Ever since it was first announced in 2015, there has been speculation as to what could account for the dimming of KIC 8462852. Credit: SentientDevelopments.com
Back in October of 2015, astronomers shook the world when they reported how the Kepler mission had noticed a strange and sudden drop in brightness coming from KIC 8462852 (aka. Tabby’s Star). This was followed by additional studies that showed how the star appeared to be consistently dimming over time. All of this led to a flurry of speculation, with possibilities ranging from large asteroids and a debris disc to an alien megastructure.
But in what may be the greatest explanation yet, a team of researchers from Columbia University and the University of California, Berkley, have suggested that the star’s strange flickering could be the result of a planet it consumed at some point in the past. This would have resulted in a big outburst of brightness from which the star is now recovering; and the remains of this planet could be transiting in front of the star, thus causing periodic drops. Continue reading “Finally, An Explanation for the Alien Megastructure?”
Once again, science fiction television and movies has let you down. They try to recreate what it might be like on an alien world, but surprise surprise, they mostly get it wrong. That’s because a truly alien world would be different in so many ways, it would blow your mind. Today we’ll help you figure out if you’re on a movie set, or you’ve actually crashlanded on an alien planet.
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Artist's impression of The Milky Way Galaxy. Based on current estimates and exoplanet data, it is believed that there could be tens of billions of habitable planets out there. Credit: NASA
On a clear night, and when light pollution isn’t a serious factor, looking up at the sky is a breathtaking experience. On occasions like these, it is easy to be blown away by the sheer number of stars out there. But of course, what we can see on any given night is merely a fraction of the number of stars that actually exist within our Galaxy.
What is even more astounding is the notion that the majority of these stars have their own system of planets. For some time, astronomers have believed this to be the case, and ongoing research appears to confirm it. And this naturally raises the question, just how many planets are out there? In our galaxy alone, surely, there must be billions!
Number of Planets per Star:
To truly answer that question, we need to crunch some numbers and account for some assumptions. First, despite the discovery of thousands of extra-solar planets, the Solar System is still the only one that we have studied deeply. So it could be that ours possesses more star systems than others, or that our Sun has a fraction of the planets that other stars do.
So let’s assume that the eight planets that exist within our Solar System (not taking into account Dwarf Planets, Centaurs, KBOs and other larger bodies) represent an average. The next step will be to multiply that number by the amount of stars that exist within the Milky Way.
Number of Stars:
To be clear, the actual number of stars in the Milky Way is subject to some dispute. Essentially, astronomers are forced to make estimates due to the fact that we cannot view the Milky Way from the outside. And given that the Milky Way is in the shape of a barred, spiral disc, it is difficult for us to see from one side to the other – thanks to light interference from its many stars.
As a result, estimates of how many stars there are come down to calculations of our galaxy’s mass, and estimates of how much of that mass is made up of stars. Based on these calculations, scientists estimate that the Milky Way contains between 100 and 400 billion stars (though some think there could be as many as a trillion).
Doing the math, we can then say that the Milky Way galaxy has – on average – between 800 billion and 3.2 trillion planets, with some estimates placing that number as high a 8 trillion! However, in order to determine just how many of them are habitable, we need to consider the number of exoplanets discovered so far for the sake of a sample analysis.
Habitable Exoplanets:
As of October 13th, 2016, astronomers have confirmed the presence of 3,397 exoplanets from a list of 4,696 potential candidates (which were discovered between 2009 and 2015). Some of these planets have been observed directly, in a process known as direct imaging. However, the vast majority have been detected indirectly using the radial velocity or transit method.
In the case of the former, the existence of planets is inferred based on the gravitational influence they have on their parent star. Essentially, astronomers measure how much the star moves back and forth to determine if it has a system of planets and how massive they are. In the case of the transit method, planets are detected when they pass directly in front of their star, causing it to dim. Here, size and mass are estimated based on the level of dimming.
