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.
Mathew Anderson, author and good friend of the Weekly Space Hangout, joins us again this week to discuss his newest book, Habitable Exoplanets: Red Dwarf Systems Like TRAPPIST-1, in which he focuses on exoplanet properties and the chances for habitable planets around Red Dwarf stars.
As he did with his two prior books, Our Cosmic Story and its followup Is Anyone Out There, Mathew will be offering a free e-copy of Habitable Exoplanets: Red Dwarf Systems Like TRAPPIST-1 to viewers of the Weekly Space Hangout, so be sure to tune in this week to find out how to get your free copy of this fascinating book.
If you would like to join the Weekly Space Hangout Crew, visit their site here and sign up. They’re a great team who can help you join our online discussions!
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When we think of other planetary systems, we tend to think that they will operate by the same basic rules as our own. In the Solar System, the planets orbit close to the equatorial plane of the Sun – meaning around its equator. The Sun’s rotational axis, the direction of its poles based to its rotation, is also the same as most of the planets’ (the exception being Uranus, which rotates on its side).
But if the study of extra-solar planets has taught us anything, it is that the Universe is full of possibilities. Consider the star known as GJ436, a red dwarf located about 33 light-years from Earth. For years, astronomers have known that this star has a planet that behaves very much like a comet. But according to a recent study led by astronomers from the University of Geneva (UNIGE), this planet also has a very peculiar orbit.
GJ436 has already been the source of much scientific interest, thanks in part to the discovery that its only confirmed exoplanet has a gaseous envelop similar a comet. This exoplanet, known as GJ436b, was first observed in 2004 using radial velocity measurements taken by the Keck Observatory. In 2007, GJ436b became the first Neptune-sized planet known to be orbiting very closely to its star (aka. a “Hot Neptune”).
And in 2015, GJ436 b made headlines again when scientists reported that its atmosphere was evaporating, resulting in a giant cloud around the planet and a long, trailing tale. This cloud was found to be the result of hydrogen in the planet’s atmosphere evaporating, thanks to the extreme radiation coming from its star. This never-before-seen phenomena essentially means that GJ436 b looks like a comet.
Another interesting fact about this planet is its orbital inclination, which astronomers have puzzled over for the past 10 years. Unlike the planets of the Solar System – whose orbits are largely circular – GJ436b follows a very eccentric, elliptical path. And as the research team indicated in their study, the planet also doesn’t orbit along the star’s equatorial plane, but passes almost above the its poles.
As Vincent Bourrier – a researcher at the Department of Astronomy of the UNIGE Faculty of Science, a member of the European Research Council project FOUR ACES, and the lead author of the study – explained in a UNIGE press release:
“This planet is under enormous tidal forces because it is incredibly close to its star, barely 3% of the Earth-Sun distance. The star is a red dwarf whose lifespan is very long, the tidal forces it induces should have since circularized the orbit of the planet, but this is not the case!”
This was an especially interesting find for many reasons. On the one hand, it is the first instance where a planet was found to have a polar orbit. On the other, studying how planets orbit around a star is a great way to learn more about how that system formed and evolved. For instance, if a planet has been disturbed by the passage of a nearby star, or is being influenced by the presence of other massive planets, that will be apparent from its orbit.
As Christophe Lovis, a UNIGE researcher and co-author of the study, explained:
“Even if we have already seen misaligned planetary orbits, we do not necessarily understand their origin, especially since here it is the first time we measure the architecture of a planetary system around a red dwarf.”
Hervé Beust, an astronomer from the University of Grenobles Alpes, was responsible for doing the orbital calculations on GJ436b. As he indicated, the likeliest explanation for GJ436b’s orbit is the existence of a more massive and more distant planet in the system. While this planet is not currently known, this could be the first indication that GJ436 is a multi-planet system.
“If that is true, then our calculations indicate that not only would the planet not move along a circle around the star, as we’ve known for 10 years, but it should also be on a highly inclined orbit,” he said. “That’s exactly what we just measured!”
Another interesting takeaway from this study was the prediction that the planet has not always orbited so closely to its star. Based on their calculations, the team hypothesizes that the GJ436b may have migrated over time to become a “evaporating planet” that it is today. Here too, the existence of an as-yet-undetected companion is believed to be the most likely cause.
As with all exoplanet studies, these findings have implications for our understanding of the Solar System as well. Looking ahead, the team hopes to conduct further studies of this system in the hopes of determining if there is an elusive planetary companion to be found. These surveys will likely benefit from the deployment of next-generation missions, particularly the James Webb Space Telescope (JWST).
As Bourier indicated, “Our next goal is to identify the mysterious planet that has upset this planetary system.” Locating it will be yet another indirect way in which astronomers discover exoplanets – determining the presence of other planets based on orbital inclination of already discovered ones. The orbital inclination method, perhaps?
