Most exoplanets orbit red dwarf stars because they're the most plentiful stars. This is an artist's illustration of what the TRAPPIST-1 system might look like from a vantage point near planet TRAPPIST-1f (at right). Credits: NASA/JPL-Caltech
In February of 2017, a team of European astronomers announced the discovery of a seven-planet system orbiting the nearby star TRAPPIST-1. Aside from the fact that all seven planets were rocky, there was the added bonus of three of them orbiting within TRAPPIST-1’s habitable zone. As such, multiple studies have been conducted that have sought to determine whether or not any planets in the system could be habitable.
When it comes to habitability studies, one of the key factors to consider is the age of the star system. Basically, young stars have a tendency to flare up and release harmful bursts of radiation while planets that orbit older stars have been subject to radiation for longer periods of time. Thanks to a new study by a pair of astronomers, it is now known that the TRAPPIST-1 system is twice as old as the Solar System.
Artist's impression of the exoplanet GJ 1132 b, which orbits the red dwarf star GJ 1132. Astronomers have managed to detect the atmosphere of this Earth-like planet. Credit: MPIA
In 2015, astronomers discovered an intriguing extrasolar planet located in a star system some 39 light years from Earth. Despite orbiting very close to its parent star, this “Venus-like” planet – known as GJ 1138b – appeared to still be cool enough to have an atmosphere. In short order, a debate ensued as to what kind of atmosphere it might have, whether it was a “dry Venus” or a “wet Venus”.
And now, thanks to the efforts of an international team of researchers, the existence of an atmosphere has been confirmed around GJ 1138b. In addition to settling the debate about the nature of this planet, it also marks the first time that an atmosphere has been detected around a low-mass Super-Earth. On top of that, GJ 1138b is now the farthest Earth-like planet that is known to have an atmosphere.
Artist’s impression of the “Venus-like” exoplanet GJ 1132b. Credit: cfa.harvard.edu
Using the GROND imager on the La Silla Observatory’s 2.2m ESO/MPG telescope, the team monitored GJ 1132b in different wavelengths as it transited in front of its parent star. Given the planet’s orbital period (1.6 days), these transits happen quite often, which presented plenty of opportunities to view it pass in front of its star. In so doing, they monitored the star for slight decreases in its brightness.
As Dr. Southworth explained to Universe via email, these observations confirmed the existence of an atmosphere:
“What we did was to measure the amount of dimming at 7 different wavelengths in optical and near-infrared light. At one of these wavelengths (IR) the planet seemed to be slightly bigger. This indicated that the planet has a large atmosphere around it which allows most of the starlight to pass through, but is opaque at one wavelength.”
The team members from the University of Cambridge and the MPIA then conducted simulations to see what this atmosphere’s composition could be. Ultimately, they concluded that it most likely has a thick atmosphere that is rich in water and/or methane – which contradicted recent theories that the planet had a thin and tenuous atmosphere (i.e. a “dry Venus”).
The ESO’s Paranal Observatory, located in the Atacama Desert of Chile. Credit: ESO
It was also the first time that an atmosphere has been confirmed around a planet that is not significantly greater in size and mass to Earth. In the past, astronomers have detected atmospheres around many other exoplanets. But in these cases, the planets were either gas giants or planets that were many times Earth’s size and mass (aka. “Super-Earths”). GJ 1132b, however, is 1.6 times as massive as Earth, and measures 1.4 Earth radii.
In addition, these findings are a significant step in the search for life beyond our Solar System. At present, astronomers seek to determine the chemical composition of a planet’s atmosphere to determine if it could be habitable. Where the right combination of chemical imbalances exist, the presence of living organisms is seen as a possible cause.
By being able to determine that a planet at lower end of the super-Earth scale has an atmosphere, we are one step closer to being able to determine exoplanet habitability. The detection of an atmosphere-bearing planet around an M-type (red dwarf) star is also good news in and of itself. Low-mass red dwarf stars are the most common star in the galaxy, and recent findings have indicated that they might be our best shot for finding habitable worlds.
Besides detecting several terrestrial planets around red dwarf stars in recent years – including seven around a single star (TRAPPIST-1) – there is also research that suggests that these stars are capable of hosting large numbers of planets. At the same time, there have been concerns about whether red dwarfs are too variable and unstable to support habitable worlds.
