They say there’s more than one way to skin an interstellar cat, and in astronomy there’s more than one way to find alien exoplanets orbiting a distant star. With the recent shut-down of NASA’s prolific Kepler mission and its windfall of discoveries, it’s time to look towards the future, and towards alternatives.
The night sky, is the night sky, is the night sky. The constellations you learned as a child are the same constellations that you see today. Ancient people recognized these same constellations. Oh sure, they might not have had the same name for it, but essentially, we see what they saw.
But when you see animations of galaxies, especially as they come together and collide, you see the stars buzzing around like angry bees. We know that the stars can have motions, and yet, we don’t see them moving?
How fast are they moving, and will we ever be able to tell?
Stars, of course, do move. It’s just that the distances are so great that it’s very difficult to tell. But astronomers have been studying their position for thousands of years. Tracking the position and movements of the stars is known as astrometry.
We trace the history of astrometry back to 190 BC, when the ancient Greek astronomer Hipparchus first created a catalog of the 850 brightest stars in the sky and their position. His student Ptolemy followed up with his own observations of the night sky, creating his important document: the Almagest.
In the Almagest, Ptolemy laid out his theory for an Earth-centric Universe, with the Moon, Sun, planets and stars in concentric crystal spheres that rotated around the planet. He was wrong about the Universe, of course, but his charts and tables were incredibly accurate, measuring the brightness and location of more than 1,000 stars.
A thousand years later, the Arabic astronomer Abd al-Rahman al-Sufi completed an even more detailed measurement of the sky using an astrolabe.
One of the most famous astronomers in history was the Danish Tycho Brahe. He was renowned for his ability to measure the position of stars, and built incredibly precise instruments for the time to do the job. He measured the positions of stars to within 15 to 35 arcseconds of accuracy. Just for comparison, a human hair, held 10 meters away is an arcsecond wide.
Also, I’m required to inform you that Brahe had a fake nose. He lost his in a duel, but had a brass replacement made.
In 1807, Friedrich Bessel was the first astronomer to measure the distance to a nearby star 61 Cygni. He used the technique of parallax, by measuring the angle to the star when the Earth was on one side of the Sun, and then measuring it again 6 months later when the Earth was on the other side.
Over the course of this period, this relatively closer star moves slightly back and forth against the more distant background of the galaxy.
And over the next two centuries, other astronomers further refined this technique, getting better and better at figuring out the distance and motions of stars.
But to really track the positions and motions of stars, we needed to go to space. In 1989, the European Space Agency launched their Hipparcos mission, named after the Greek astronomer we talked about earlier. Its job was to measure the position and motion of the nearby stars in the Milky Way. Over the course of its mission, Hipparcos accurately measured 118,000 stars, and provided rough calculations for another 2 million stars.
That was useful, and astronomers have relied on it ever since, but something better has arrived, and its name is Gaia.
Launched in December 2013, the European Space Agency’s Gaia in is in the process of mapping out a billion stars in the Milky Way. That’s billion, with a B, and accounts for about 1% of the stars in the galaxy. The spacecraft will track the motion of 150 million stars, telling us where everything is going over time. It will be a mind bending accomplishment. Hipparchus would be proud.
With the most precise measurements, taken year after year, the motions of the stars can indeed be calculated. Although they’re not enough to see with the unaided eye, over thousands and tens of thousands of years, the positions of the stars change dramatically in the sky.
The familiar stars in the Big Dipper, for example, look how they do today. But if you go forward or backward in time, the positions of the stars look very different, and eventually completely unrecognizable.
When a star is moving sideways across the sky, astronomers call this “proper motion”. The speed a star moves is typically about 0.1 arc second per year. This is almost imperceptible, but over the course of 2000 years, for example, a typical star would have moved across the sky by about half a degree, or the width of the Moon in the sky.
The star with the fastest proper motion that we know of is Barnard’s star, zipping through the sky at 10.25 arcseconds a year. In that same 2000 year period, it would have moved 5.5 degrees, or about 11 times the width of your hand. Very fast.
When a star is moving toward or away from us, astronomers call that radial velocity. They measure this by calculating the doppler shift. The light from stars moving towards us is shifted towards the blue side of the spectrum, while stars moving away from us are red-shifted.
Between the proper motion and redshift, you can get a precise calculation for the exact path a star is moving in the sky.