NASA’s announcement last week of 7 new exoplanets is still causing great excitement. Any time you discover 7 “Earth-like” planets around a distant star, with 3 of them “potentially” in the habitable zone, it’s a big deal. But now that we’re over some of our initial excitement, let’s look at some of the questions that need to be answered before we can all get excited again.
What About That Star?
The star that the planets orbit, called Trappist-1, is a Red Dwarf star, much dimmer and cooler than our Sun. The three potentially habitable planets—TRAPPIST-1e, f, and g— get about the same amount of energy as Earth and Mars do from the Sun, because they’re so close to it. Red Dwarfs are very long-lasting stars, and their lifetimes are measured in the trillions of years, rather than billions of years, like our Sun is.
But Red Dwarfs themselves can have some unusual properties that are problematic when it comes to supporting life on nearby planets.
Red Dwarfs can be covered in starspots, or what we call sunspots when they appear on our Sun. On our Sun, they don’t have much affect on the amount of energy received by the Earth. But on a Red Dwarf, they can reduce the energy output by up to 40%. And this can go on for months at a time.
Other Red Dwarfs can emit powerful flares of energy, causing the star to double in brightness in mere minutes. Some Red Dwarfs constantly emit these flares, along with powerful magnetic fields.
Part of the excitement surrounding the Trappist planets is that they show multiple rocky planets in orbit around a Red Dwarf. And Red Dwarfs are the most common type of star in the Milky Way. So, the potential for life-supporting, rocky planets just grew in a huge way.
But we don’t know yet how the starspots and flaring of Red Dwarfs will affect the potential habitability of planets orbiting them. It could very well render them uninhabitable.
Will Tidal Locking Affect the Planets’ Habitability?
The planets orbiting Trappist-1 are very likely tidally locked to their star. This means that they don’t rotate, like Earth and the rest of the planets in our Solar System. This has huge implications for the potential habitability of these planets. With one side of the planet getting all the energy from the star, and the other side in perpetual darkness, these planets would be nothing like Earth.
One side would be constantly roasted by the star, while the other would be frigid. It’s possible that some of these planets could have atmospheres. Depending on the type of atmosphere, the extreme temperature effects of tidal locking could be mitigated. But we just don’t know if or what type of atmosphere any of the planets have. Yet.
So, Do They Have Atmospheres?
We just don’t know yet. But we do have some constraints on what any atmospheres might be.
Preliminary data from the Hubble Space Telescope suggests that TRAPPIST 1b and 1c don’t have extended gas envelopes. All that really tells us is that they aren’t gaseous planets. In any case, those two planets are outside of the habitable zone. What we really need to know is if TRAPPIST 1e, 1f, and 1g have atmospheres. We also need to know if they have greenhouse gases in their atmospheres. Greenhouse gases could help make tidally locked planets hospitable to life.
On a tidally locked planet, the termination line between the sunlit side and the dark side is considered the most likely place for life to develop. The presence of greenhouse gases could expand the habitable band of the termination line and make more of the dark side warmer.
We won’t know much about any greenhouse gases in the atmospheres of these planets until the James Webb Space Telescope (JWST) and the European Extremely Large Telescope (EELT) are operating. Those two ‘scopes will be able to analyze the atmospheres for greenhouse gases. They might also be able to detect biosignatures like ozone and methane in the atmospheres.
We’ll have to wait a while for that though. The JWST doesn’t launch until October 2018, and the EELT won’t see first light until 2024.
Do They Have Liquid Water?
We don’t know for sure if life requires liquid water. We only know that’s true on Earth. Until we find life somewhere else, we have to be guided by what we know of life on Earth. So we always start with liquid water.
A study published in 2016 looked at planets orbiting ultra-cool dwarfs like TRAPPIST-1. They determined that TRAPPIST 1b and 1c could have lost as much as 15 Earth oceans of water during the early hot phase of their solar system. TRAPPIST 1d might have lost as much as 1 Earth ocean of water. If they had any water initially, that is. But the study also shows that they may have retained some of that water. It’s not clear if the three habitable planets in the TRAPPIST system suffered the same loss of initial water. But if they did, they could have retained a similar amount of water.
There are still a lot of questions here. The word “habitable” only means that they are receiving enough energy from their star to keep water in liquid form. Since the planets are tidally locked, any water they did retain could be frozen on the planets’ dark side. To find out for sure, we’ll have to point other instruments at them.
Are Their Orbits Stable?
Planets require stable orbits over a biologically significant period of time in order for life to develop. Conditions that change too rapidly make it impossible for life to survive and adapt. A planet needs a stable amount of solar radiation, and a stable temperature, to support life. If the solar radiation, and the planet’s temperature, fluctuates too rapidly or too much due to orbital instability, then life would not be able to adapt to those changes.
Right now, there’s no indication that the orbits of the TRAPPIST 1 planets are unstable. But we are still in the preliminary stage of investigation. We need a longer sampling of their orbits to know for sure.
Pelted by Interlopers?