Artist’s impression of Kepler-1649b, the “Venus-like” world orbiting an M-class star 219 light-years from Earth. Credit: Danielle Futselaar
As Southworth explained, spotting an atmosphere around a planet that closely orbits a red dwarf could help bolster the case for red dwarf habitability:
“One of the big issues has been that very-low-mass stars typically have strong magnetic fields and thus throw out a lot of X-ray and ultraviolet light. These high-energy photons tend to destroy molecules in atmospheres, and might also evaporate them completely. The fact that we have detected an atmosphere around GJ 1132b means that this kind of planet is indeed capable of retaining an atmosphere for billions of years, even whilst being bombarded by the high-energy photons from their host stars.
In the future, GJ 1132b is expected to be a high-priority target for study with the Hubble Space Telescope, the Very Large Telescope (VLT) at the Paranal Observatory in Chile, and next-generation telescopes like the James Webb Space Telescope (scheduled for launch in 2018). Already, observations are being made, and the results are being eagerly anticipated.
I’m sure I’m not the only one who would like to hear what astronomers discover as they set their sights on this nearby star system and it’s Venus-like world! In the meantime, be sure to check out this video about GJ 1132b, courtesy of MIT news:
Artist's concept of the TRAPPIST-1 star system, an ultra-cool dwarf that has seven Earth-size planets orbiting it. We're going to keep finding more and more solar systemsl like this, but we need observatories like WFIRST, with starshades, to understand the planets better. Credits: NASA/JPL-Caltech
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.
This illustration shows TRAPPIST-1 in relation to our Sun. Image: By ESO – http://www.eso.org/public/images/eso1615e/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=48532941
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.
Tidal locking is not rare. For example, Pluto and its moon Charon (above) are tidally locked to each other, as are the Earth and the Moon. But can life appear and survive on a planet tidally locked to its star? Credit: NASA/JHUAPL/SwRI
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.
Artist’s impression of an “eyeball” planet, a water world where the sun-facing side is able to maintain a liquid-water ocean. Credit and Copyright: eburacum45/ DeviantArt
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.
Computer generated simulation of an asteroid strike on the Earth. Credit: Don Davis/AFP/Getty Images
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.
This artist’s impression shows the European Extremely Large Telescope (E-ELT) in its enclosure. The E-ELT will be a 39-metre aperture optical and infrared telescope. ESO/L. Calçada
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.
Artist's depiction of a waterworld. A new study suggests that Earth is in a minority when it comes to planets, and that most habitable planets may be greater than 90% ocean. Credit: David A. Aguilar (CfA)
The discovery of an exoplanet candidate orbiting around nearby Proxima Centauri has certainly been exciting news. In addition to being the closest exoplanet to our Solar System yet discovered, all indications point to it being terrestrial and located within the stars’ circumstellar habitable zone. However, this announcement contained its share of bad news as well.
For one, the team behind the discovery indicated that given the nature of its orbit around Proxima Centauri, the planet likely in terms of how much water it actually had on its surface. But a recent research study by scientists from the University of Marseilles and the Carl Sagan Institute may contradict this assessment. According to their study, the exoplanet’s mass may consist of up to 50% water – making it an “ocean planet”.
According to the findings of the Pale Red Dot team, Proxima Centauri b orbits its star at an estimated distance of 7 million kilometers (4.35 million mi) – only 5% of the Earth’s distance from the Sun. It also orbits Proxima Centauri with an orbital period of 11 days, and either has a synchronous rotation, or a 3:2 orbital resonance (i.e. three rotations for every two orbits).
Artist’s impression of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. Credit: ESO/M. Kornmesser
Because of this, liquid water is likely to be confined to either the sun-facing side of the planet (in the case of a synchronous rotation), or in its tropical zone (in the case of a 3:2 resonance). In addition, the radiation Proxima b receives from its red dwarf star would be significantly higher than what we are used to here on Earth.
However, according to a study led by Bastien Brugger of the Astrophysics Laboratory at the University of Marseilles, Proxima b may be wetter than we previously thought. For the sake of their study, titled “Possible Internal Structures and Compositions of Proxima Centauri b” (which was accepted for publication in The Astrophysical Journal Letters), the research team used internal structure models to compute the radius and mass of Proxima b.
