The most common type of star in the galaxy is the red dwarf star. None of these small, dim stars can be seen from Earth with the naked eye, but they can emit flares far more powerful than anything our Sun emits. Two astronomers using the Hubble space telescope saw a red dwarf star give off a powerful type of flare called a superflare. That’s bad news for any planets in these stars’ so-called habitable zones.
Red dwarfs make up about 75% of the stars in the Milky Way, so they probably host many exoplanets. In fact, scientists think most of the planets that are in habitable zones are orbiting red dwarfs. But the more astronomers observe these stars, the more they’re becoming aware of just how chaotic and energetic it can be in their neighbourhoods. That means we might have to re-think what habitable zone means.
“When I realized the sheer amount of light the superflare emitted, I sat looking at my computer screen for quite some time just thinking, ‘Whoa.'” – Parke Loyd, Arizona State University.
What exactly is a “normal” solar system? If we thought we had some idea in the past, we definitely don’t now. And a new study led by astronomers at Cambridge University has reinforced this fact. The new study found four gas giant planets, similar to our own Jupiter and Saturn, orbiting a very young star called CI Tau. And one of the planets has an extreme orbit that takes it more than a thousand times more distant from the star than the innermost planet.
A pair of astronomers combing through data from the Kepler spacecraft have discovered the first exomoon. The moon is in the Kepler 1625 system about 8,000 light years away, in the constellation Cygnus. It orbits the gas giant Kepler 1625b, and, unlike all the moons in our Solar System, this one is a “gas moon.”
It was only a matter of time before we found an exomoon. We’ve found thousands of exoplanets, thanks mostly to the Kepler spacecraft. And where there are planets, we can expect moons. But even though it seemed inevitable, the first confirmed exomoon is still exciting.
In the past few decades, thousands of extra-solar planets have been discovered within our galaxy. As of July 28th, 2018, a total of 3,374 extra-solar planets have been confirmed in 2,814 planetary systems. While the majority of these planets have been gas giants, an increasing number have been terrestrial (i.e. rocky) in nature and were found to be orbiting within their stars’ respective habitable zones (HZ).
However, as the case of the Solar System shows, HZs do not necessary mean a planet can support life. Even though Venus and Mars are at the inner and the outer edge of the Sun’s HZ (respectively), neither is capable of supporting life on its surface. And with more potentially-habitable planets being discovered all the time, a new study suggests that it might be time to refine our definition of habitable zones.
As Dr. Ramirez indicated in his study, the most generic definition of a habitable zone is the circular region around a star where surface temperatures on an orbiting body would be sufficient to maintain water in a liquid state. However, this alone does not mean a planet is habitable, and additional considerations need to be taken into account to determine if life could truly exist there. As Dr. Ramirez told Universe Today via email:
“The most popular incarnation of the HZ is the classical HZ. This classical definition assumes that the most important greenhouse gases in potentially habitable planets are carbon dioxide and water vapor. It also assumes that habitability on such planets is sustained by the carbonate-silicate cycle, as is the case for the Earth. On our planet, the carbonate-silicate cycle is powered by plate tectonics.
“The carbonate-silicate cycle regulates the transfer of carbon dioxide between the atmosphere, surface, and interior of the Earth. It acts as a planetary thermostat over long timescales and ensures that there is not too much CO2 in the atmosphere (the planet gets too hot) or too little (the planet gets too cold). The classical HZ also (typically) assumes that habitable planets possess total water inventories (e.g. total water in the oceans and seas) similar in size to that on the Earth.”
This is what can be referred to as the “low-hanging fruit” approach, where scientists have looked for signs of habitability based on what we as humans are most familiar with. Given that the only example we have of habitability is planet Earth, exoplanet studies have been focused on finding planets that are “Earth-like” in composition (i.e. rocky), orbit, and size.
However, in recent years this definition has come to be challenged by newer studies. As exoplanet research has moved away from merely detecting and confirming the existence of bodies around other stars and moved into characterization, newer formulations of HZs have emerged that have attempted to capture the diversity of potentially-habitable worlds.