Our Solar System is a relatively placid place when it comes to meteors and asteroids. But it wasn’t always that way. Evidence from lunar rock samples show that it may have suffered through a period called the “Late Heavy Bombardment.” During this time, the inner Solar System was like a shooting gallery, with Earth, Venus, Mercury, Mars, and our Moon being struck continuously by asteroids.
The cause of this period of Bombardment, so the theory goes, was the migration of the giant planets through the solar system. Their gravity would have dislodged asteroids from the asteroid belt and the Kuiper Belt, and sent them into the path of the inner, terrestrial planets.
We know that Earth has been hit by meteorites multiple times, and that at least one of those times, a mass extinction was the result.
The TRAPPIST 1 system has no giant planets. But we don’t know if it has an asteroid belt, a Kuiper Belt, or any other organized, stable body of asteroids. It may be populated by asteroids and comets that are unstable. Perhaps the planets in the habitable zone are subjected to regular asteroid strikes which wipes out any life that gets started there. Admittedly, this is purely speculative, but so are a lot of other things about the TRAPPIST 1 system.
How Will We Find Out More?
We need more powerful telescopes to probe exoplanets like those in the TRAPPIST 1 system. It’s the only way to learn more about them. Sending some kind of probe to a solar system 40 light years away is something that might not happen for generations, if ever.
Luckily, more powerful telescopes are on the way. The James Webb Space Telescope should be in operation by April of 2019, and one of its objectives is to study exoplanets. It will tell us a lot more about the atmospheres of distant exoplanets, and whether or not they can support life.
Other telescopes, like the Giant Magellan Telescope (GMT) and the European Extremely Large Telescope (E-ELT), have the potential to capture images of large exoplanets, and possibly even Earth-sized exoplanets like the ones in the TRAPPIST system. These telescopes will see their first light within ten years.
What these questions show is that we can’t get ahead of ourselves. Yes, it’s exciting that the TRAPPIST planets have been discovered. It’s exciting that there are multiple terrestrial worlds there, and that 3 of them appear to be in the habitable zone.
It’s exciting that a Red Dwarf star—the most common type of star in our neighborhood—has been found with multiple rocky planets in the habitable zone. Maybe we’ll find a bunch more of them, and the prospect of finding life somewhere else will grow.
But it’s also possible that Earth, with all of its life supporting and sustaining characteristics, is an extremely unlikely occurrence. Special, rare, and unrepeatable.
Ever since the ESO announced the discovery of an extra-solar planet orbiting Proxima Centauri, scientists have been trying to determine what the conditions are like on this world. This has been especially important given the fact that while Proxima b orbits within the habitable zone of its sun, red dwarfs like Proxima Centauri are known to be somewhat inhospitable.
And while some research has cast doubt on the possibility that Proxima b could indeed support life, a new research study offers a more positive picture. The research comes from the Blue Marble Space Institute of Science (BMSIS) in Seattle, Washington, where astrobiologist Dimitra Atri has conducted simulations that show that Proxima b could indeed be habitable, assuming certain prerequisites were met.
Dr. Atri is a computational physicist whose work with the BMSIS includes the impacts of antiparticles and radiation on biological systems. For the sake of his study – “Modelling stellar proton event-induced particle radiation dose on close-in exoplanets“, which appeared recently in the Monthly Notices of the Royal Astronomical Society Letters – he conducted simulations to measure the impact stellar flares from its sun would have on Proxima b.
To put this perspective, it is important to note how the Kepler mission has found a plethora of planets orbiting red dwarf stars in recent years, many of which are believed to be “Earth-like” and close enough to their suns to have liquid water on their surfaces. However, red dwarfs have a number of issues that do not bode well for habitability, which include their variable nature and the fact they are cooler and fainter than other classes of stars.
This means that any planet close enough to orbit within a red dwarf’s habitable zone would be subject to powerful solar flares – aka. Stellar Proton Events (SPEs) – and would likely be tidally-locked with the star. In other words, only one side would be getting the light and heat necessary to support life, but it would be exposed to a lot of solar protons, which would interact with its atmosphere to create harmful radiation.
As such, the astronomical community is interested in what kinds of conditions are there for planets like Proxima b so they might know if life has (or had) a shot at evolving there. For the sake of his study, Dr. Atri conducted a series of probability (aka. Monte Carlo) simulations that took into account three factors – the type and size of stellar flares, various thicknesses of the planet’s atmosphere and the strength of its magnetic field.
As Dr. Atri explained to Universe Today via email, the results were encouraging – as far as the implications for extra-terrestrial life are concerned:
“I used Monte Carlo simulations to study the radiation dose on the surface of the planet for different types of atmospheres and magnetic field configurations. The results are optimistic. If the planet has both a good magnetic field and a sizable atmosphere, the effects of stellar flares are insignificant even if the star is in an active phase.”