In the course of its mission, the Kepler mission has observed about 150,000 stars, which during its initial four year mission consisted primarily of M-class stars. Also known as red dwarfs, these low-mass, lower-luminosity stars are harder to observe than our own Sun.
Histogram showing the number of exoplanets discovered by year. Credit: NASA Ames/W. Stenzel, Princeton/T. Morton
Since that time, Kepler has entered a new phase, also known as the K2 mission. During this phase, which began in November of 2013, Kepler has been shifting its focus to observe more in the way of K- and G-class stars – which are nearly as bright and hot as our Sun.
According to a recent study from NASA Ames Research Center, Kepler found that about 24% of M-class stars may harbor potentially habitable, Earth-size planets (i.e. those that are smaller than 1.6 times the radius of Earth’s). Based upon the number of M-class stars in the galaxy, that alone represents about 10 billion potentially habitable, Earth-like worlds.
Meanwhile, analyses of the K2 phase suggests that about one-quarter of the larger stars surveyed may also have Earth-size planet orbiting within their habitable zones. Taken together, the stars observed by Kepler make up about 70% of those found within the Milky Way. So one can estimate that there are literally tens of billions of potentially habitable planets in our galaxy alone.
In the coming years, new missions will be launching, like the James Webb Space Telescope (JWST) and the Transitting Exoplanet Survey Satellite (TESS). These missions will be able to detect smaller planets orbiting fainter stars, and maybe even determine if there’s life on any of them.
Once these new missions get going, we’ll have better estimates of the size and number of planets that orbit a typical star, and we’ll be able to come up with better estimates of just many planets there are in the galaxy. But until then, the numbers are still encouraging, as they indicate that the chances for extra-terrestrial intelligence are high!
Artist's depiction of the interior of a low-mass star, such as the one seen in an X-ray image from Chandra in the inset. Credit: NASA/CXC/M.Weiss
In August of 2016, the European Southern Observatory announced that the nearest star to our own – Proxima Centauri – had an exoplanet. Since that time, considerable attention has been focused on this world (Proxima b) in the hopes of determining just how “Earth-like” it really is. Despite all indications of it being terrestrial and similar in mass to Earth, there are some lingering doubts about its ability to support life.
This is largely due to the fact that Proxima b orbits a red dwarf. Typically, these low mass, low temperature, slow fusion stars are not known for being as bright and warm as our Sun. However, a new study produced by researchers at the Harvard Smithsonian Center for Astrophysics (CfA) has indicated that Proxima Centauri might be more like our star than we thought.
For instance, our Sun has what is known as a “Solar Cycle“, an 11-year period in which it experiences changes in the levels of radiation it emits. This cycle is driven by changes in the Sun’s own magnetic field, and corresponds to the appearance of Sunspots on its surface. During a “solar minimum”, the Sun’s surface is clear of spots, while at a solar maximum, one hundred sunspots can appear on an area the size of 1% the Sun’s surface area.
Composite of 25 separate images spanning the period of April 16, 2012, to April 15, 2013, revealing active regions during this part of the Solar Cycle. Credit: NASA/SDO/AIA/S. Wiessinger
For the sake of their research, the Harvard Smithsonian team examined Proxima Centauri over the course of several years to see if it too had a cycle. As they explain in their research paper, titled “Optical, UV, and X-Ray Evidence for a 7-Year Stellar Cycle in Proxima Centauri” they relied on several years worth optical, UV, and X-ray observations made of the star.
What they found was that Proxima Centauri does indeed have a cycle that involves changes in its minimum and maximum amount of emitting radiation, which corresponds to “starspots” on its surface. As Dr. Wargelin told Universe Today via email:
“The optical/ASAS data showed a nice 7-year cycle, as well as an 83-day rotation period. When we broke down that data by year we saw the period vary from around 77 to 90 days. We interpret that as ‘differential rotation’ like that found on the Sun. The rotation rate differs at different latitudes; on the Sun it’s around 35 days at the poles and 24.5 at the equator. The “average” rotation is usually given as 27.3 days.”