As Dr. Ramirez explained, these newer formulations have complimented traditional notions of HZs by considering that habitable planets may have different atmospheric compositions:
“For instance, they consider the influence of additional greenhouses gases, like CH4 and H2, both of which have been considered important for early conditions on both Earth and Mars. The addition of these gases makes the habitable zone wider than what would be predicted by the classical HZ definition. This is great, because planets thought to be outside the HZ, like TRAPPIST-1h, may now be within it. It has also been argued that planets with dense CO2-CH4 atmospheres near the outer edge of the HZ of hotter stars may be inhabited because it is hard to sustain such atmospheres without the presence of life.”
One such study was conducted by Dr. Ramirez and Lisa Kaltenegger, an associate professor with the Carl Sagan Institute at Cornell University. According to a paper they produced in 2017, which appeared in the Astrophysical Journal Letters, exoplanet-hunters could find planets that would one day become habitable based on the presence ofvolcanic activity – which would be discernible through the presence of hydrogen gas (H2) in their atmospheres.
This theory is a natural extension of the search for “Earth-like” conditions, which considers that Earth’s atmosphere was not always as it is today. Basically, planetary scientists theorize that billions of years ago, Earth’s early atmosphere had an abundant supply of hydrogen gas (H2) due to volcanic outgassing and interaction between hydrogen and nitrogen molecules in this atmosphere is what kept the Earth warm long enough for life to develop.
In Earth’s case, this hydrogen eventually escaped into space, which is believed to be the case for all terrestrial planets. However, on a planet where there is sufficient levels of volcanic activity, the presence of hydrogen gas in the atmosphere could be maintained, thus allowing for a greenhouse effect that would keep their surfaces warm. In this respect, the presence of hydrogen gas in a planet’s atmosphere could extend a star’s HZ.
According to Ramirez, there is also the factor of time, which is not typically taken into account when assessing HZs. In short, stars evolve over time and put out varying levels of radiation based on their age. This has the effect of altering where a star’s HZ reaches, which may not encompass a planet that is currently being studied. As Ramirez explained:
“[I]t has been shown that M-dwarfs (really cool stars) are so bright and hot when they first form that they can desiccate any young planets that are later determined to be in the classical HZ. This underscores the point that just because a planet is currently located in the habitable zone, it doesn’t mean that it is actually habitable (let alone inhabited). We should be able to watch out for these cases.
Finally, there is the issue of what kinds of star system astronomers have been observing in the hunt for exoplanets. Whereas many surveys have examined G-type yellow dwarf star (which is what our Sun is), much research has been focused on M-type (red dwarf) stars of late because of their longevity and the fact that they believed to be the most likely place to find rocky planets that orbit within their stars’ HZs.
“Whereas most previous studies have focused on single star systems, recent work suggests that habitable planets may be found in binary star systems or even red giant or white dwarf systems, potentially habitable planets may also take the form of desert worlds or even ocean worlds that are much wetter than the Earth,” says Ramirez. “Such formulations not only greatly expand the parameter space of potentially habitable planets to search for, but they allow us to filter out the worlds that are most (and least) likely to host life.”
In the end, this study shows that the classical HZ is not the only tool that can be used to asses the possibility of extra-terrestrial life. As such, Ramirez recommends that in the future, astronomers and exoplanet-hunters should supplement the classical HZ with the additional considerations raised by these newer formulations. In so doing, they just may be able to maximize their chances for finding life someday.
“I recommend that scientists pay real special attention to the early stages of planetary systems because that helps determine the likelihood that a planet that is currently located in the present day habitable zone is actually worth studying further for more evidence of life,” he said. “I also recommend that the various HZ definitions are used in conjunction so that we can best determine which planets are most likely to host life. That way we can rank these planets and determine which ones to spend most of our telescope time and energy on. Along the way we would also be testing how valid the HZ concept is, including determining how universal the carbonate-silicate cycle is on a cosmic scale.”
Younger stars have a cloud of dusty debris encircling them, called a circumstellar disk. This disk is material left over from the star’s formation, and it’s out of this material that planets form. But scientists using the Hubble have been studying an enormous dust structure some 150 billion miles across. Called an exo-ring, this newly imaged structure is much larger than a circumstellar disk, and the vast structure envelops the young star HR 4796A and its inner circumstellar disk.
Discovering a dust structure around a young star is not new, and the star in this new paper from Glenn Schneider of the University of Arizona is probably our most (and best) studied exoplanetary debris system. But Schneider’s paper, along with capturing this new enormous dust structure, seems to have uncovered some of the interplay between the bodies in the system that has previously been hidden.