In other words, Atri found that the existence of a strong magnetic field, which would also ensure that the planet has a viable atmosphere, would lead to survivable conditions. While the planet would still experience a spike in radiation whenever a superflare took place, life could survive on a planet like Proxima b in the long run. On the other hand, a weak atmosphere or magnetic field would foretell doom.
“If the planet does not have a significant magnetic field, chances of having any atmosphere and moderate temperatures are negligible,” he said. “The planet would be bombarded with extinction level superflares. Although in case of Proxima b, the star is in a stable condition and does not have violent flaring activity any more – past activity in its history would make the planet a hostile place for a biosphere to originate/evolve.”
History is the key word here, since red dwarf stars like Proxima Centauri have incredible longevity (as noted, up to 10 trillion years). According to some research, this makes red dwarf stars good candidates for finding habitable exoplanets, since it takes billions of years for complex life to evolve. But in order for life to be able to achieve complexity, planets need to maintain their atmospheres over these long periods of time.
Naturally, Atri admits that his study cannot definitively answer whether our closest exoplanet-neighbor is habitable, and that the debate on this is likely to continue for some time. “It is premature to think that Proxima b is habitable or otherwise,” he says. “We need more data about its atmosphere and the strength of its magnetic field.”
In the future, missions like the James Webb Space Telescope should tell us more about this system, its planet, and the kinds of conditions that are prevalent there. By aiming its extremely precise suite of instruments at this neighboring star, it is sure to detect transits of the planet around this faint sun. One can only hope that it finds evidence of a dense atmosphere, which will hint at the presence of a magnetic field and life-supporting conditions.
Hope is another key word here. Not only would a habitable Proxima b be good news for those of us hoping to find life beyond Earth, it would also be good news as far as the existence of life throughout the Universe is concerned. Red dwarf stars make up 70% of the stars in spiral galaxies and more than 90% of all stars in elliptical galaxies. Knowing that even a fraction of these could support life greatly increases the odds of finding intelligence out there!
Last year, astronomers discovered a terrestrial exoplanet orbiting GJ 1132, a red dwarf star located just 12 parsecs (39 light years) away from Earth. Though too close to its parent star to be anything other than extremely hot, astronomers were intrigued to note that it appeared to still be cool enough to have an atmosphere. This was quite exciting, as it represented numerous opportunities for research.
In essence, the planet appeared to be “Venus-like” – i.e. very hot, but still in possession of an atmosphere. What’s more, it was close enough to our Solar System that its atmosphere could be studied in detail. However, a debate began over whether its atmosphere would be hot and wet, or thin and tenuous. And after a year of study, a team of astronomers from the CfA believe they have unlocked that mystery.
In addition to being relatively close to our own Solar System in astronomical terms, the Venus-like exoplanet GJ 1132b also has a relatively small orbital period around its star. This means that opportunities to spot it as it passes in front of its star (i.e. the Transit Method), occur quite often.
This makes it an excellent target for detailed observation and study, which in turn will help astronomers to learn more about terrestrial exoplanets that orbit close to red dwarf stars. But as noted already, astronomers were divided on the issue of GJ 1132b’s atmosphere.
Thanks to the research efforts of Laura Schaefer and her colleagues from the Harvard-Smithsonian Center for Astrophysics (CfA), it now appears that the case for a thin atmosphere is the far more likely. Interestingly enough, this was confirmed by determining just how much oxygen the planet has in its atmosphere.
For the sake of their study, which was outlined in a paper that approved for publication in The Astrophysical Journal – titled “Predictions of the atmospheric composition of GJ 1132b” – they explain how they used a “magma ocean-atmosphere” model to determine what would happen to GJ 1132b over time if it began with a water-rich atmosphere.
They began with the knowledge that a planet like GJ 1132b – which orbits its star at a distance of 2.25 million km (1.4 million mi) – would be subjected to intense amounts of ultraviolet light. This would result in any water vapor in the atmosphere being broken down into hydrogen and oxygen (a process known as photolysis), with the hydrogen escaping into space and the oxygen being retained.
At the same time, they determined that the planet’s atmosphere and proximity to its star would lead to a severe greenhouse effect that would leave the surface molten for a long time. This “magma ocean” would likely interact with the atmosphere by absorbing some of the oxygen. How much would be absorbed and how much would be retained was the big question.
They concluded that the planet’s magma ocean would absorb about one-tenth of the oxygen in the atmosphere. The majority of the remaining 90 percent, according to their model, would be lost to space while a small margin would linger around the planet. This proved to be very much consistent with measurements made of the planet thus far.
As Dr. Laura Schaefer explained to Universe Today via email:
“We determined that the planet would likely have a thin atmosphere by doing a suite of models looking at atmospheric loss and interaction with a surface magma ocean. For the allowable composition range (esp. the abundance of water) based on the current mass measurement, nearly all of the allowed compositions resulted in thin atmospheres, except at the very extreme upper end of the range.”