Their models were based on the assumptions that Proxima b is both a terrestrial planet (i.e. composed of rocky material and minerals) and did not have a massive atmosphere. Based on these assumptions, and mass estimates produced by the Pale Red Dot survey (~1.3 Earth masses), they concluded that Proxima b has a radius that is between 0.94 and 1.4 times that of Earth, and a mass that is roughly 1.1 to 1.46 times that of Earth.
As Brugger told Universe Today via email:
“We listed all compositions that Proxima b could have, and ran the model for each of them (that makes about 5000 simulations), giving us each time the corresponding planet radius. We finally excluded all the results that were not compatible with a planetary body, basing on the formation conditions of our solar system (since we do not know these conditions for the Proxima Centauri system). And thus, we obtained a range of possible planet radii for Proxima b, going from 0.94 to 1.40 times the radius of the Earth.”
Tidally-locked planets like Gliese 581 g (artist’s impression) are likely to be “eyeball” worlds, with a warm-water ocean on the sun-facing side surrounded by ice. Credit: Lynette Cook/NSF
This range in size allows for some very different planetary compositions. At the lower end, being slightly smaller but a bit more massive than Earth, Proxima b would likely be a Mercury-like planet with a 65% core mass fraction. However, at the higher end of the radii and mass estimates, Proxima b would likely be half water by mass.
“If the radius is 0.94 Earth radii, then Proxima b is fully rocky with a huge metallic core (like Mercury in the solar system),” said Brugger. “On the opposite, Proxima b can reach a radius of 1.40 only if it harbors a massive amount of water (50% of the total planet mass), and in this case it would be an ocean planet, with a 200 km deep liquid ocean! Below that, the pressure is so high that the water would turn into ice, forming a ~3000 km thick ice layer (Under which there would be a core made of rocks).”
In other words, Proxima b could be an “eyeball planet”, where the sun-facing side has a liquid ocean surface, while the dark side is covered in frozen ice. Recent studies have suggested that this may be the case with planet’s that orbit within the habitable zones of red dwarf stars, where tidal-locking ensures that only one side gets the heat necessary to maintain liquid water on the surface.
On the other hand, if it has an orbital resonance of 3:2, its likely to have a double-eyeball pattern – with liquid oceans in both the eastern and western hemispheres – while remaining frozen at the terminators and poles. However, if the lower estimates should be true, then Proxima b is likely to be a rocky, dense planet where liquid water is rare on one side, and frozen on the other.
Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. New research suggest the planet may be more watery than previously thought. Credit: ESO
But perhaps the most interesting aspect of the the research is that it offers a glimpse into the likelihood of Proxima b being habitable. Ever since its discovery, the question of whether or not the planet can support life has remained contentious. But as Brugger explained:
“The interesting part is that all the cases we considered are compatible with a habitable planet. So if the planet radius is finally measured (in some months or years), two cases are possible: either (i) the measurement lies within the 0.94-1.40 range and we will be able to give the exact composition of the planet (and not only a range of possibilities), or (ii) the measured radius is out of this range, and we will know that the planet is not habitable. The case where Proxima b is an ocean planet is particularly interesting, because this kind of planet does not need an atmosphere of oxygen and nitrogen (like on the Earth) to harbor life, since it can develop in its huge ocean.”
But of course, these scenarios are based on the assumption that Proxima b has a lot in common with the planets of our own Solar System. It’s also based on the assumption that the planet is indeed about 1.3 Earth masses. Until the planet can be observed making a transit of Proxima Centauri, astronomers won’t know for sure how massive it is.
Ultimately, we’re still a long ways away from determining Proxima b’s exact size, composition, and surface features – to say nothing about whether or not it can actually support life. Nevertheless, research like this is beneficial in that it helps us to come up with constrains on what kind of planetary conditions could exist there.
And who knows? Someday, we may be able to send probes or crewed missions to the planet, and perhaps they will beam back images of sentient beings navigating vast oceans, looking for some fabled parcel of land they heard about? God I hope not! Once was more than enough!
Artist’s impression of a sunset seen from the surface of an Earth-like exoplanet. Credit: ESO/L. Calçada
The hunt for exoplanets has been heating up in recent years. Since it began its mission in 2009, over four thousand exoplanet candidates have been discovered by the Kepler mission, several hundred of which have been confirmed to be “Earth-like” (i.e. terrestrial). And of these, some 216 planets have been shown to be both terrestrial and located within their parent star’s habitable zone (aka. “Goldilocks zone”).