Schneider used the Space Telescope Imaging Spectrograph (STIS) on the Hubble to study the system. The system’s inner disk was already well-known, but studying the larger structure has revealed more complexity.
The origin of this vast structure of dusty debris is likely collisions between newly forming planets within the smaller inner ring. Outward pressure from the star HR 4769A then propelled the dust outward into space. The star is 23 times more luminous than our Sun, so it has the necessary energy to send the dust such a great distance.
A press release from NASA describes this vast exo-ring structure as a “donut-shaped inner tube that got hit by a truck.” It extends much further in one direction than the other, and looks squashed on one side. The paper presents a couple possible causes for this asymmetric extension.
It could be a bow wave caused by the host star travelling through the interstellar medium. Or it could be under the gravitational influence of the star’s binary companion (HR 4796B), a red dwarf star located 54 billion miles from the primary star.
“The dust distribution is a telltale sign of how dynamically interactive the inner system containing the ring is'” – Glenn Schneider, University of Arizona, Tucson.
The asymmetrical nature of the vast exo-structure points to complex interactions between all of the stars and planets in the system. We’re accustomed to seeing the radiation pressure from the host star shape the gas and dust in a circumstellar disk, but this study presents us with a new level of complexity to account for. And studying this system may open a new window into how solar systems form over time.
“We cannot treat exoplanetary debris systems as simply being in isolation. Environmental effects, such as interactions with the interstellar medium and forces due to stellar companions, may have long-term implications for the evolution of such systems. The gross asymmetries of the outer dust field are telling us there are a lot of forces in play (beyond just host-star radiation pressure) that are moving the material around. We’ve seen effects like this in a few other systems, but here’s a case where we see a bunch of things going on at once,” Schneider further explained.
The paper suggests that the location and brightness of smaller rings within the larger dust structure places constraints on the masses and orbits of planets within the system, even when the planets themselves can’t be seen. But that will require more work to determine with any specificity.
This paper represents a refinement and advancement of the Hubble’s imaging capabilities. The paper’s author is hopeful that the same methods using in this study can be used on other similar systems to better understand these larger dust structures, how they form, and what role they play.
As he says in the paper’s conclusion, “With many, if not most, technical challenges now understood and addressed, this capability should be used to its fullest, prior to the end of the HST mission, to establish a legacy of the most robust images of high-priority exoplanetary debris systems as an enabling foundation for future investigations in exoplanetary systems science.”
When we finally find life somewhere out there beyond Earth, it’ll be at the end of a long search. Life probably won’t announce its presence to us, we’ll have to follow a long chain of clues to find it. Like scientists keep telling us, at the start of that chain of clues is water.
The discovery of the TRAPPIST-1 system last year generated a lot of excitement. 7 planets orbiting the star TRAPPIST-1, only 40 light years from Earth. At the time, astronomers thought at least some of them were Earth-like. But now a new study shows that some of the planets could hold more water than Earth. About 250 times more.
This new study focuses on the density of the 7 TRAPPIST-1 planets. Trying to determine that density is a challenging task, and it involved some of the powerhouses in the world of telescopes. The Spitzer Space Telescope, the Kepler Space Telescope, and the SPECULOOS (Search for habitable Planets EClipsing ULtra-cOOl Stars) facility at ESO’s Paranal Observatory were all used in the study.
In this study, the observations from the three telescopes were subjected to complex computer modelling to determine the densities of the 7 TRAPPIST planets. As a result, we now know that they are all mostly made of rock, and that some of them could be 5% water by mass. (Earth is only about 0.02% water by mass.)
Finding the densities of these planets was not easy. To do so, scientists had to determine both the mass and the size. The TRAPPIST-1 planets were found using the transit method, where the light of the host star dips as the planets pass between their star and us. The transit method gives us a pretty good idea of the size of the planets, but that’s it.
It’s a lot harder to find the mass, because planets with different masses can have the same orbits and we can’t tell them apart. But in multi-planet systems like TRAPPIST-1, there is a way.
As the planets orbit the TRAPPIST-1 star, more massive planets disturb the orbits of the other planets more than lighter ones. This changes the timing of the transits. These effects are “complicated and very subtle” according to the team, and it took a lot of observation and measurement of the transit timing—and very complex computer modelling—to determine their densities.