This magma ocean-atmosphere model could not only help scientists to study terrestrial exoplanets that orbit close to their parent stars, but also to understand how our own planet Venus came to be. For some time, scientists have theorized that Venus began with significant amounts of water on its surface, but that it then underwent a significant change.
This ocean is believed to have evaporated due to Venus’ closer proximity to the Sun, with the ensuing water vapor triggering a runaway greenhouse effect. Over time, ultraviolet radiation from the Sun broke apart the water molecules, resulting in the hot, virtually waterless atmosphere we see today. However, what happened to all the oxygen has remained a mystery.
“We also have plans to use this model in the future to study Venus, which may have once had about the same amount of water as the Earth but is now very dry,” said Schaefer. “There is very little O2 left in Venus’ atmosphere, so this model would help us understand what happened to that oxygen (whether it was lost to space or absorbed by the planet’s mantle).”
Schaefer predicts that their model will also assist researchers with the study of other, similar exoplanets. One example is the TRAPPIST-1 system, which contains three planets that may lie with the star’s the habitable zone. But as Schaefer put it, the real value lies in the fact that we are more likely to find “Venus-like” worlds down the road:
“Most of the rocky planets that we know of and will discover in the near future will likely be hotter than the Earth or even Venus, just because it is easier to detect hotter planets. So there are a lot of planets out there similar to GJ 1132b just waiting to be studied!”
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. It’s scientists are dedicated to studying the origin, evolution and future of the universe.
And be sure to check out this video, courtesy of MIT news:
If you’ve heard me say “oot and aboot”, you know I’m a Canadian. And we Canadians are accustomed to a little cold. Okay, a LOT of cold. It’s not so bad here on the West Coast, but folks from Winnepeg can endure temperatures colder than the surface of Mars. Seriously, who lives like that?
And on one of those cold days, even on a clear sunny day, the Sun is pointless and worthless. As the bone chilling cold numbs your fingers and toes, it’s as if the Sun itself has gone cold, sapping away all the joy and happiness in the world. And don’t get me started about the rain. Clearly, I need to take more tropical vacations.
But we know the Sun isn’t cold at all, it’s just that the atmosphere around you feels cold. The surface of the Sun is always the same balmy 5,500 degrees Celsius. Just to give you perspective, that’s hot enough to melt iron, nickel. Even carbon melts at 2500 C. So, no question, the Sun is hot.
And you know that the Sun is hot because it’s bright. There are actually photons streaming from the Sun at various wavelengths, from radio, infrared, through the visible spectrum, and into the ultraviolet. There are even X-ray photons blasting off the Sun.
If the Sun was cooler, it would look redder, just like a cooler red dwarf star, and if the Sun was hotter, it would appear more blue. But could you have a star that’s cooler, or even downright cold?
The answer is yes, you just have to be willing to expand your definition of what a star is.
Under the normal definition, a star is a collection of hydrogen, helium and other elements that came together by mutual gravity. The intense gravitational pressure of all that mass raised temperatures at the core of the star to the point that hydrogen could be fused into helium. This reaction releases more energy than it takes, which causes the Sun to emit energy.
The coolest possible red dwarf star, one with only 7.5% the mass of the Sun, will still have a temperature of about 2,300 C, a little less than the melting point of carbon.
But if a star doesn’t have enough mass to ignite fusion, it becomes a brown dwarf. It’s heated by the mechanical action of all that mass compressing inward, but it’s cooler. Average brown dwarfs will be about 1,700 C, which actually, is still really hot. Like, molten rock hot.
Brown dwarfs can actually get a lot cooler, a new class of these “stars” were discovered by the WISE Space Observatory that start at 300 degrees, and go all the way down to about 27 degrees, or room temperature. This means there are stars out there that you could touch.
Except you couldn’t, because they’d still have more than a dozen times the mass of Jupiter, and would tear your arm off with their intense gravity. And anyway, they don’t a solid surface. No, you can’t actually touch them.
That’s about as cold as stars get, today, in the Universe.
But if you’re willing to be very very patient, then it’s a different story. Our own Sun will eventually run out of fuel, die and become a white dwarf. It’ll start out hot, but over the eons, it’ll cool down, eventually becoming the same temperature as the background level of the Universe – just a few degrees above absolute zero. Astronomers call these black dwarfs.
We’re talking a long long time, though, in fact, in the 13.8 billion years that the Universe has been around, no white dwarfs have had enough time to cool down significantly. In fact, it would take about a quadrillion years to get within a few degrees of the cosmic microwave background radiation temperature.
This article was originally published in 2008, but has been updated several times now to keep track with our advancing knowledge of the cosmos!
My six-year old daughter is a question-asking machine. We were driving home from school a couple of days ago, and she was grilling me about the nature of the Universe. One of her zingers was, “What’s the Biggest Star in the Universe”? I had an easy answer. “The Universe is a big place,” I said, “and there’s no way we can possibly know what the biggest star is”. But that’s not a real answer.