But in what may prove to be the most exciting find to date, the German weekly Der Spiegel announced recently that astronomers have discovered an Earth-like planet orbiting Proxima Centauri, just 4.25 light-years away. Yes, in what is an apparent trifecta, this newly-discovered exoplanet is Earth-like, orbits within its sun’s habitable zone, and is within our reach. But is this too good to be true?
For over a century, astronomers have known about Proxima Centauri and believed that it is likely to be part of a trinary star system (along with Alpha Centauri A and B). Located just 0.237 ± 0.011 light years from the binary pair, this low-mass red dwarf star is also 0.12 light years (~7590 AUs) closer to Earth, making it the closest star system to our own.
In the past, the Kepler mission has revealed several Earth-like exoplanets that were deemed to be likely habitable. And recently, an international team of researchers narrowed the number of potentially-habitable exoplanets in the Kepler catalog down to the 20 that are most likely to support life. However, in just about all cases, these planets are hundreds (if not thousands) of light years away from Earth.
Knowing that there is a habitable planet that a mission from Earth could reach within our own lifetimes is nothing short of amazing! But of course, there is reason to be cautiously optimistic. Citing anonymous sources, the magazine stated:
“The still nameless planet is believed to be Earth-like and orbits at a distance to Proxima Centauri that could allow it to have liquid water on its surface — an important requirement for the emergence of life. Never before have scientists discovered a second Earth that is so close by.”
In addition, they claim that the discovery was made by the European Southern Observatory (ESO) using the La Silla Observatory‘s reflecting telescope. Coincidentally, it was this same observatory that announced the discovery of Alpha Centauri Bb back in 2012, which was also declared to be “the closest exoplanet to Earth”. Unfortunately, subsequent analysis cast doubt on its existence, claiming it was a spurious artifact of the data analysis.
Artist’s impression of the Earth-like exoplanet discovered orbiting Alpha Centauri B by the European Southern Observatory on October 17, 2012. Credit: ESO
However, according to Der Spiegel’s unnamed source – whom they claim was involved with the La Silla team that made the find – this latest discovery is the real deal, and was the result of intensive work. “Finding small celestial bodies is a lot of hard work,” the source was quoted as saying. “We were moving at the technically feasible limit of measurement.”
The article goes on to state that the European Southern Observatory (ESO) will be announcing the finding at the end of August. But according to numerous sources, in response to a request for comment by AFP, ESO spokesman Richard Hook refused to confirm or deny the discovery of an exoplanet around Proxima Centauri. “We are not making any comment,” he is reported as saying.
What’s more, the folks at Project Starshot are certainly excited by the news. As part of Breakthrough Initiatives – a program founded by Russian billionaire Yuri Milner to search for intelligent life (with backing from Stephen Hawking and Mark Zuckerberg) – Starshot intends to send a laser-sail driven-nanocraft to Alpha Centauri in the coming years.
This craft, they claim, will be able to reach speeds of up to 20% the speed of light. At this speed, it will able to traverse the 4.37 light years that lie between Earth and Alpha Centauri in just 20 years. But with the possible discovery of an Earth-like planet orbiting Proxima Centauri, which lies even closer, they may want to rethink that objective.
Project Starshot, an initiative sponsored by the Breakthrough Foundation, is intended to be humanity’s first interstellar voyage. Credit: breakthroughinitiatives.org
As Professor Phillip Lubin – a professor at the University of California, Santa Barbara, the brains behind Project Starshot, and a key advisor to NASA’s DEEP-IN program – told Universe Today via email:
“The discovery of possible planet around Proxima Centauri is very exciting. It makes the case of visiting nearby stellar systems even more compelling, though we know there are many exoplanets around other nearby stars and it is very likely that the Alpha Centauri system will also have planets.”
Naturally, there is the desire (especially amongst exoplanet enthusiasts) to interpret the ESO’s refusal to comment either way as a sort of tacit confirmation. And knowing that industry professionals are excited it about it does lend an air of legitimacy. But of course, assuming anything at this point would be premature.
If the statements made by the unnamed source, and quoted by Der Speigel, are to be taken at face value, then confirmation (or denial) will be coming shortly. In the meantime, we’ll all just need to be patient. Still, you have to admit, it’s an exciting prospect: an Earth-like planet that’s actually within reach! And with a mission that could make it there within our own lifetimes. This is the stuff good science fiction is made of, you know.