We know, for example, that the dwarf star Hipparcos 85605 is moving rapidly towards us. It’s 16 light-years away right now, but in the next few hundred thousand years, it’s going to get as close as .13 light-years away, or about 8,200 times the distance from the Earth to the Sun. This won’t cause us any direct effect, but the gravitational interaction from the star could kick a bunch of comets out of the Oort cloud and send them down towards the inner Solar System.
The motions of the stars is fairly gentle, jostling through gravitational interactions as they orbit around the center of the Milky Way. But there are other, more catastrophic events that can make stars move much more quickly through space.
When a binary pair of stars gets too close to the supermassive black hole at the center of the Milky Way, one can be consumed by the black hole. The other now has the velocity, without the added mass of its companion. This gives it a high-velocity kick. About once every 100,000 years, a star is kicked right out of the Milky Way from the galactic center.
Another situation can happen where a smaller star is orbiting around a supermassive companion. Over time, the massive star bloats up as supergiant and then detonates as a supernova. Like a stone released from a sling, the smaller star is no longer held in place by gravity, and it hurtles out into space at incredible speeds.
Astronomers have detected these hypervelocity stars moving at 1.1 million kilometers per hour relative to the center of the Milky Way.
All of the methods of stellar motion that I talked about so far are natural. But can you imagine a future civilization that becomes so powerful it could move the stars themselves?
In 1987, the Russian astrophysicist Leonid Shkadov presented a technique that could move a star over vast lengths of time. By building a huge mirror and positioning it on one side of a star, the star itself could act like a thruster.
Photons from the star would reflect off the mirror, imparting momentum like a solar sail. The mirror itself would be massive enough that its gravity would attract the star, but the light pressure from the star would keep it from falling in. This would create a slow but steady pressure on the other side of the star, accelerating it in whatever direction the civilization wanted.
Over the course of a few billion years, a star could be relocated pretty much anywhere a civilization wanted within its host galaxy.
This would be a true Type III Civilization. A vast empire with such power and capability that they can rearrange the stars in their entire galaxy into a configuration that they find more useful. Maybe they arrange all the stars into a vast sphere, or some kind of geometric object, to minimize transit and communication times. Or maybe it makes more sense to push them all into a clean flat disk.
Amazingly, astronomers have actually gone looking for galaxies like this. In theory, a galaxy under control by a Type III Civilization should be obvious by the wavelength of light they give off. But so far, none have turned up. It’s all normal, natural galaxies as far as we can see in all directions.
For our short lifetimes, it appears as if the sky is frozen. The stars remain in their exact positions forever, but if you could speed up time, you’d see that everything is in motion, all the time, with stars moving back and forth, like airplanes across the sky. You just need to be patient to see it.
In 2013, the European Space Agency deployed the long-awaited Gaia space observatory. As one of a handful of next-generation space observatories that will be going up before the end of the decade, this mission has spent the past few years cataloging over a billion astronomical objects. Using this data, astronomers and astrophysicists hope to create the largest and most precise 3D map of the Milky Way to date.
Though it is almost to the end of its mission, much of its earliest information is still bearing fruit. For example, using the mission’s initial data release, a team of astrophysicists from the University of Toronto managed to calculate the speed at which the Sun orbits the Milky Way. From this, they were able to obtain a precise distance estimate between our Sun and the center of the galaxy for the first time.
For some time, astronomers have been unsure as to exactly how far our Solar System is from the center of our galaxy. Much of this has to do with the fact that it is impossible to view it directly, due to a combination of factors (i.e. perspective, the size of our galaxy, and visibility barriers). As a result, since the year 2000, official estimates have varied between 7.2 and 8.8 kiloparsecs (~23,483 to 28,700 light years).
For the sake of their study, the team – which was led by Jason Hunt, a Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto – combined Gaia’s initial release with data from the RAdial Velocity Experiment (RAVE). This survey, which was conducted between 2003 and 2013 by the Australian Astronomical Observatory (AAO), measured the positions, distances, radial velocities and spectra of 500,000 stars.
Over 200,000 of these stars were also observed by Gaia and information on them was included in its initial data release. As they explain in their study, which was published in the Journal of Astrophysical Letters in November 2016, they used this to examined the speeds at which these stars orbit the center of the galaxy (relative to the Sun), and in the process discovered that there was an apparent distribution in their relative velocities.