So she refined the question. “What’s the biggest star that we know of?” Of course, I was stuck in the car, and without access to the Internet. But once I got back home, and was able to do some research, I learned the answer and thought I’d share it with the rest of you But to answer it fully, some basic background information needs to be covered first. Ready?
Solar Radius and Mass:
When talking about the size of stars, it’s important to first take a look at our own Sun for a sense of scale. Our familiar star is a mighty 1.4 million km across (870,000 miles). That’s such a huge number that it’s hard to get a sense of scale. Speaking of which, the Sun also accounts for 99.9% of all the matter in our Solar System. In fact, you could fit one million planet Earths inside the Sun.
Using these values, astronomers have created the terms “solar radius” and “solar mass”, which they use to compare stars of greater or smaller size and mass to our own. A solar radius is 690,000 km (432,000 miles) and 1 solar mass is 2 x 1030 kilograms (4.3 x 1030 pounds). That’s 2 nonillion kilograms, or 2,000,000,000,000,000,000,000,000,000,000 kg.
Another thing worth considering is the fact that our Sun is pretty small, as stars go. As a G-type main-sequence star (specifically, a G2V star), which is commonly known as a yellow dwarf, its on the smaller end of the size chart (see above). While it is certainly larger than the most common type of star – M-type, or Red Dwarfs – it is itself dwarfed (no pun!) by the likes of blue giants and other spectral classes.
To break it all down, stars are grouped based on their essential characteristics, which can be their spectral class (i.e. color), temperature, size, and brightness. The most common method of classification is known as the Morgan–Keenan (MK) system, which classifies stars based on temperature using the letters O, B, A, F, G, K, and M, – O being the hottest and M the coolest. Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g. O1 to M9 are the hottest to coldest stars).
In the MK system, a luminosity class is added using Roman numerals. These are based on the width of certain absorption lines in the star’s spectrum (which vary with the density of the atmosphere), thus distinguishing giant stars from dwarfs. Luminosity classes 0 and I apply to hyper- or supergiants; classes II, III and IV apply to bright, regular giants, and subgiants, respectively; class V is for main-sequence stars; and class VI and VII apply to subdwarfs and dwarf stars.
There is also the Hertzsprung-Russell diagram, which relates stellar classification to absolute magnitude (i.e. intrinsic brightness), luminosity, and surface temperature. The same classification for spectral types are used, ranging from blue and white at one end to red at the other, which is then combined with the stars Absolute Visual Magnitude (expressed as Mv) to place them on a 2-dimensional chart (see above).
On average, stars in the O-range are hotter than other classes, reaching effective temperatures of up to 30,000 K. At the same time, they are also larger and more massive, reaching sizes of over 6 and a half solar radii and up to 16 solar masses. At the lower end, K and M type stars (orange and red dwarfs) tend to be cooler (ranging from 2400 to 5700 K), measuring 0.7 to 0.96 times that of our Sun, and being anywhere from 0.08 to 0.8 as massive.
Based on the full of classification of our Sun (G2V), we can therefore say that it a main-sequence star with a temperature around 5,800K. Now consider another famous star system in our galaxy – Eta Carinae, a system containing at least two stars located around 7500 light-years away in the direction of the constellation Carina. The primary of this system is estimated to be 250 times the size of our Sun, a minimum of 120 solar masses, and a million times as bright – making it one of the biggest and brightest stars ever observed.
There is some controversy over this world’s size though. Most stars blow with a solar wind, losing mass over time. But Eta Carinae is so large that it casts off 500 times the mass of the Earth every year. With so much mass lost, it’s very difficult for astronomers to accurately measure where the star ends, and its stellar wind begins. Also, it is believed that Eta Carinae will explode in the not-too-distant future, and it will be the most spectacular supernovae humans have ever seen.
In terms of sheer mass, the top spot goes to R136a1, a star located in the Large Magellanic Cloud, some 163,000 light-years away. It is believed that this star may contain as much as 315 times the mass of the Sun, which presents a conundrum to astronomers since it was believed that the largest stars could only contain 150 solar masses. The answer to this is that R136a1 was probably formed when several massive stars merged together. Needless to say, R136a1 is set to detonate as a hypernova, any day now.
In terms of large stars, Betelgeuse serves as a good (and popular) example. Located in the shoulder of Orion, this familiar red supergiant has a radius of 950-1200 times the size of the Sun, and would engulf the orbit of Jupiter if placed in our Solar System. In fact, whenever we want to put our Sun’s size into perspective, we often use Betelgeuse to do it (see below)!
Yet, even after we use this hulking Red Giant to put us in our place, we are still just scratching the surface in the game of “who’s the biggest star”. Consider WOH G64, a red supergiant star located in the Large Magellanic Cloud, approximately 168,000 light years from Earth. At 1.540 solar radii in diameter, this star is currently one of the largest in the known universe.