In essence, Proxima Centauri has its own cycle, but one that is a lot more dramatic than our Sun’s. Besides lasting 7 years from peak to peak, it involves spots covering over 20% of its surface at one time. These spots are apparently much bigger than the ones we regularly observe on our Sun as well.
An X-Ray image of Proxima Centauri. Credit: Chandra/Harvard/NASA
This was surprising, given that Proxima’s interior is very different from our Sun’s. Because of its low mass, the interior of Proxima Centauri is convective, where material in the core is transferred outward. In contrast, only the outer layer of our Sun undergoes convection while the core remains relatively still. This means that, unlike our Sun, energy is transferred to the surface through physical movement, and not radiative processes.
While these findings cannot tell us anything directly about whether or not Proxima b might be habitable, the existence of this solar cycle is an interesting find that might be leading in that general direction. As Dr. Wargelin explained:
“Magnetic fields are what drive high energy emission (UV and X rays) and stellar winds (like the solar wind) in solar-type and smaller stars, AND a stellar cycle (if it has one). That X-ray/UV emission and stellar wind can ionize/evaporate/strip the atmosphere of close-in planets, particularly if the planet doesn’t have a protective magnetic field of its own.
“Therefore….. a necessary but not sufficient requirement for understanding (i.e., modeling) the evolution of a planet’s atmosphere is understanding the magnetic field of the host star. If you don’t understand why a star has a cycle (and standard theory says fully convective stars like Proxima can NOT have cycles) then you don’t understand its magnetic field.”
As always, further observations and research will be necessary before we can fully understand Proxima Centauri, and whether or not any planets that orbit it could support life. But then again, we’ve only known about Proxima b for a short time, and the rate at which we are learning new things about it is quite impressive!
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!
This artist's illustration shows a planet circling a pair of distant red dwarf stars, representing the the system OGLE-2007-BLG-349 system, about 8,000 lightyears from Earth. Credit: NASA, ESA, and G. Bacon (STScI).
Binary stars are common throughout the galaxy, as it has been estimated about half the stars in our sky consist of two stars orbiting each other. Therefore, it’s also thought that about half of all exoplanet host stars are binaries as well. However, only about 10 of these so called circumbinary planets have been found so far in the 3,000-plus confirmed extrasolar planets that have been discovered.
But chalk up one more circumbinary planet, and this one bodes well for a technique that could help scientists find planets that orbit far away from their stars. Astronomers using the Hubble Space Telescope have confirmed a very interesting “three-body” system where two very close stars have a planet that orbits them both at a rather large distance.
The two red dwarf stars are just 7 million miles apart, or about 14 times the diameter of the Moon’s orbit around Earth. The planet orbits roughly 300 million miles from the stellar duo, about the distance of the asteroid belt from the Sun. The planet completes an orbit around both stars roughly every seven years.
The Hubble Space Telescope. Image: NASA
Hubble used the a technique called gravitational microlensing, where the gravity of a foreground star bends and amplifies the light of a background star that momentarily aligns with it. The light magnification can reveal clues to the nature of the foreground star and any associated planets.
The system, called OGLE-2007-BLG-349, was originally detected in 2007 by the Optical Gravitational Lensing Experiment (OGLE), a telescope at the Las Campanas Observatory in Chile that searches for and observes microlensing effects from small distortions of spacetime, caused by stars and exoplanets.
However, the original OGLE observations could not confirm the details of the OGLE-2007-BLG-349 system. OGLE and several other ground-based observations determined there was a star and a planet in this system, but they couldn’t positively identify what the observed third body was.
“The ground-based observations suggested two possible scenarios for the three-body system: a Saturn-mass planet orbiting a close binary star pair or a Saturn-mass and an Earth-mass planet orbiting a single star,” said David Bennett, from NASA’s Goddard Space Flight Center, who is the first author in a new paper about the system, to be published in the Astrophysical Journal.
