Earthlings are fortunate. Our planet has a robust magnetic shield. Without out magnetosphere, the Sun’s radiation would’ve probably ended life on Earth before it even got going. And our Sun is rather tame, in stellar terms.
What’s it like for exoplanets orbiting more active stars?
In order to be considered habitable, a planet needs to have liquid water. Cells, the smallest unit of life, need water to carry out their functions. For liquid water to exist, the temperature of the planet needs to be right. But how about the size of the planet?
Without sufficient mass a planet won’t have enough gravity to hold onto its water. A new study tries to understand how size affects the ability of a planet to hold onto its water, and as a result, its habitability.
I’ve said many times in the past that the Earth is the best planet in the Universe. No matter where we go, we’ll never find a planet that’s a better home to Earth life than Earth. Of course, that’s because we, and all other Earth life evolved in this environment. Evolution adapted us to this planet, and it’s unlikely we could ever find another planet this good for us.
However, is it the best planet? Are there places in the Universe which might have the conditions for more diversity of life?
When searching for potentially habitable exoplanets, scientists are forced to take the low-hanging fruit approach. Since Earth is the only planet we know of that is capable of supporting life, this search basically comes down to looking for planets that are “Earth-like”. But what if Earth is not the meter stick for habitability that we all tend to think it is?
That was the subject of a keynote lecture that was recently made at the Goldschmidt Geochemistry Congress, which took place from Aug. 18th to 23rd, in Barcelona, Spain. Here, a team of NASA-supported researchers explained how an examination of what goes into defining habitable zones (HZs) shows that some exoplanets may have better conditions for life to thrive than Earth itself has.
When astronomers discover a new exoplanet, one of the first considerations is if the planet is in the habitable zone, or outside of it. That label largely depends on whether or not the temperature of the planet allows liquid water. But of course it’s not that simple. A new study suggests that frozen, icy worlds with completely frozen oceans could actually have livable land areas that remain habitable.
In August of 2016, astronomers from the European Southern Observatory (ESO) announced the discovery of an exoplanet in the neighboring system of Proxima Centauri. The news was greeted with consider excitement, as this was the closest rocky planet to our Solar System that also orbited within its star’s habitable zone. Since then, multiple studies have been conducted to determine if this planet could actually support life.
Unfortunately, most of the research so far has indicated that the likelihood of habitability are not good. Between Proxima Centauri’s variability and the planet being tidally-locked with its star, life would have a hard time surviving there. However, using lifeforms from early Earth as an example, a new study conducted by researchers from the Carl Sagan Institute (CSI) has shows how life could have a fighting chance on Proxima b after all.
Looking to the future, NASA and other space agencies have high hopes for the field of extra-solar planet research. In the past decade, the number of known exoplanets has reached just shy of 4000, and many more are expected to be found once next-generations telescopes are put into service. And with so many exoplanets to study, research goals have slowly shifted away from the process of discovery and towards characterization.
Unfortunately, scientists are still plagued by the fact that what we consider to be a “habitable zone” is subject to a lot of assumptions. Addressing this, an international team of researchers recently published a paper in which they indicated how future exoplanet surveys could look beyond Earth-analog examples as indications of habitability and adopt a more comprehensive approach.
How many exoplanets are there? Not that long ago, we didn’t know if there were any. Then we detected a few around pulsars. Then the Kepler spacecraft was launched and it discovered a couple thousand more. Now NASA’s TESS (Transiting Exoplanet Survey Satellite) is operational, and a new study predicts its findings.
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.”
Shortly after Einstein published his Theory of General Relativity in 1915, physicists began to speculate about the existence of black holes. These regions of space-time from which nothing (not even light) can escape are what naturally occur at the end of most massive stars’ life cycle. While black holes are generally thought to be voracious eaters, some physicists have wondered if they could also support planetary systems of their own.
Looking to address this question, Dr. Sean Raymond – an American physicist currently at the University of Bourdeaux – created a hypothetical planetary system where a black hole lies at the center. Based on a series of gravitational calculations, he determined that a black hole would be capable of keeping nine individual Suns in a stable orbit around it, which would be able to support 550 planets within a habitable zone.
As Raymond indicates, one of the immediate advantages of having this black hole at the center of a system is that it can support a large number of Suns. For the sake of his system, Raymond chose 9, thought he indicates that many more could be sustained thanks to the sheer gravitational influence of the central black hole. As he wrote on his website:
“Given how massive the black hole is, one ring could hold up to 75 Suns! But that would move the habitable zone outward pretty far and I don’t want the system to get too spread out. So I’ll use 9 Suns in the ring, which moves everything out by a factor of 3. Let’s put the ring at 0.5 AU, well outside the innermost stable circular orbit (at about 0.02 AU) but well inside the habitable zone (from about 2.7 to 5.4 AU).”