Artist’s impression of how an an Earth-like exoplanet might look. Credit: ESO.
The ongoing hunt for exoplanets has yielded some very interesting returns in recent years. All told, the Kepler mission has discovered more than 4000 candidates since it began its mission in March of 2009. Amidst the many “Super-Jupiters” and assorted gas giants (which account for the majority of Kepler’s discoveries) astronomers have been particularly interested in those exoplanets which resemble Earth.
And now, an international team of scientists has finished perusing the Kepler catalog in an effort to determine just how many of these planets are in fact “Earth-like”. Their study, titled “A Catalog of Kepler Habitable Zone Exoplanet Candidates” (which will be published soon in the AstrophysicalJournal), explains how the team discovered 216 planets that are both terrestrial and located within their parent star’s “habitable zone” (HZ).
The international team was made up of researchers from NASA, San Francisco State University, Arizona State University, Caltech, University of Hawaii-Manoa, the University of Bordeaux, Cornell University and the Harvard-Smithsonian Center for Astrophysics. Having spent the past three years looking over the more than 4000 entries, they have determined that 20 of the candidates are most like Earth (i.e. likely habitable).
Figure showing the habitable zone for different types of stars, as well as the location of terrestrial size Kepler candidates. Credit: Chester Harman
As Stephen Kane, an associate professor of physics and astronomy at San Fransisco University and lead author of the study, explained in a recent statement:
“This is the complete catalog of all of the Kepler discoveries that are in the habitable zone of their host stars. That means we can focus in on the planets in this paper and perform follow-up studies to learn more about them, including if they are indeed habitable.”
In addition to isolating 216 terrestrial planets from the Kepler catalog, they also devised a system of four categories to determine which of these were most like Earth. These included “Recent Venus”, where conditions are like that of Venus (i.e. extremely hot); “Runaway Greenhouse”, where planets are undergoing serious heating; “Maximum Greenhouse”, where planets are within their star’s HZ; and “Recent Mars”, where conditions approximate those of Mars.
From this, they determined that of the Kepler candidates, 20 had radii less than twice that of Earth (i.e. on the smaller end of the Super-Earth category) and existed within their star’s HZ. In other words, of all the planets discovered in our local Universe, they were able to isolate those where liquid water can exist on the surface, and the gravity would likely be comparable to Earth’s and not crushing!
Earlier today, NASA announced that Kepler had confirmed the existence of 1,284 new exoplanets, the most announced at any given time. Credit: NASA
This is certainly exciting news, since one of the most important aspects of exoplanet hunting has been finding worlds that could support life. Naturally, it might sound a bit anthropocentric or naive to assume that planets which have similar conditions to our own would be the most likely places for it to emerge. But this is what is known as the “low-hanging fruit” approach, where scientists seek out conditions which they know can lead to life.
“There are a lot of planetary candidates out there, and there is a limited amount of telescope time in which we can study them,” said Kane. “This study is a really big milestone toward answering the key questions of how common is life in the universe and how common are planets like the Earth.”
Professor Kane is renowned for being one of the world’s leading “planet-hunters”. In addition to discovering several hundred exoplanets (using data obtained by the Kepler mission) he is also a contributor to two upcoming satellite missions – the NASA Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency’s Characterizing ExOPLanet Satellite (CHEOPS).
These next-generation exoplanet hunters will pick up where Kepler left off, and are likely to benefit greatly from this recent study.
Artist's concept of the prototype starshade, a giant structure designed to block the glare of stars so that future space telescopes can take pictures of planets. Credit: NASA/JPL
For countless generations, people have looked up at the stars and wondered if life exists somewhere out there, perhaps on planets much like ours. But it has only been in recent decades that we have been able to confirm the existence of extrasolar planets (aka. exoplanets) in other star systems. In fact, between 1988 and April 20th of 2016, astronomers have been able to account for the existence of 2108 planets in 1350 different star systems, including 511 multiple planetary systems.
Most of these discoveries have taken place within just the past three years, thanks to improvements in our detection methods, and the deployment of the Kepler space observatory in 2009. Looking ahead, astronomers hope to improve on these methods even further with the introduction of the Starshade, a giant space structure designed to block the glare of stars, thus making it easier to find planets – and perhaps another Earth!