Lead author Simon Grimm explains how it was done: “The TRAPPIST-1 planets are so close together that they interfere with each other gravitationally, so the times when they pass in front of the star shift slightly. These shifts depend on the planets’ masses, their distances and other orbital parameters. With a computer model, we simulate the planets’ orbits until the calculated transits agree with the observed values, and hence derive the planetary masses.”
So, what about the water?
First of all, this study didn’t detect water. It detected volatile material which is probably water.
Whether or not they’ve confirmed the presence of water, these are still very important results. We’re getting good at finding exoplanets, and the next step is to determine the properties of any atmospheres that exoplanets have.
Team member Eric Agol comments on the significance: “A goal of exoplanet studies for some time has been to probe the composition of planets that are Earth-like in size and temperature. The discovery of TRAPPIST-1 and the capabilities of ESO’s facilities in Chile and the NASA Spitzer Space Telescope in orbit have made this possible — giving us our first glimpse of what Earth-sized exoplanets are made of!”
This study doesn’t tell us if any of the TRAPPIST planets have life on them, or even if they’re habitable. It’s just one more step on the path to hopefully, maybe, one day, finding life somewhere. Study co-author Brice-Olivier Demory, at the University of Bern, said as much: “Densities, while important clues to the planets’ compositions, do not say anything about habitability. However, our study is an important step forward as we continue to explore whether these planets could support life.”
This is what the study determined about the different planets in the TRAPPIST system:
TRAPPIST 1-b and 1c are the two innermost planets and are likely to have rocky cores and be surrounded by atmospheres much thicker than Earth’s.
TRAPPIST-1d is the lightest of the planets at about 30 percent the mass of Earth. We’re uncertain whether it has a large atmosphere, an ocean or an ice layer.
TRAPPIST-1e is a bit of a surprise. It’s the only planet in the system slightly denser than Earth. It may have a denser iron core, and it does not necessarily have a thick atmosphere, ocean or ice layer. TRAPPIST-1e is a mystery because it appears to be so much rockier than the rest of the planets. It’s the most similar to Earth, in size, density and the amount of radiation it receives from its star.
TRAPPIST-1f, g and h might have frozen surfaces. If they have thin atmospheres, they would be unlikely to contain the heavy molecules that we find on Earth, such as carbon dioxide.
The TRAPPIST-1 system is going to be studied for a very long time. It promises to be one of the first targets for the James Webb Space Telescope (we hope.) It’s a very intriguing system, and whether or not any of the planets are deemed habitable, studying them will teach us a lot about our search for water, habitability, and life.
In February of 2017, astronomers from the European Southern Observatory (ESO) announced the discovery of seven rocky planets around the nearby star of TRAPPIST-1. Not only was this the largest number of Earth-like planets discovered in a single star system to date, the news was also bolstered by the fact that three of these planets were found to orbit within the star’s habitable zone.
Since that time, multiple studies have been conducted to ascertain the likelihood that these planets are actually habitable. Thanks to an international team of scientists who used the Hubble Space Telescope to study the system’s planets, we now have the first clues as to whether or not water (a key ingredient to life as we know it) exists on any of TRAPPIST-1s rocky worlds.
As Bourrier explained in a Hubble press release, this helped them to determine the water content of the system’s seven planets:
“Ultraviolet radiation is an important factor in the atmospheric evolution of planets. As in our own atmosphere, where ultraviolet sunlight breaks molecules apart, ultraviolet starlight can break water vapor in the atmospheres of exoplanets into hydrogen and oxygen.”
How ultraviolet radiation interacts with a planet’s atmosphere is important when it comes to assessing the potential habitability of a planet. Whereas lower-energy UV radiation causes photodissociation, a process where water molecules break down into oxygen and hydrogen, extreme ultraviolet rays (XUV radiation) and x-rays cause the upper atmosphere of a planet to heat up – which causes the hydrogen and oxygen to escape.
Since hydrogen is lighter than oxygen, it is more easily lost to space where its spectra can be observed. This is precisely what Bourrier and his team did. By monitoring the TRAPPIST-1 planets spectra for signs of hydrogen loss, the team was effectively able to gauge their water content. What they found was that the UV radiation emitted by TRAPPIST-1 suggests that its planets could have lost quite a lot of water during their history.