With Hubble’s sharp eyesight, the research team was able to separate the background source star and the lensing star from their neighbors in the very crowded star field. The Hubble observations revealed that the starlight from the foreground lens system was too faint to be a single star, but it had the brightness expected for two closely orbiting red dwarf stars, which are fainter and less massive than our sun.
“So, the model with two stars and one planet is the only one consistent with the Hubble data,” Bennett said.
“OGLE has detected over 17,000 microlensing events, but this is the first time such an event has been caused by a circumbinary planetary system,” explains Andrzej Udalski from the University of Warsaw, Poland, co-author of the study and leader of the OGLE project.
The team said this first-ever confirmation of an exoplanet system using the gravitational microlensing technique suggests some intriguing possibilities. While data from the Kepler Space Telescope is more likely to reveal planets that orbit close to their stars, microlensing allows planets to be found at distances far from their host stars.
“This discovery, suggests we need to rethink our observing strategy when it comes to stellar binary lensing events,” said Yiannis Tsapras, another member of the team, from the Astronomisches Recheninstitut in Heidelberg, Germany. “This is an exciting new discovery for microlensing”.
The team said that since this observation has shown that microlensing can successfully detect circumbinary planets, Hubble could provide an essential new role in the continued search for exoplanets.
OGLE-2007-BLG-349 is located 8,000 light-years away, towards the center of our galaxy.
(And, you’re welcome… I didn’t mention Tatooine in this article, until now!)
Artist’s conception of the binary system with three giant planets discovered in this study. One star hosts two planets and the other hosts the third. The system represents the smallest-separation binary in which both stars host planets that has ever been observed. Image courtesy of Robin Dienel/Carnegie.
The more we look, the more we see the great diversity in planetary systems around other stars. And curiously, planet hunters are finding that most star systems are very different from our own.
An example is a recently discovered system that is extremely crowded. It consists of a three giant planets in a binary (two stars) system. One star hosts two planets and the other hosts the third. The system represents the smallest-separation binary in which both stars host planets that has ever been observed.
“The probability of finding a system with all these components was extremely small,” said Johanna Teske from the Carnegie Institution for Science, “so these results will serve as an important benchmark for understanding planet formation, especially in binary systems.”
An illustration of this highly unusual system, which features the smallest-separation binary stars that both host planets ever discovered. Only six other metal-poor binary star systems with exoplanets have ever been found. Illustration courtesy of Timothy Rodigas/Carnegie.
Teske and her team said this busy system might help explain the influence that giant planets like Jupiter have over a solar system’s architecture.
“We are trying to figure out if giant planets like Jupiter often have long and, or eccentric orbits,” Teske explained. “If this is the case, it would be an important clue to figuring out the process by which our Solar System formed, and might help us understand where habitable planets are likely to be found.”
The twin stars are named HD 133131A and HD 133131B. The former hosts two Jupiter-sized worlds and the latter a planet with a mass at least 2.5 times Jupiter’s. All three planets have “eccentric” or highly elliptical orbits. So far no smaller, rocky worlds have been detected but the team said those type of planets could be part of the system, or may have been part of the system in the past.
The two stars themselves are separated by only 360 astronomical units (AU – the distance between the Earth and the Sun, approximately 150,000,000 km or 93,000,000 miles). This is extremely close for twin stars with detected planets orbiting the individual stars. The next-closest known binary star system with planets has stars about 1,000 AU apart.
The two stars are more like fraternal twins rather than identical because they have slight different chemical compositions. The team said this could indicate that one star swallowed some baby planets early in its life, changing its composition slightly. Or another option is that the gravitational forces of the detected giant planets may have had a strong effect on fully-formed small planets, flinging them in towards the star or out into space.