In short, our Sun moves around the center of the Milky Way at a speed of 240 km/s (149 mi/s), or 864,000 km/h (536,865 mph). Naturally, some of the more than 200,000 candidates were moving faster or slower. But for some, there was no apparent angular momentum, which they attributed to these stars being scattering onto “chaotic, halo-type orbits when they pass through the Galactic nucleus”.
As Hunt explained in Dunlap Institute press release:
“Stars with very close to zero angular momentum would have plunged towards the Galactic center where they would be strongly affected by the extreme gravitational forces present there. This would scatter them into chaotic orbits taking them far above the Galactic plane and away from the Solar neighbourhood… By measuring the velocity with which nearby stars rotate around our Galaxy with respect to the Sun, we can observe a lack of stars with a specific negative relative velocity. And because we know this dip corresponds to 0 km/sec, it tells us, in turn, how fast we are moving.”
The next step was to combine this information with proper motion calculations of Sagittarius A* – the supermassive black hole believed to be at the center of our galaxy. After correcting for its motion relative to background objects, they were able to effectively triangulate the Earth’s distance from the center of the galaxy. From this, they derived a refined distance of estimate of 7.6 to 8.2 kpc – which works out to about 24,788 to 26,745 light years.
This study builds upon previous work conducted by the study’s co-authors – Prof. Ray Calberg, the current chair of the Department of Astronomy & Astrophysics at the University of Toronto. Years ago, he and Prof. Kimmo Innanen of the Department of Physics and Astronomy at York University conducted a similar study using radial velocity measurement from 400 of the Milky Way’s stars.
But by incorporating data from the Gaia observatory, the UofT team was able to obtain a much more comprehensive data set and narrow the distance to galactic center by a significant amount. And this was based on only the initial data released by the Gaia mission. Looking ahead, Hunt anticipates that further data releases will allow his team and other astronomers to refine their calculations even more.
“Gaia’s final release in late 2017 should enable us to increase the precision of our measurement of the Sun’s velocity to within approximately one km/sec,” he said, “which in turn will significantly increase the accuracy of our measurement of our distance from the Galactic center.”
As more next-generation space telescopes and observatories are deployed, we can expect them to provide us with a wealth of new information about our Universe. And from this, we can expect that astronomers and astrophysicists will begin to shine the light on a number of unresolved cosmological questions.
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.
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.
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.
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.
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!
Further Reading: NASA Exoplanet Archive
For years, exoplanet hunters have been busy searching for planets that are similar to Earth. And when earlier this month, an unnamed source indicated that the European Southern Observatory (ESO) had done just that – i.e. spotted a terrestrial planet orbiting within the star’s habitable zone – the response was predictably intense.
The unnamed source also indicated that the ESO would be confirming this news by the end of August. At the time, the ESO offered no comment. But on the morning of Monday, August 22nd, the ESO broke its silence and announced that it will be holding a press conference this Wednesday, August 24th.
No mention was made as to the subject of the press conference or who would be in attendance. However, it is safe to assume at this point that it’s main purpose will be to address the burning question that’s on everyone’s mind: is there an Earth-analog planet orbiting the nearest star to our own?
For years, the ESO has been studying Proxima Centauri using the La Silla Observatory’s High Accuracy Radial velocity Planet Searcher (HARPS). It was this same observatory that reported the discovery of a planet around Alpha Centauri B back in 2012 – which was the “closest planet to Earth” at the time – which has since been cast into doubt.
Relying on a technique known as the Radial Velocity (or Doppler) Method, they have been monitoring this star for signs of movement. Essentially, as planets orbit a star, they exert a gravitational influence of their own which causes the star to move in a small orbit around the system’s center of mass.
Ordinarily, a star would require multiple exoplanets, or a planet of significant size (i.e. a Super-Jupiter) in order for the signs to be visible. In the case of terrestrial planets, which are much smaller than gas giants, the effect on a star’s orbit would be rather negligible. But given that Proxima Centauri is the closest star system to Earth – at a distance of 4.25 light years – the odds of discerning its radial velocity are significantly better.
According to the source cited by the German weekly Der Speigel, which was the first to report the story, the unconfirmed exoplanet is not only believed to be “Earth-like” (in the sense that it is a rocky body) but also orbits within it’s stars habitable zone (i.e. “Goldilocks Zone”).