The losses were most severe for the innermost planets – TRAPPIST-1b and 1c – which receive the most UV radiation from their star. In fact, the team estimates that these planets could have lost more than 20 Earth-oceans worth of water in the course of the system’s history – which is estimated to be between 5.4 and 9.8 billion years old. In other words, these inner planets would be bone dry and most definitely sterile.
However, these same findings also suggest that the outer planets of the system have lost significantly less water over time, which could mean that they still possess abundant amounts on their surfaces. This includes the three planets that are within the star’s habitable zone – TRAPPIST-1e, f and g – which indicates that these planets could be habitable after all.
These findings are bolstered by the calculated water loss and geophysical water release rates, which also favor the idea that the more-massive and outermost planets have retained most of their water over time. These findings are very significant, in that they further demonstrate that atmospheric escape and evolution are closely linked on the planets of the TRAPPIST-1 system.
The findings are also encouraging, since previous studies that considered atmospheric loss in this system painted a rather grim picture. These include those that indicated that TRAPPIST-1 experiences too much flare, that even calm red dwarfs subject their planets to intense radiation over time, and that the distance between TRAPPIST-1 and its respective planets would mean that solar wind would be deposited directly onto their atmospheres.
In other words, these studies cast doubt on whether or not stars that orbit M-type (red dwarf) stars would be able to retain their atmospheres over time – even if they had an Earth-like atmosphere and magnetosphere. Like Mars, this research indicated that atmospheric stripping caused by solar wind would inevitably render their surfaces cold, desiccated, and lifeless.
In short, this is one of the few pieces of good news we’ve received since the existence of seven planets in the TRAPPIST-1 system (and three potentially habitable ones) was first announced. It’s also a positive indication as far as the habitability of red dwarf star systems go. In recent years, many of those impressive exoplanet finds have taken place around red dwarf stars – i.e. Proxima b, LHS 1140b, Gliese 581g, Gliese 625b, and Gliese 682c.
Given the number of rocky planets that have been detected orbiting this type of star – and the fact that they are the most common in in the Universe (accounting for 70% of stars in the Milky Way alone) – knowing that they could support habitable planets is certainly welcome! But of course, Bourrier and his colleagues emphasize that the study is not conclusive, and further research is needed to determine if any of the TRAPPIST-1 planets are actually watery.
As Bourieer indicated, this will most likely involve next-generation telescopes:
“While our results suggest that the outer planets are the best candidates to search for water with the upcoming James Webb Space Telescope, they also highlight the need for theoretical studies and complementary observations at all wavelengths to determine the nature of the TRAPPIST-1 planets and their potential habitability.”
Rocky planets around the most common type of star, the potential to retain water, and 1oo billion potential planets in the Milky Way Galaxy alone. One thing is for sure: the James Webb Space Telescope is going to have its hands full once it is deployed in October of 2018!
And be sure to check out this animation of the TRAPPIST-1 system as well, courtesy of L. Calçada and the ESO:
Ever since scientists confirmed the existence of seven terrestrial planets orbiting TRAPPIST-1, this system has been a focal point of interest for astronomers. Given its proximity to Earth (just 39.5 light-years light-years away), and the fact that three of its planets orbit within the star’s “Goldilocks Zone“, this system has been an ideal location for learning more about the potential habitability of red dwarf stars systems.
This is especially important since the majority of stars in our galaxy are red dwarfs (aka. M-type dwarf stars). Unfortunately, not all of the research has been reassuring. For example, two recent studies performed by two separate teams from Harvard-Smithsonian Center for Astrophysics (CfA) indicate that the odds finding life in this system are less likely than generally thought.
The first study, titled “Physical Constraints on the Likelihood of Life on Exoplanets“, sought to address how radiation and stellar wind would affect any planets located within TRAPPIST-1s habitable zone. Towards this end, the study’s authors – Professors Manasvi Lingam and Avi Loeb – constructed a model that considered how certain factors would affect conditions on the surface of these planets.
This model took into account how the planets distance from their star would affect surface temperatures and atmospheric loss, and how this might affect the changes life would have to emerge over time. As Dr. Loeb told Universe Today via email:
“We considered the erosion of the atmosphere of the planets due to the stellar wind and the role of temperature on ecological and evolutionary processes. The habitable zone around the faint dwarf star TRAPPIST-1 is several tens of times closer in than for the Sun, hence the pressure of the stellar wind is several orders of magnitude higher than on Earth. Since life as we know it requires liquid water and liquid water requires an atmosphere, it is less likely that life exists around TRAPPIST-1 than in the solar system.”