But both stars are “metal poor,” meaning that most of their mass is hydrogen and helium, as opposed to other elements like iron or oxygen. This is another curious thing about this system, as most stars that host giant planets are “metal rich.”
The system was found using the Planet Finder Spectrograph, an instrument developed by Carnegie scientists and mounted on the Magellan Clay Telescopes at Carnegie’s Las Campanas Observatory. This finding represents the first exoplanet detection made based solely on data from the. PFS is able to find large planets with long-duration orbits or orbits that are very elliptical rather than circular.
Artist's impression of a system of exoplanets orbiting a low mass, red dwarf star. Credit: NASA/JPL
Finding exoplanets is hard work. In addition to requiring seriously sophisticated instruments, it also takes teams of committed scientists; people willing to pour over volumes of data to find the evidence of distant worlds. Professor Kipping, an astronomer based at the Harvard-Smithsonian Center for Astrophysics, is one such person.
Within the astronomical community, Kipping is best known for his work with exomoons. But his research also extends to the study and characterization of exoplanets, which he pursues with his colleagues at the Cool Worlds Laboratory at Columbia University. And what has interested him most in recent years is finding exoplanets around our Sun’s closest neighbor – Proxima Centauri.
Kipping describes himself as a “modeler”, combining novel theoretical modeling with modern statistical data analysis techniques applied to observations. He is also the Principal Investigator (PI) of The Hunt for Exomoons with Kepler (HEK) project and a fellow at the Harvard College Observatory. For the past few years, he and his team have been taking the hunt for exoplanets to the local stellar neighborhood.
The inspiration for this search goes back to 2012, when Kipping was at a conference and heard the news about a series of exoplanets being discovery around Kepler 42 (aka. KOI-961). Using data from the Kepler mission, a team from the California Institute of Technology discovered three exoplanets orbiting this red dwarf star, which is located about 126 light years from Earth.
At the time, Kipping recalled how the author of the study – Professor Philip Steven Muirhead, now an associate professor at the Institute for Astrophysical Research at Boston University – commented that this star system looked a lot like our nearest red dwarf stars – Barnard’s Star and Proxima Centauri.
In addition, Kepler 42’s planets were easy to spot, given that their proximity to the star meant that they completed an orbital period in about a day. Since they pass regularly in front of their star, the odds of catching sight of them using the Transit Method were good.
As Prof. Kipping told Universe Today via email, this was the “ah-ha moment” that would inspire him to look at Proxima Centauri to see if it too had a system of planets:
“We were inspired by the discovery of planets transiting KOI-961 by Phil Muirhead and his team using the Kepler data. The star is very similar to Proxima, a late M-dwarf harboring three sub-Earth sized planets very close to the star. It made me realize that if that system was around Proxima, the transit probability would be 10% and the star’s small size would lead to quite detectable signals.”
The MOST satellite, a Canadian built space telescope. Credit: Canadian Space Agency
In essence, Kipping realized that if such a planetary system also existed around Proxima Centauri, a star with similar characteristics, then they would very easy to detect. After that, he and his team began attempting to book time with a space telescope. And by 2014-15, they had been given permission to use the Canadian Space Agency’s Microvariability and Oscillation of Stars (MOST) satellite.
Roughly the same size as a suitcase, the MOST satellite weighs only 54 kg and is equipped with an ultra-high definition telescope that measures just 15 cm in diameter. It is the first Canadian scientific satellite to be placed in orbit in 33 years, and was the first space telescope to be entirely designed and built in Canada.
Despite its size, MOST is ten times more sensitive than the Hubble Space Telescope. In addition, Kipping and his team knew that a mission to look for transiting exoplanets around Proxima Centauri would be too high-risk for something like Hubble. In fact, the CSA initially rejected their applications for this same reason.
“MOST initially denied us because they wanted to look at Alpha Centauri following the announcement by Dumusque et al. of a planet there,” said Kipping. “So understandably Proxima, for which no planets were known at the time, was not as high priority as Alpha Cen. We never even tried for Hubble time, it would be a huge ask to stare HST at a single star for months on end with just a a 10% chance for success.”