The terrestrial planets of our Solar System at approximately relative sizes. From left, Mercury, Venus, Earth and Mars. Credit: Lunar and Planetary Institute
In studying our Solar System over the course of many centuries, astronomers learned a great deal about the types of planets that exist in our universe. This knowledge has since expanded thanks to the discovery of extrasolar planets, many of which are similar to what we have observed here at home.
For example, while hundreds of gas giants of varying size have been detected (which are easier to detect because of their size), numerous planets have also been spotted that are similar to Earth – aka. “Earth-like”. These are what is known as terrestrial planets, a designation which says a lot about a planet how it came to be.
Definition:
Also known as a telluric or rocky planet, a terrestrial planet is a celestial body that is composed primarily of silicate rocks or metals and has a solid surface. This distinguishes them from gas giants, which are primarily composed of gases like hydrogen and helium, water, and some heavier elements in various states.
The term terrestrial planet is derived from the Latin “Terra” (i.e. Earth). Terrestrial planets are therefore those that are “Earth-like”, meaning they are similar in structure and composition to planet Earth.
Artist’s concept for the range of Earth-like extrasolar planets that have been discovered in recent years. Credit: NASA/JPL
Composition and Characteristics:
All terrestrial planets have approximately the same type of structure: a central metallic core composed of mostly iron, with a surrounding silicate mantle. Such planets have common surface features, which include canyons, craters, mountains, volcanoes, and other similar structures, depending on the presence of water and tectonic activity.
Terrestrial planets also have secondary atmospheres, which are generated through volcanism or comet impacts. This also differentiates them from gas giants, where the planetary atmospheres are primary and were captured directly from the original solar nebula.
Terrestrial planets are also known for having few or no moons. Venus and Mercury have no moons, while Earth has only the one (the Moon). Mars has two satellites, Phobos and Deimos, but these are more akin to large asteroids than actual moons. Unlike the gas giants, terrestrial planets also have no planetary ring systems.
The Earth’s interior structure, shown here as consisting of multiple “layers”. Credit: discovermagazine.com
Solar Terrestrial Planets:
All those planets found within the Inner Solar System – Mercury, Venus, Earth and Mars – are examples of terrestrial planets. Each are composed primarily of silicate rock and metal, which is differentiated between a dense, metallic core and a silicate mantle. The Moon is similar, but has a much smaller iron core.
Io and Europa are also satellites that have internal structures similar to that of terrestrial planets. In the case of the former, models of the moon’s composition suggest that the mantle is composed primarily of silicate rock and iron, which surrounds a core of iron and iron sulphide. Europa, on the other hand, is believed to have an iron core that is surrounded by an outer layer of water.
Dwarf planets, like Ceres and Pluto, and other large asteroids are similar to terrestrial planets in the fact that they do have a solid surface. However, they differ in that they are, on average, composed of more icy materials than rock.
Extrasolar Terrestrial Planets:
Most of the planets detected outside of the Solar System have been gas giants, owing to the fact that they are easier to spot. However, since 2005, hundreds of potentially terrestrial extrasolar planets have been found – mainly by the Kepler space mission. Most of these have been what is known as “super-Earths” (i.e. planets with masses between Earth’s and Neptune’s).
Examples of extrasolar terrestrial planets include Gliese 876 d, a planet that has a mass 7 to 9 times that of Earth. This planet orbits the red dwarf Gliese 876, which is located approximately 15 light years from Earth. The existence of three (or possibly four) terrestrial exoplanets was also confirmed between 2007 and 2010 in the Gliese 581 system, another red dwarf roughly 20 light years from Earth.
The smallest of these, Gliese 581 e, is only about 1.9 Earth masses, but orbits very close to the star. Two others, Gliese 581 c and Gliese 581 d, as well as a proposed fourth planet (Gliese 581 g) are more-massive super-Earths orbiting in or close to the habitable zone of the star. If true, this could mean that these worlds are potentially habitable Earth-like planets.
The first confirmed terrestrial exoplanet, Kepler-10b – a planet with between 3 and 4 Earth masses and located some 460 light years from Earth – was found in 2011 by the Kepler space mission. In that same year, the Kepler Space Observatory team released a list of 1235 extrasolar planet candidates, including six that were “Earth-size” or “super-Earth-size” (i.e. less than 2 Earth radii) and which were located within their stars’ habitable zones.
Since then, Kepler has discovered hundreds of planets ranging from Moon-sized to super-Earths, with many more candidates in this size range. As of January, 2013, 2740 planet candidates have been discovered.