Essentially, Dr. Lingam and Dr, Loeb found that planets in the TRAPPIST-1 system would be barraged by UV radiation with an intensity far greater than that experienced by Earth. This is a well-known hazard when it comes to red dwarf stars, which are variable and unstable when compared to our own Sun. They concluded that compared to Earth, the chances of complex life existing on planets within TRAPPIST-1’s habitable zone were less than 1%.
“We showed that Earth-sized exoplanets in the habitable zone around M-dwarfs display much lower prospects of being habitable relative to Earth, owing to the higher incident ultraviolet fluxes and closer distances to the host star,” said Loeb. “This applies to the recently discovered exoplanets in the vicinity of the Sun, Proxima b (the nearest star four light years away) and TRAPPIST-1 (ten times farther), which we find to be several orders of magnitude smaller than that of Earth.”
Essentially, the team found that TRAPPIST-1, like our Sun, sends streams of charged particles outwards into space – i.e. stellar wind. Within the Solar System, this wind exerts force on the planets and can have the effect of stripping away their atmospheres. Whereas Earth’s atmosphere is protected by its magnetic field, planets like Mars are not – hence why it lost the majority of its atmosphere to space over the course of hundreds of million of years.
As the research team found, when it comes to TRAPPIST-1, this stream exerts a force on its planets that is between 1,000 to 100,000 times greater than what Earth experiences from solar wind. Furthermore, they argue that TRAPPIST-1’s magnetic field is likely connected to the magnetic fields of the planets that orbit around it, which would allow particles from the star to directly flow onto the planet’s atmosphere.
In other words, if TRAPPIST-1’s planets do have magnetic fields, they will not afford them any protection. So if the flow of charged particles is strong enough, it could strip these planets’ atmospheres away, thus rendering them uninhabitable. As Garraffo put it:
“The Earth’s magnetic field acts like a shield against the potentially damaging effects of the solar wind. If Earth were much closer to the Sun and subjected to the onslaught of particles like the TRAPPIST-1 star delivers, our planetary shield would fail pretty quickly.”
As you can imagine, this is not exactly good news for those who were hoping that the TRAPPIST-1 system would hold the first evidence of life beyond our Solar System. Between the fact that its planets orbit a star that emits varying degrees of intense radiation, and the proximity its seven planets have to the star itself, the odds of life emerging on any planet within it’s “habitable zone” are not significant.
The findings of the second study are particularly significant in light of other recent studies. In the past, Prof. Loeb and a team from the University of Chicago have both addressed the possibility that the TRAPPIST-1 system’s seven planets – which are relatively close together – are well-suited to lithopanspermia. In short, they determined that given their close proximity to each other, bacteria could be transferred from one planet to the next via asteroids.
But if the proximity of these planets also means that they are unlikely to retain their atmospheres in the face of stellar wind, the likelihood of lithopanspermia may be a moot point. However, before anyone gets to thinking that this is bad news as far as the hunt for life goes, it is important to note that this study does not rule out the possibility of life emerging in all red dwarf star systems.
As Dr. Jeremy Drake – a senior astrophysicist from the CfA and one of Garraffo’s co-authors – indicated, the results of their study simply mean that we need to cast a wide net when searching for life in the Universe. “We’re definitely not saying people should give up searching for life around red dwarf stars,” he said. “But our work and the work of our colleagues shows we should also target as many stars as possible that are more like the Sun.”
And as Dr. Loeb himself has indicated in the past, red dwarf stars are still the most statistically-likely place to find habitable worlds:
“By surveying the habitability of the Universe throughout cosmic history from the birth of the first stars 30 million years after the Big Bang to the death of the last stars in 10 trillion years, one reaches the conclusion that unless habitability around low-mass stars is suppressed, life is most likely to exist near red dwarf stars like Proxima Centauri or TRAPPIST-1 trillions of years from now.”
If there is one takeaway from these studies, it is that the existence of life within a star system does not simply require planets orbiting within the circumstellar habitable zones. The nature of the stars themselves and the role played by solar wind and magnetic fields also have to be taken into account, since they can mean the difference between a life-bearing planet and a sterile ball of rock!