Because of this, it would be possible for this planet to have liquid water on its surface, and an atmosphere capable of supporting life. However, we won’t know any of this for certain until we can direct the next-generation of telescopes – like the James Webb Space Telescope or Transiting Exoplanet Survey Satellite (TESS) – to study it more thoroughly.
This is certainly an exciting development, as confirmation will mean that there is planet similar to Earth that is within our reach. Given time and the development of more advanced propulsion systems, we might even be able to mount a mission there to study it up close!
The press conference will start at 1 p.m. Central European Time (CET) – 1 p.m. EDT/10 a.m. PDT. And you bet that we will be reporting on the results shortly thereafter! Stay tuned!
Further Reading: Seeker
Every planet in the Solar System has its own peculiar orbit, and these vary considerably. Whereas planet Earth takes 365.25 days to complete a single orbit about our Sun, Mars takes almost twice as long – 686.971 days. Then you have Jupiter and the other gas giants, which take between 11.86 and 164.8 years to orbit our Sun. But even with these serving as examples, astronomers were not prepared for what they found when they looked at CVSO 30.
This star system, which lies some 1200 light years from Earth, has been found in recent years to have two candidate exoplanets. These planets, which are many times the mass of Jupiter, were discovered by an international team of astronomers using both the Transit Method and Direct Imaging. And what they found was very interesting: one planet has an orbital period of less than 11 days while the other takes a whopping 27,000 years to orbit its parent star!
In addition to being a big surprise, the detection of these two planets using different methods was an historic achievement. Up until now, the vast majority of the over 2,000 exoplanets discovered have been detected thanks to indirect methods. These include the aforementioned Transit Method, which detects planets by measuring the dimming effect they cause when crossing their parent star’s path, and the Radial Velocity Method, which measures the gravitational effect planets have on their parent star.
In 2012, astronomers used the Transit Method to detect CVSO 30b, a planet with 5 to 6 times the mass of Jupiter, and which orbits its star at a distance of only 1.2 million kilometers (by comparison, Mercury orbits our Sun at a distance of 58 million kilometers). From these characteristics, CVSO 30b can be described as a particularly “hot-Jupiter”.
In contrast, Direct Imaging has been used to spot only a few dozen exoplanets. The reason for this is because it is typically quite difficult to detect the light reflected by a planet’s atmosphere due it being drowned out by the light of its parent star. It can also be quite demanding when it comes to the instrument involved. Still, compared to indirect methods, it can be more effective when it comes to exploring the remote regions of a star.
Thanks to the efforts of an international team of astronomers, who combined the use of the Keck Observatory in Hawaii, the ESO’s Very Large Telescope in Chile, and the Spanish National Research Council’s (CSIC) Calar Alto Observatory, CVSO 30c was spotted in remote regions around its parent star, orbiting at a distance of around 666 AU.
The details of the discovery were published in a paper titled “Direct Imaging discovery of a second planet candidate around the possibly transiting planet host CVSO 30“. In it, the researchers – who hail from such prestigious institutions as the Cerro Tololo Inter-American Observatory, the Jena Observatory, the European Space Agency and the Max Planck Institute for Astronomy – explained the methods used to find the exoplanet, and the significance of its discovery.
As Tobias Schmidt – of the University of Hamburg, the Astrophysical Institute and University Observatory Jena, and the lead author of the paper – told Universe Today via email:
“[30b and 30c] are both unusual on their own. CVSO 30b is the first transiting planet around a star as young as 2.5 million years. Published in 2012, all previously detected transiting planets were older than few hundred million years… It has been a surprise to find a planetary mass companion at 662 AU, or 662 times the distance from Earth to the Sun, from a primary star having only about 0.4 solar masses. According to the standard model, planets form in disks around the star. But none of the observed disks around such low-mass stars is large enough to form such an object.”
In other words, it is surprising to find two exoplanet candidates with several times the mass of Jupiter (aka. Super-Jupiters) orbiting a star as small as CVSO 30. But to find two exoplanets with such a disparity in terms of their respective distance from their star (despite being similar in mass) was particularly surprising.
Relying on high-contrast photometric and spectroscopic observations from the Very Large Telescope, the Keck telescopes and the Calar Alto observatory, the international team was able to spot 30c using a technique known as lucky imaging. This process, which is used by ground-based telescopes, involves many high-speed, quick exposure photos being taken to minimize atmospheric interference.