Categories:
Scientists have proposed several categories for classifying terrestrial planets. Silicate planets are the standard type of terrestrial planet seen in the Solar System, which are composed primarily of a silicon-based rocky mantle and a metallic (iron) core.
Iron planets are a theoretical type of terrestrial planet that consists almost entirely of iron and therefore has a greater density and a smaller radius than other terrestrial planets of comparable mass. Planets of this type are believed to form in the high-temperature regions close to a star, and where the protoplanetary disk is rich in iron. Mercury is possible example, which formed close to our Sun and has a metallic core equal to 60–70% of its planetary mass.
Coreless planets are another theoretical type of terrestrial planet, one that consists of silicate rock but has no metallic core. In other words, coreless planets are the opposite of an iron planet. Coreless planets are believed to form farther from the star where volatile oxidizing material is more common. Though the Solar System has no coreless planets, chondrite asteroids and meteorites are common.
And then there are Carbon planets (aka. “diamond planets”), a theoretical class of planets that are composed of a metal core surrounded by primarily carbon-based minerals. Again, the Solar System has no planets that fit this description, but has an abundance of carbonaceous asteroids.
Until recently, everything scientists knew about planets – which included how they form and the different types that exist – came from studying our own Solar System. But with the explosion that has taken place in exoplanet discovery in the past decade, what we know about planets has grown significantly.
For one, we have come to understand that the size and scale of planets is greater than previously thought. What’s more, we’ve seen for the first time that many planets similar to Earth (which could also include being habitable) do in fact exist in other Solar Systems.
Who knows what we will find once we have the option of sending probes and manned missions to other terrestrial planets?
Astronomy Cast has episodes on the terrestrial planets including Mars, and an interview with Darin Ragozzine, one of the Kepler Space Mission scientists.
A conceptual image of the Transiting Exoplanet Survey Satellite.
Image Credit: MIT
The search for exoplanets is heating up, thanks to the deployment of space telescopes like Kepler and the development of new observation methods. In fact, over 1800 exoplanets have been discovered since the 1980s, with 850 discovered just last year. That’s quite the rate of progress, and Earth’s scientists have no intention of slowing down!
Hot on the heels of the Kepler mission and the ESA’s deployment of the Gaia space observatory last year, NASA is getting ready to launch TESS (the Transiting Exoplanet Survey Satellite). And to provide the launch services, NASA has turned to one of its favorite commercial space service providers – SpaceX.
The launch will take place in August 2017 from the Cape Canaveral Air Force Station in Florida, where it will be placed aboard a Falcon 9 v1.1 – a heavier version of the v 1.0 developed in 2013. Although NASA has contracted SpaceX to perform multiple cargo deliveries to the International Space Station, this will be only the second time that SpaceX has assisted the agency with the launch of a science satellite.
This past September, NASA also signed a lucrative contract with SpaceX worth $2.6 billion to fly astronauts and cargo to the International Space Station. As part of the Commercial Crew Program, SpaceX’s Falcon 9 and Dragon spacecraft were selected by NASA to help restore indigenous launch capability to the US.
Artist’s impression of the James Webb Space Telescope, the space observatory scheduled for launch in 2018. Image Credit: NASA/JPL
The total cost for TESS is estimated at approximately $87 million, which will include launch services, payload integration, and tracking and maintenance of the spacecraft throughout the course of its three year mission.
As for the mission itself, that has been the focus of attention for many years. Since it was deployed in 2009, the Kepler spacecraft has yielded more and more data on distant planets, many of which are Earth-like and potentially habitable. But in 2013, two of four reaction wheels on Kepler failed and the telescope has lost its ability to precisely point toward stars. Even though it is now doing a modified mission to hunt for exoplanets, NASA and exoplanet enthusiasts have been excited by the prospect of sending up another exoplanet hunter, one which is even more ideally suited to the task.
Once deployed, TESS will spend the next three years scanning the nearest and brightest stars in our galaxy, looking for possible signs of transiting exoplanets. This will involve scanning nearby stars for what is known as a “light curve”, a phenomenon where the visual brightness of a star drops slightly due to the passage of a planet between the star and its observer.