Artist’s impression of the Earth-like exoplanet discovered orbiting Alpha Centauri B iby the European Southern Observatory on October 17, 2012. Credit: ESO
By 2014 and 2015, they secured permission to use MOST and observed Proxima Centauri twice – in May of both years. From this, they acquired a month and half’s-worth of space-based photometry, which they are currently processing to look for transits. As Kipping explained, this was rather challenging, since Proxima Centauri is a very active star – subject to star flares.
“The star flares very frequently and prominently in our data,” he said. “Correcting for this effect has been one the major obstacles in our analysis. On the plus side, the rotational activity is fairly subdued. The other issue we have is that MOST orbits the Earth once every 100 minutes, so we get data gaps every time MOST goes behind the Earth.”
Their efforts to find exoplanets around Proxima Centauri are especially significant in light of the European Southern Observatory’s recent announcement about the discovery of a terrestrial exoplanet within Proxima Centauri’s habitable zone (Proxima b). But compared to the ESO’s Pale Red Dot project, Kipping and his team were relying on different methods.
“Essentially, we seek planets which have the right alignment to transit (or eclipse) across the face of the star, whereas radial velocities look for the wobbling motion of a star in response to the gravitational influence of an orbiting planet. Transits are always less likely to succeed for a given star, because we require the alignment to be just right. However, the payoff is that we can learn way more about the planet, including things like it’s size, density, atmosphere and presence of moons and rings.”
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
In the coming months and years, Kipping and his team may be called upon to follow up on the success of the ESO’s discovery. Having detected Proxima b using the Radial Velocity method, it now lies to astronomers to confirm the existence of this planet using another detection method.
In addition, much can be learned about a planet through the Transit Method, which would be helpful considering all the things we still don’t know about Proxima b. This includes information about its atmosphere, which the Transit Method is often able to reveal through spectroscopic measurements.
Suffice it to say, Kipping and his colleagues are quite excited by the announcement of Proxima b. As he put it:
“This is perhaps the most important exoplanet discovery in the last decade. It would be bitterly disappointing if Proxima b does not transit though, a planet which is paradoxically so close yet so far in terms of our ability to learn more about it. For us, transits would not just be the icing on the cake, serving merely as a confirmation signal – rather, transits open the door to learning the intimate secrets of Proxima, changing Proxima b from a single, anonymous data point to a rich world where each month we would hear about new discoveries of her nature and character.”
This coming September, Kipping will be joining the faculty at Columbia University, where he will continue in his hunt for exoplanets. One can only hope that those he and his colleagues find are also within reach!
Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO
The ESO’s recent announcement that they have discovered an exoplanet candidate orbiting Proxima Centauri – thus confirming weeks of speculation – has certainly been exciting news! Not only is this latest find the closest extra-solar planet to our own Solar System, but the ESO has also indicated that it is rocky, similar in size and mass to Earth, and orbits within the star’s habitable zone.
However, in the midst of this news, there has been some controversy regarding certain labels. For instance, when a planet like Proxima b is described as “Earth-like”, “habitable”, and/or “terrestrial“, there are naturally some questions as to what this really means. For each term, there are particular implications, which in turn beg for clarification.
For starters, to call a planet “Earth-like” generally means that it is similar in composition to Earth. This is where the term “terrestrial” really comes into play, as it refers to a rocky planet that is composed primarily of silicate rock and metals which are differentiated between a metal core and a silicate mantle and crust.
This applies to all planets in the inner Solar System, and is often used in order to differentiate rocky exoplanets from gas giants. This is important within the context of exoplanet hunting, as the majority of the 4,696 exoplanet candidates – of which 3,374 have been confirmed (as of August 18th, 2016) – have been gas giants.