What they found was an exoplanet with a wide orbit that was between 4 and 5 Jupiter masses, and was also very young – less than 10 million years old. What’s more, the spectroscopic data indicated that it is unusually blue for a planet, as most other planet candidates of its kind are very red. The researchers concluded from this that it is likely that 30c is the first young planet of its kind to be directly imaged.
They further concluded that 30 c is also likely the first “L-T transition object” younger than 10 million years to be found orbiting a star. L-T transition objects are a type of brown dwarf – objects that are too large to be considered planets, but too small to be considered stars. Typically they are found embedded in large clouds of gas and dust, or on their own in space.
Paired with its companion – 30 b, which is impossibly close to its parent star – 30 c is not believed to have formed at its current position, and is likely not stable in the long-term. At least, not where current models of planetary formation and orbit are concerned. However, as Prof. Schmidt indicated, this offers a potential explanation for the odd nature of these exoplanets.
“We do think this is a very good hint,” he said, “that the two objects might have formed regularly around the star at a separation comparable to Jupiter or Saturn’s separation from the Sun, then interacted gravitationally and were scattered to their current orbits. However this is still speculation, further investigations will try to prove this. Both have about the same mass of few Jupiter masses, the inner one might be even lower.”
The discovery is also significant since it was the first time that these two detection methods – Transit and Direct Imaging – were used to confirm exoplanet candidates around the same star. In this case, the methods were quite complimentary, and present opportunities to learn more about exoplanets. As Professor Schmidt explained:
“Both Transit method and radial velocity method have problems finding planets around young stars, as the activity of young stars is disturbing the search for them. CVSO 30 b was the first very young planet found with these methods, currently a hand full of candidates exist. Direct imaging, on the other hand, is working best for young planets as they still contract and are thus self-luminous. It is therefore great luck that a far out planet was found around the very first young star hosting a inner planet…
“However, the real advantage of transit and direct imaging methods is that the two objects can now be investigated in greater detail. While we can use the direct light from the imaging for spectroscopy, i.e. split the light according to its wavelength, we hope to achieve the same for the inner planet candidate. This is possible as the light passes through the atmosphere of the planet during transits and some of the elements are absorbed by the composition material of the atmosphere. So we do hope to learn a lot about planet formation, thus also formation of the early Solar System and about young planets in particular from the CVSO 30 system.”
Since astronomers first began began to find exoplanet candidates in distant star systems, we have come to learn just how diverse our Universe really is. Many of the discoveries have challenged our notions about where planets can form around their parent star, while others have showed us that planets can take many different forms.
As time goes on and our exploration of the local Universe advances, we will be challenged to find explanations for how it all fits together. And from that, new and more comprehensive models will no doubt emerge.
Early astronomers realized some of the “stars” in the sky were planets in our Solar System, and really, only then did we realize Earth is a planet too. Now, we’re finding planets around other stars, and thanks to the Kepler Space Telescope, we’re able to find planets that are even smaller than Earth.
This great new graphic of the history of planetary detection was put together by Hugh Osborn, a PhD student at the University of Warwick, who works with data from the WASP (Wide Angle Search for Planets) and NGTS (Next Generation Transit Survey) telescope surveys to discover exoplanets. It starts with the first real “discovery’ of a planet — Uranus in 1781 by William and Caroline Herschel.
“The idea of this plot is to compare our own Solar System (with planets plotted in dark blue) against the newly-discovered extrasolar worlds,” wrote Osborn on his website. “Think of this plot as a projection of all 1873 worlds onto our own solar system, with the Sun (and all other stars) at the far left. As you move out to the right, the orbital period of the planets increases, and correspondingly (thanks to Kepler’s Third Law), so does the distance from the star. Moving upwards means the mass of the worlds increase, from Moon-sized at the base to 10,000 times that of Earth at the top (30 Jupiter Masses).”
You’ll notice a few “clusters” as time moves along. The circles in dark blue are the planets in our Solar System; light blue are planets found by radial velocity. Then in maroon are planets found by direct imaging, followed by orange for microlensing and green for transits.
The first batch of exoplanets were the massive ‘Hot Jupiters’, which were the first exoplanets found “simply because they are easiest to find,” using the radial velocity method. Then you’ll see clusters found by the other methods ending with the big batch found by Kepler.