Another major advantage of having a black hole at the center of a system is that it shrinks what is known as the “Hill radius” (aka. Hill sphere, or Roche sphere). This is essentially the region around a planet where its gravity is dominant over that of the star it orbits, and can therefore attract satellites. According to Raymond, a planet’s Hill radius would be 100 times smaller around a million-sun black hole than around the Sun.
This means that a given region of space could stably fit 100 times more planets if they orbited a black hole instead of the Sun. As he explained:
“Planets can be super close to each other because the black hole’s gravity is so strong! If planets are little toy Hot wheels cars, most planetary systems are laid out like normal highways (side note: I love Hot wheels). Each car stays in its own lane, but the cars are much much smaller than the distance between them. Around a black hole, planetary systems can be shrunk way down to Hot wheels-sized tracks. The Hot wheels cars — our planets — don’t change at all, but they can remain stable while being much closer together. They don’t touch (that would not be stable), they are just closer together.”
This is what allows for many planets to be placed with the system’s habitable zone. Based on the Earth’s Hill radius, Raymond estimates that about six Earth-mass planets could fit into stable orbits within the same zone around our Sun. This is based on the fact that Earth-mass planets could be spaced roughly 0.1 AU from each other and maintain a stable orbit.
Given that the Sun’s habitable zone corresponds roughly to the distances between Venus and Mars – which are 0.3 and 0.5 AU away, respectively – this means there is 0.8 AUs of room to work with. However, around a black hole with 1 million Solar Masses, the closest neighboring planet could be just 1/1000th (0.001) of an AU away and still have a stable orbit.
Doing the math, this means that roughly 550 Earths could fit in the same region orbiting the black hole and its nine Suns. There is one minor drawback to this whole scenario, which is that the black hole would have to remain at its current mass. If it were to become any larger, it would cause the Hill radii of its 550 planets to shrink down further and further.
Once the Hill radius got down to the point where it was the same size as any of the Earth-mass planets, the black hole would begin to tear them apart. But at 1 million Solar masses, the black hole is capable of supporting a massive system of planets comfortably. “With our million-Sun black hole the Earth’s Hill radius (on its current orbit) would already be down to the limit, just a bit more than twice Earth’s actual radius,” he says.
Lastly, Raymond considers the implications that living in such a system would have. For one, a year on any planet within the system’s habitable zone would be much shorter, owing to the fact their orbital periods would be much faster. Basically, a year would last roughly 1.6 days for planets at the inner edge of the habitable zone and 4.6 days for planets at the outer edge of the habitable zone.
In addition, on the surface of any planet in the system, the sky would be a lot more crowded! With so many planets in close orbit together, they would pass very close to one another. That essentially means that from the surface of any individual Earth, people would be able to see nearby Earths as clear as we see the Moon on some days. As Raymond illustrated:
“At closest approach (conjunction) the distance between planets is about twice the Earth-Moon distance. These planets are all Earth-sized, about 4 times larger than the Moon. This means that at conjunction each planet’s closest neighbor appears about twice the size of the full Moon in the sky. And there are two nearest neighbors, the inner and outer one. Plus, the next-nearest neighbors are twice as far away so they are still as big as the full Moon during conjunction. And four more planets that would be at least half the full Moon in size during conjunction.”
He also indicates that conjunctions would occur almost once per orbit, which would mean that every few days, there would be no shortage of giant objects passing across the sky. And of course, there would be the Sun’s themselves. Recall that scene in Star Wars where a young Luke Skywalker is watching two suns set in the desert? Well, it would a little like that, except way more cool!
According to Raymond’s calculations, the nine Suns would complete an orbit around the black hole every three hours. Every twenty minutes, one of these Suns would pass behind the black hole, taking just 49 seconds to do so. At this point, gravitational lensing would occur, where the black hole would focus the Sun’s light toward the planet and distort the apparent shape of the Sun.
To illustrate what this would look like, he provides an animation (shown above) created by @GregroxMun – a planet modeller who develops space graphics for Kerbal and other programs – using Space Engine.
While such a system may never occur in nature, it is interesting to know that such a system would be physically possible. And who knows? Perhaps a sufficiently advanced species, with the ability to tow stars and planets from one system and place them in orbit around a black hole, could fashion this Ultimate Solar System. Something for SETI researchers to be on the lookout for, perhaps?
This hypothetical exercise was the second installment in two-part series by Raymond, titled “Black holes and planets”. In the first installment, “The Black Hole Solar System“, Raymond considered what it would be like if our system orbited around a black hole-Sun binary. As he indicated, the consequences for Earth and the other Solar planets would be interesting, to say the least!