What this does not mean, at least not automatically, is that the planet is habitable in the way Earth is. Simply being terrestrial in nature is not an indication that the planet has a suitable atmosphere or a warm enough climate to support the existence of liquid water or microbial life on its surface.
What’s more, Earth-like generally implies that a planet will be similar in mass and size to Earth. But this is not the same as composition, as many exoplanets that have been discovered have been labeled as “Earth-sized” or “Super-Earths” – i.e. planets with around 10 times the mass of Earth – based solely on their mass.
This term also distinguishes an exoplanet candidate from those that are 15 to 17 masses (which are often referred to as “Neptune-sized”) and those that have masses similar to, or many times greater than that of Jupiter (i.e. Super-Jupiters). In all these cases, size and mass are the qualifiers, not composition.
Ergo, finding a planet that is greater in size and mass than Earth, but significantly less than that of a gas giant, does not mean it is terrestrial. In fact, some scientists have recommended that the term “mini-Neptune” be used to describe planets that are more massive than Earth, but not necessarily composed of silicate minerals and metals.
And estimates of size and mass are not exactly metrics for determining whether or not a planet is “habitable”. This term is especially sticky when it comes to exoplanets. When scientists attach this word to extra-solar planets like Proxima b, Gliese 667 Cc, Kepler-452b, they are generally referring to the fact that the planet exists within its parent star’s “habitable zone” (aka. Goldilocks zone).
This term describes the region around a star where a planet will experience average surface temperatures that allow for liquid water to exist on its surface. For those planets that orbit too close to their star, they will experience intense heat that transforms surface water into hydrogen and oxygen – the former escaping into space, the latter combining with carbon to form CO².
This is what scientists believe happened to Venus, where thick clouds of CO² and water vapor triggered a runaway greenhouse effect. This turned Venus from a world that once had oceans into the hellish environment we know today, where temperatures are hot enough to melt lead, atmospheric density if off the charts, and sulfuric acid rains from its thick clouds.
Kepler-62f, an exoplanet that is about 40% larger than Earth. It’s located about 1,200 light-years from our solar system in the constellation Lyra. Credit: NASA/Ames/JPL-Caltech
For planets that orbit beyond a star’s habitable zone, water ice will become frozen solid, and the only liquid water will likely be found in underground reservoirs (this is the case on Mars). As such, finding planets that are just right in terms of average surface temperature is intrinsic to the “low-hanging fruit” approach of searching for life in our Universe.
But of course, just because a planet is warm enough to have water on its surface doesn’t mean that life can thrive on it. As our own Solar System beautifully demonstrates, a planet can have the necessary conditions for life, but still become a sterile environment because it lacks a protective magnetosphere.
This is what scientists believe happened to Mars. Located within our Sun’s Goldilocks zone (albeit on the outer edge of it), Mars is believed to have once had an atmosphere and liquid water on its surface. But today, atmospheric pressure on the surface of Mars is only 1% that of Earth’s, and the surface is dry, cold, and devoid of life.
The reason for this, it has been determined, is because Mars lost its magnetosphere 4.2 Billion years ago. According to NASA’s MAVEN mission, this resulted in Mars’ atmosphere being slowly stripped away over the course of the next 500 million years by solar wind. What little atmosphere it had left was not enough to retain heat, and its surface water evaporated.
Billions of years ago, Mars was a very different world. Liquid water flowed in long rivers that emptied into lakes and shallow seas. A thick atmosphere blanketed the planet and kept it warm. Credit: NASA
By the same token, planets that do not have protective magnetospheres are also subject to an intense level of radiation on their surfaces. On the Martian surface, the average dose of radiation is about 0.67 millisieverts (mSv) per day, which is about a fifth of what people are exposed to here on Earth in the course of a year.
We can expect similar situations on extra-solar planets where a magnetosphere does not exist. Essentially, Earth is fortunate in that it not only orbits in a pretty cushy spot around our Sun, but that its core is differentiated between a solid inner core and a liquid, rotating outer core. This rotation, it is believed, is responsible for creating a dynamo effect that in turn creates Earth’s magnetic field.
