identical, then that would be a strong argument for a common origin. Conversely, different biologies would indicate unique abiogeneses in each system. we are looking at something like those molecules in the lecture room all in a stack one day. Maybe even with life beyond bacteria.
Out of the range of planets with “too much water”, what percent of them might be stripped down to being “just right”? And for how long? Also, what would be the effect on ocean water of being concentrated by intense radiation shining through the atmosphere?
Someone had said that while science can span a continuum from the subatomic realm through molecular cell machinery to sentient behavior, it is difficult to pin the beginning of life to any particular point. Life is a subset, with an ill-defined starting point. Survival, growth and replication are characteristics that segregate from the primordial soup, forming the imperatives of life. Stirring the universe occasionally produces the fire, the cooking pot and ingredients for the soups: with just the right fire, pot and ingredients, one may have the right soups, quite possibly many instances of them, although we know of only one.
Proposed recipes can start at any point, but the earlier they start, the more complexity they include and the iffier they get.
Some of these posts got me to speculating. Let’s suppose Earth was the recipient of a life-seeding program – not just random archeal/bacterial last common ancestors trying to surf their way into the atmosphere like Lieutenant Doolittle, but rather riding aboard little automated probes at the 13th stage of a formerly massive interstellar colonization ship. I suppose there is some chance of this, and if someone thought this was worth doing, the chance that Earth has the perfect level of water for life may be a little higher than we think.
But if there was a ship, it needed a propellant, and when founding a new colony, what better propellant can you think of than libraries? Perhaps there are little bacterium-sized glass (sensu lato) beads lying around packed full of information at the molecular level, just waiting for someone to scoop up a few km^2 of lunar soil and pore over every single last grain at AFM levels of detail. How long can it take??? :)
I have made a similar speculation about information stored in tiny grains scattered throughout the geologic record. One issue is that the amount of information stored is dependent on size – should the effort go into lots of tiny grains, or into fewer, larger, storage mechanisms? Then there is the issue of how to decode it. Life itself is at least a signal about information that is “self-decoding” in that its presence is observable through its actions. bacterial spores are about as compact as you can get.
I don’t think you need a self-propelled starship, but rather huge fleets of tiny light/laser sailcraft blown outwards away from the home star. The sails and their cargo could even be created by automation that lasts for a long time without control by sentient beings who initiated the plan.
Just looked at a brief article in the 14 October 2022 AAAS Science.
On page 123 there was an article about a widely detected meteor entering the atmosphere over western Canada, 22 February 2021. “Fireball Traced to Far Edge of the Solar System”. There were several lines of evidence or reasonig that made this fall significant:
1.) It appeared to contain significant rock.
2.) Traceback suggested a semi major axis of about 1000 AUs.
3.) Oort Cloud models have generally assumed a composition of icy comets.
Estimates of how far the Oort Cloud extends vary. The article suggested halfway to the next star, assuming perhaps that a similar cloud extended from the Centauri system. My introductory astronomy text would suggest about 50,000 AUs. Our previous discussions have noted that Scholz’s Star with Brown Dwarf companion passed near the edge of a lower bound cloud about 70,000 years ago.
4. If we give credence to this event the Oort Cloud might be made up of 1 to 20 percent of rocky material, whatever its total mass might be.
Then if other stars have Oort clouds, then from time to time they mix.
Whether it is in cluster nurseries or in close passages later orbiting the galactic center.
I can’t see what Jupiter would have had to do with such an object – unless there were an inner solar system passage that caused some material to be ejected. But with the velocities and perturbation forces involved, it would be easier to have an object drop into the inner solar system after deceleration rather than the other way around.
And consequently, I would suspect that Oort Cloud enrichment of near stellar objects such as terrestrial planets in HZs could be achieved with or without jovian planets in the vicinity. In addition, with Oort Clouds of wide extent, and as leftovers of collapsing
protostars, the interaction of passing stars could result in exchange of organic compounds transferred as well as rocks. At least anything detectable in the interstellar medium. Not all of this would survive to show up on planetary surfaces, but it does suggest alternate paths to “abiogenesis”.