But there’s also RW Cephei, an orange hypergiant star in the constellation Cepheus, located 3,500 light years from Earth and measuring 1,535 solar radii in diameter. Westerlund 1-26 is also pretty huge, a red supergiant (or hypergiant) located within the Westerlund 1 super star cluster 11,500 light-years away that measures 1,530 solar radii in diameter. Meanwhile, V354 Cephei and VX Sagittarii are tied when it comes to size, with both measuring an estimated 1,520 solar radii in diameter.
The Largest Star: UY Scuti
As it stands, the title of the largest star in the Universe (that we know of) comes down to two contenders. For example, UY Scuti is currently at the top of the list. Located 9.500 light years away in the constellation Scutum, this bright red supergiant and pulsating variable star has an estimated average median radius of 1,708 solar radii – or 2.4 billion km (1.5 billion mi; 15.9 AU), thus giving it a volume 5 billion times that of the Sun.
However, this average estimate includes a margin of error of ± 192 solar radii, which means that it could be as large as 1900 solar radii or as small as 1516. This lower estimate places it beneath stars like as V354 Cephei and VX Sagittarii. Meanwhile, the second star on the list of the largest possible stars is NML Cygni, a semiregular variable red hypergiant located in the Cygnus constellation some 5,300 light-years from Earth.
Due to the location of this star within a circumstellar nebula, it is heavily obscured by dust extinction. As a result, astronomers estimate that its size could be anywhere from 1,642 to 2,775 solar radii, which means it could either be the largest star in the known Universe (with a margin of 1000 solar radii) or indeed the second largest, ranking not far behind UY Scuti.
And up until a few years ago, the title of biggest star went to VY Canis Majoris; a red hypergiant star in the Canis Major constellation, located about 5,000 light-years from Earth. Back in 2006, professor Roberta Humphrey of the University of Minnesota calculated its upper size and estimated that it could be more than 1,540 times the size of the Sun. Its average estimated mass, however, is 1420, placing it in the no. 8 spot behind V354 Cephei and VX Sagittarii.
These are the biggest star that we know of, but the Milky way probably has dozens of stars that are even larger, obscured by gas and dust so we can’t see them. But even if we cannot find these stars, it is possible to theorize about their likely size and mass. So just how big can stars get? Once again, Professor Roberta Humphreys of the University of Minnesota provided the answer.
As she explained when contacted, the largest stars in the Universe are the coolest. So even though Eta Carinae is the most luminous star we know of, it’s extremely hot – 25,000 Kelvin – and therefore only 250 solar radii big. The largest stars, in contrast, will be cool supergiants. Case in point, VY Canis Majoris is only 3,500 Kelvin, and a really big star would be even cooler.
At 3,000 Kelvin, Humphreys estimates that cool supergiant would be as big as 2,600 times the size of the Sun. This is below the upper estimates for NML Cygni, but above the average estimates for both it and UY Scutii. Hence, this is the upper limit of a star (at least theoretically and based on all the information we have to date).
But as we continue to peer into the Universe with all of our instruments, and explore it up close through robotic spacecraft and crewed missions, we are sure to find new and exciting things that will confound us further!
And be sure to check out this great animation that shows the size of various objects in space, starting with our Solar System’s tiny planets and finally getting to UY Scuti. Enjoy!
Three more potentially Earthlike worlds have been discovered in our galactic backyard, announced online today by the European Southern Observatory. Researchers using the 60-cm TRAPPIST telescope at ESO’s La Silla observatory in Chile have identified three Earth-sized exoplanets orbiting a star just 40 light-years away.
The star, originally classified as 2MASS J23062928-0502285 but now known more conveniently as TRAPPIST-1, is a dim “ultracool” red dwarf star only .05% as bright as our Sun . Located in the constellation Aquarius, it’s now the 37th-farthest star known to host orbiting exoplanets.
The exoplanets were discovered via the transit method (TRAPPIST stands for Transiting Planets and Planetesimals Small Telescope) through which the light from a star is observed to dim slightly by planets passing in front of it from our point of view. This is the same method that NASA’s Kepler spacecraft has used to find over 1,000 confirmed exoplanets.
As an ultracool dwarf TRAPPIST-1 is a very small and dim and isn’t easily visible from Earth, but it’s its very dimness that has allowed its planets to be discovered with existing technology. Their subtle silhouettes may have been lost in the glare of larger, brighter stars.
Follow-up measurements of the three exoplanets indicated that they are all approximately Earth-sized and have temperatures ranging from Earthlike to Venuslike (which is, admittedly, a fairly large range.) They orbit their host star very closely with periods measured in Earth days, not years.
“With such short orbital periods, the planets are between 20 and 100 times closer to their star than the Earth to the Sun,” said Michael Gillon, lead author of the research paper. “The structure of this planetary system is much more similar in scale to the system of Jupiter’s moons than to that of the Solar System.”