However, using our own Solar System again as a model, we find that magnetic fields are not entirely uncommon. While Earth is the only terrestrial planet in our Solar System to have on (all the gas giants have powerful fields), Jupiter’s moon Ganymede also has a magnetosphere of its own.
Similarly, there are orbital parameters to consider. For instance, a planet that is similar in size, mass and composition could still have a very different climate than Earth due to its orbit. For one, it may be tidally-locked with its star, which would mean that one side is permanently facing towards it, and is therefore much warmer.
An artist’s depiction of planets transiting a red dwarf star in the TRAPPIST-1 System. Credit: NASA/ESA/STScl
On the other hand, it may have a slow rotational velocity, and a rapid orbital velocity, which means it only experiences a few rotations per orbit (as is the case with Mercury). Last, but certainly not least, its distance from its respective star could mean it receives far more radiation than Earth does – regardless of whether or not it has a magnetosphere.
This is believed to the be the case with Proxima Centauri b, which orbits its red dwarf star at a distance of 7 million km (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).
Because of this, the climate is likely to be very different than Earth’s, with water confined to either its sun-facing side (in the case of a synchronous rotation), or in its tropical zone (in the case of a 3:2 resonance). In addition, the radiation it receives from its red dwarf star would be significantly higher than what we are used to here on Earth.
So what exactly does “Earth-like” mean? The short answer is, it can mean a lot of things. And in this respect, its a pretty dubious term. If Earth-like can mean similarities in mass, size, composition, and can allude to the fact that planet orbits within its star’s habitable zone – but not necessarily all of the above – then its not a very reliable term.
Artist’s impression of the Earth-like planets that have been observed in other star systems. Image Credit: JPL
In the end, the only way to keep things clear would be to describe a planet as “Earth-like” if it in fact shows similarities in terms of size, mass and composition, all at the same time. The word “terrestrial” can certainly be substituted in a pinch, but only where the composition of the planet is known with a fair degree of certainty (and not just its size and mass).
And words like “habitable” should probably only be used when chaperoned by words like “potentially”. After all, being within a star’s habitable zone certainly means there’s the potential for life. But it doesn’t not necessarily entail that life could have emerged there, or that humans could live there someday.
And should these words apply to Proxima b? Perhaps, but one should consider the fact that the ESO has announced the detection of a exoplanet using the Radial Velocity method. Until such time as it is confirmed using direct detection methods, its remains a candidate exoplanet (not a confirmed one).
But even these simple measures would likely not be enough to erase all the ambiguity or controversy. When it comes right down to it, planet-hunting – like all aspects of space exploration and science – is a divisive issue. And new findings always have a way of drawing criticism and disagreement from several quarters at once.
And you thought Pluto’s classification confused things! Well, Pluto has got nothing on the exoplanet database! So be prepared for many years of classification debates and controversy!
Artist's impression of the "Venus-like" exoplanet GJ 1132b. Credit: cfa.harvard.edu
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.
Artist’s concept of exoplanets orbiting a young, red dwarf star. Credit: NASA/JPL-Caltech
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.
Size comparison of the exoplanet GJ 1132 b with Earth, as reported in the Open Exoplanet Catalogue as of 2015-11-14. Credit: Open Exoplanet Catalogue/AldaronAt 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.
Artist’s impression of three newly-discovered exoplanets orbiting an ultracool dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).
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:
![Comparison of best-fit size of the exoplanet GJ 1132 b with the Solar System planet Earth, as reported in the Open Exoplanet Catalogue[1] as of 2015-11-14. Open Exoplanet Catalogue (2015-11-14). Retrieved on 2015-11-14. Aldaron, a.k.a. Aldaro](https://www.universetoday.com/wp-content/uploads/2016/08/Exoplanet_Comparison_GJ_1132_b-e1471628608145.png)