By measuring the rate at which the star dims, scientists are able to estimate the size of the planet passing in front of it. Combined with measurements the star’s radial velocity, they are also able to determine the density and physical structure of the planet. Though it has some drawbacks, such as the fact that stars rarely pass directly in front of their host stars, it remains the most effective means of observing exoplanets to date.
Number of extrasolar planet discoveries on up to Sept. 2014, with colors indicating method of detection. Blue: radial velocity; Green: transit; Yellow: timing, Red: direct imaging; Orange: microlensing. Image Credit: Alderon/Wikimedia Commons
In fact, as of 2014, this method became the most widely used for determining the presence of exoplanets beyond our Solar System. Compared to other methods – such as measuring a star’s radial velocity, direct imaging, the timing method, and microlensing – more planets have been detected using the transit method than all the other methods combined.
In addition to being able to spot planets by the comparatively simple method of measuring their light curve, the transit method also makes it possible to study the atmosphere of a transiting planet. Combined with the technique of measuring the parent star’s radial velocity, scientists are also able to measure a planet’s mass, density, and physical characteristics.
With TESS, it will be possible to study the mass, size, density and orbit of exoplanets. In the course of its three-year mission, TESS will be looking specifically for Earth-like and super-Earth candidates that exist within their parent star’s habitable zone.
This information will then be passed on to Earth-based telescopes and the James Webb Space Telescope – which will be launched in 2018 by NASA with assistance from the European and Canadian Space Agencies – for detailed characterization.
The TESS Mission is led by the Massachusetts Institute of Technology – who developed it with seed funding from Google – and is overseen by the Explorers Program at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
An array of Earth-like planets. Image Credit: NASA
Since 1995, astronomers have detected thousands of worlds orbiting nearby stars, sparking a race to find the one that most resembles Earth. The discovery of habitable exoplanets and even extraterrestrial life is often referred to as the Holy Grail of science. So with the gold rush of exoplanet discoveries these days, it’s pretty tempting in news articles to lose readers in a fantastical narrative.
This month I’m launching a project on Beacon — a new independent platform for journalism — that will go behind the sensational headlines covering the search for Earth 2.0.
But I can’t do it without your help. In order to commit to writing about this on a regular basis, I need to raise $4,000 from subscribers who are willing to support my work over this month. Don’t worry, subscriptions are available for only $5 per month. This will supply the funding necessary to write for six months.
By Kepler’s definition, to be Earth-like a planet must be both Earth-size (less than 1.25 times Earth’s radius and less than twice Earth’s mass) and must circle its host star within the habitable zone: the band where liquid water can exist.
Image Credit: xkcd
This simple, and yet variant, definition is a crucial starting point. But one glance at our Solar System (namely Venus and Mars) demonstrates that just because a planet is Earth-like doesn’t mean it’s an Earth twin.
So even if we do find Earth-like planets, we still don’t have the ability to know if they’re water worlds with luscious green planets and civilizations peering back at us.
But should we scale our definition of Earth-like planets up or down? Examples in the Solar System suggest that we should scale it down. Maybe planets located nearer to the center of the habitable zone are more congenial to life.
But can we base our definition on a single example — even if it’s the only example we know — alone? Theoretical astronomers suggest the picture is much more complicated. Life might arise on larger worlds, ones up to three times as massive as Earth, because they’re more likely to have an atmosphere due to more volcanic activity. Or life might arise on older worlds, where there’s simply more time for life to evolve.
It’s a crucial debate in astronomy research today, and it’s one that the media needs to handle with care. I am proud to be a part of Universe Today’s team, bringing readers up-to-date with the on goings in our local Universe. And Beacon will allow me to spend even more time, focusing on such a critical topic.
For each article, I will gather news, opinions and commentary from astronomers in the field. Not only do I have training as an astronomer, but my graduate school research focused on detecting exoplanet atmospheres from ground-based telescopes. With this deep-rooted understanding of the field at hand, I am able to parse complex information by directly reading peer-reviewed journal articles and interviewing astronomers I’ve met through my previous research.
But I really do need your help. Subscriptions are available for only $5 per month, and there are special rewards — such as gorgeous astronomy photos printed on canvas and gift subscriptions for friends — for people who subscribe at higher levels. You can directly subscribe here.
But here’s the best part: when you subscribe to my work, you’ll get access not only to all the stories I write, but the work of over 100 additional writers, based all over the world. This month Beacon is launching a series of astronomy projects, including one by Universe Today writer Elizabeth Howell.