Although these three new exoplanets are Earth-sized they do not yet classify as “potentially habitable,” at least by the standards of the Planetary Habitability Laboratory (PHL) operated by the University of Puerto Rico at Arecibo. The planets fall outside PHL’s required habitable zone; two are too close to the host star and one is too far away.
In addition there are certain factors that planets orbiting ultracool dwarfs would have to contend with in order to be friendly to life, not the least of which is the exposure to energetic outbursts from solar flares.
This does not guarantee that the exoplanets are completely uninhabitable, though; it’s entirely possible that there are regions on or within them where life could exist, not unlike Mars or some of the moons in our own Solar System.
The exoplanets are all likely tidally locked in their orbits, so even though the closest two are too hot on their star-facing side and too cold on the other, there may be regions along the east or west terminators that maintain a climate conducive to life.
“Now we have to investigate if they’re habitable,” said co-author Julien de Wit at MIT in Cambridge, Mass. “We will investigate what kind of atmosphere they have, and then will search for biomarkers and signs of life.”
Discovering three planets orbiting such a small yet extremely common type of star hints that there are likely many, many more such worlds in our galaxy and the Universe as a whole.
“So far, the existence of such ‘red worlds’ orbiting ultra-cool dwarf stars was purely theoretical, but now we have not just one lonely planet around such a faint red star but a complete system of three planets,” said study co-author Emmanuel Jehin.
The team’s research was presented in a paper entitled “Temperate Earth-sized planets transiting a nearby ultracool dwarf star” and will be published in Nature.
Note: the original version of this article described 2MASS J23062928-0502285 (TRAPPIST-1) as a brown dwarf based on its classification on the Simbad archive. But at M8V it is “definitely a star,” according to co-author Julien de Wit in an email, although at the very low end of the red dwarf line. Corrections have been made above.
While our Sun will only survive for about 5 billion more years, smaller, cooler red dwarfs can last for trillions of years. What’s the secret to their longevity?
You might say our Sun will last a long time. And sure, another 5 billion years or so of main sequence existence does sound pretty long lived. But that’s nothing compared to the least massive stars out there, the red dwarfs.
These tiny stars can have just 1/12th the mass of the Sun, but instead of living for a paltry duration, they can last for trillions of years. What’s the secret to their longevity? Is it Botox?
To understand why red dwarfs have such long lifespans, we’ll need to take a look at main sequence stars first, and see how they’re different. If you could peel back the Sun like a grapefruit, you’d see juicy layers inside.
In the core, immense pressure and temperature from the mass of all that starstuff bears down and fuses atoms of hydrogen into helium, releasing gamma radiation.
Outside the core is the radiative zone, not hot enough for fusion. Instead, photons of energy generated in the core are emitted and absorbed countless times, taking a random journey to the outermost layer of the star.
And outside the radiative zone is the convective zone, where superheated globs of hot plasma float up to the surface, where they release their heat into space.
Then they cool down enough to sink back through the Sun and pick up more heat. Over time, helium builds up in the core. Eventually, this core runs out of hydrogen and it dies. Even though the core is only a fraction of the total mass of hydrogen in the Sun, there’s no mechanism to mix it in.
A red dwarf is fundamentally different than a main sequence star like the Sun. Because it has less mass, it has a core, and a convective zone, but no radiative zone. This makes all the difference.
The convective zone connects directly to the core of the red dwarf, the helium byproduct created by fusion is spread throughout the star. This convection brings fresh hydrogen into the core of the star where it can continue the fusion process.
By perfectly using all its hydrogen, the lowest mass red dwarf could sip away at its hydrogen fuel for 10 trillion years.
One of the biggest surprises in modern astronomy is just how many of these low mass red dwarf worlds have planets. And some of the most Earthlike worlds ever seen have been found around red dwarf stars. Planets with roughly the mass of Earth, orbiting within their star’s habitable zone, where liquid water could be present.
One of the biggest problems with red dwarfs is that they can be extremely variable. For example, 40% of a red dwarf’s surface could be covered with sunspots, decreasing the amount of radiation it produces, changing the size of its habitable zone.
Other red dwarfs produce powerful stellar flares that could scour a newly forming world of life. DG Canes Venaticorum recently generated a flare 10,000 times more powerful than anything ever seen from the Sun. Any life caught in the blast would have a very bad day.
Fortunately, red dwarfs only put out these powerful flares in the first billion years or so of their lives. After that, they settle down and provide a nice cozy environment for trillions of years. Long enough for life to prosper we hope.
In the distant future, some superintelligent species may figure out how to properly mix the hydrogen back into the Sun, removing the helium, if they do, they’ll add billions of years to the Sun’s life.
It seems like such a shame for the Sun to die with all that usable hydrogen sitting just a radiative zone away from fusion.
Have you got any ideas on how we could mix up the hydrogen in the Sun and remove the helium? Post your wild ideas in the comments!