“This clustering shows that there are more Earth and super-Earth sized planets than any other,” said Osborn. “Hopefully we can begin to probe below it’s limit and into the Earth-like regime, where thousands more worlds should await!”
On reddit, Osborn also provided great, short explanations of the various methods used to detect planets, which we’ll include below:
Planets orbit thanks to gravitational attraction from their star’s mass. But the mass of the planet also has an effect on the star – pulling it around in a tiny circle once every orbit. Astronomers can split the light from a star up into it’s colours, which have an atomic barcode of absorption lines in. These lines shift position as the star moves – the light is effectively compressed to bluer colours when moving towards and pulled to redder colours when moving away.
So, by measuring this to-and-fro (radial) velocity, and finding periodic signals, astronomers can detect the tug of distant exoplanets.
This is easier to get your head around – point a big telescope at a star and directly image a planet around it. This only work for the biggest young planets as these are warmest, so glow brightest in the infra-red (like a red-hot piece of Iron). To find the planet in the glare of it’s star, the starlight needs to be suppressed. This is done by either blocking it out with a starshade, or digitally combining the images in such a way to remove the central star, revealing new exoplanets.
Einstein’s general theory of relativity shows that mass bends space time. This means that light can be bent by massive objects, and even act like a lens. Occasionally a star with a planetary system passes in front of a distant star. The light from the distant star is bent and lensed by both the star and the planet, giving two sharp increases in brightness over a few days – one for the star and one for the planet. The amount of lensing gives the mass of the planets, and the time between the events gives us the distance from their star. More info
When a planet crosses in front of it’s star, it blocks out a small portion of sunlight depending on it’s size. We only see the star as a single point, but we can infer the presence of a planet from the dip in light. When this repeats, we get a period. This is how we have found more than 1000 of the current crop of ~1800 exoplanets!
Thanks to Hugh Osborn for sharing his expertise with Universe Today!
Able to achieve an astounding precision of 0.97 m/s (3.5 km/h), with an effective precision of the order of 30 cms-1, the High Accuracy Radial velocity Planet Searcher (HARPS) echelle spectrograph has already discovered 16 planetary objects in the southern hemisphere and has now logged four more. And that’s only the beginning…
“A long-period companion, probably a second planet, is also found orbiting HD7449. Planets around HD137388, HD204941, and HD7199 have rather low eccentricities (less than 0.4) relative to the 0.82 eccentricity of HD7449b. All these planets were discovered even though their hosting stars have clear signs of activity.” says X. Dumusque (et al). “Solar-like magnetic cycles, characterized by long-term activity variations, can be seen for HD137388, HD204941 and HD7199, whereas the measurements of HD7449 reveal a short-term activity variation, most probably induced by magnetic features on the stellar surface.”
Using radial velocity is currently the preferred method for detecting new planets. But, despite the quality of the equipment, low mass planets placed at a great distance from the host star become problematic because of the star’s own “noise”. RV is an indirect method which utilizes the presence of star wobble to spot orbiting bodies. Unfortunately, normal star activity such as magnetic cycles, spots and plagues can produce similar signals, but now long term variables like these are being fine tuned into the equation.
“The planets announced in this paper for the first time have been discovered even though their host stars display clear signs of activity. We have found that HD7449 exhibits signs of short term activity, whereas HD7199, HD137388, and HD204941 have solar-like magnetic cycles.” says Dumusque. “When examining the RVs and the fitted planets for HD7199, HD137388, and HD204941, it is clear that magnetic cycles induce RV variations that could be misinterpreted as long-period planetary signature. Therefore, the long-term variations in the activity index have to be studied properly to distinguish between the real signature of a planet and long-term activity noise.”
The paper then goes on to explain our Sun should show RV variations of 10ms?1 over its cycle and that it is typical behavior for solar-like stars. Perhaps all stars which display magnetic cycles also have long-term RV variations? “The high precision HARPS sample, composed of 451 stars, provides a good set of measurements to search for this activity-RV correlation.” says Lovis (et al). “A more complete study is in progress and will be soon published.”
Factual Information Courtesy of Wikipedia. Further Reading: The HARPS search for southern extra-solar planets. XXX. Planetary systems around stars with solar-like magnetic cycles and short-term activity variation.