More than 1,000 exoplanets have been confirmed and cataloged (PHL @ UPR Arecibo)
It was just last week that we reported on the oh-so-close approach to 1,000 confirmed exoplanets discovered thus far, and now it’s official: the Extrasolar Planets Encyclopedia now includes more than 1,000! (1,010, to be exact.)
21 years after the first planets beyond our own Solar System were even confirmed to exist, it’s quite a milestone!
The milestone of 1,000 confirmed exoplanets was surpassed on October 22, 2013 after twenty-one years of discoveries. The long-established and well-known Extrasolar Planets Encyclopedia now lists 1,010 confirmed exoplanets.
Not all current exoplanet catalogs list the same numbers as this depends on their particular criteria. For example, the more recent NASA Exoplanet Archive lists just 919. Nevertheless, over 3,500 exoplanet candidates are waiting for confirmation.
The first confirmed exoplanets were discovered by the Arecibo Observatory in 1992. Two small planets were found around the remnants of a supernova explosion known as a pulsar. They were the surviving cores of former planets or newly formed bodies from the ashes of a dead star. This was followed by the discovery of exoplanets around sun-like stars in 1995 and the beginning of a new era of exoplanet hunting.
A “Periodic Table of Exoplanets” as listed by the Extrasolar Planets Encyclopedia (PHL)
(The first exoplanets to be confirmed were two orbiting pulsar PSR B1257+12, 1,000 light-years away. A third was found in 2007.)
Exoplanet discoveries have been full of surprises from the outset. Nobody expected exoplanets around the remnants of a dead star (i.e. PSR 1257+12), nor Jupiter-size orbiting close to their stars (i.e. 51 Pegasi). We also know today of stellar systems packed with exoplanets (i.e. Kepler-11), around binary stars (i.e. Kepler-16), and with many potentially habitable exoplanets (i.e. Gliese 667C).
“The discovery of many worlds around others stars is a great achievement of science and technology. The work of scientists and engineers from many countries were necessary to achieve this difficult milestone. However, one thousand exoplanets in two decades is still a small fraction of those expected from the billions of stars in our galaxy. The next big goal is to better understand their properties, while detecting many new ones.”
– Prof. Abel Mendéz, Associate Professor of Physics and Astrobiology, UPR Arecibo
Source: Press release by Professor Abel Méndez at the Planetary Habitability Laboratory (PHL) at Arecibo
While not illustrating the full 1,010 lineup, this is still a mesmerizing visualization by Daniel Fabrycky of 885 planetary candidates in 361 systems as found by the Kepler mission. (I for one am looking forward to the third installment!)
Of course, scientists are still hunting for the “Holy Grail” of extrasolar planets: an Earth-sized, rocky world orbiting a Sun-like star within its habitable zone. But with new discoveries and confirmations happening almost every week, it’s now only a matter of time. Read more in this recent article by Universe Today writer David Dickinson.
This week for the Weekly Space Hangout, we were joined by an impressive team of space journalists and special guest John Zeller, the Founder of Space Advocates – they’re best known for their Penny4NASA campaign.
We discussed the government shutdown, cool reusable spacecraft and electric aircraft, exoplanets, non-killer asteroids, tilted planets and much much more.
We organize the Weekly Space Hangout every Friday afternoon at 12:00 pm Pacific / 3:00 pm Eastern. You can watch it on Universe Today, Google+ or the Universe Today YouTube channel.
As far as our understanding of life in the Universe goes, right now, we’re it. But the past decade has brought discoveries of hundreds of planets orbiting other stars, some of which could potentially host life. Fellow science journalist Lee Billings has written a new book about the exciting field of searching for extrasolar planets. Five Billion Years of Solitude (read our review here) takes a look at some of the remarkable scientists and the incredible discoveries being made.
Earlier this week, we talked with Lee about the book and the future of how we might find a mirror of Earth.
Universe Today: What was the impetus behind writing this book – was there a specific event or moment where you said, ‘I want to write about astrobiology and the search for exoplanets,’ or was it a more gradual thing over time, where you were just intrigued by the whole expanding field?
Lee Billings
Lee Billings: A little bit of both. I was definitely intrigued by the expanding field of searching for exoplanets, but it came all together for me after interviewing astronomer Greg Laughlin from the University of California, Santa Cruz in 2007 for an infographic about exoplanets. Near the end of our conversation, he mentioned — rather off the cuff — that if you tracked the smallest exoplanets found year by year and graphed them out over time, the trend-line would indicate that we would find an Earth-sized exoplanet by 2011. And I thought, “Holy crap, that’s just four years away!”
I was struck by the disconnect where we could see this plain-as-day data, but the wider world didn’t realize or appreciate this. It also bothered me that we’d soon be finding potentially habitable other worlds, and yet have great difficulty actually determining if they were habitable or even inhabited. And so there was this observational disconnect too, and a lot of people who didn’t seem to care there was this disconnect.
UT: And now that finding exoplanets has made front page news, are you encouraged by how people from afar are viewing this field?
LB: Yes and no. Exoplanets have been in the news for years now. 10 to 15 years ago when astronomers like Geoff Marcy and Michel Mayor were finding the first exoplanets — honkin’ huge balls of gas orbiting close to their stars — it would make front page news. Right now, there’s kind of been ‘exoplanet fatigue,’ where every couple of days a new exoplanet is being announced and exoplanets are even less in the news now because of this overload. And it’s going to keep happening, and I feel like by 2020 finding an Earth-sized planet in the habitable zone isn’t going to make front-page news because it’s going to be happening all the time and people are getting used to it.
UT: Kind of like the Apollo program all over again, where people soon got tired of watching people walk on the Moon?
LB: Yes! Even though I feel like more people in the public are aware of exoplanets being discovered and they even think exoplanets are cool, many think that finding thousands of exoplanets is just like stamp collecting — oh, we found another planet, let’s put it in the book and isn’t that one really pretty — that’s not what this is about. It’s about finding signs of life, finding a sense of context for ourselves in the wider universe, figuring out where Earth and all life upon it fits in this greater picture. I don’t think people are attuned to that side, but are being seduced by the stamp-collecting, horse-race nature of how finding exoplanets are depicted in the media. The emphasis isn’t on what it is going to take to really go out and find out more details about these exoplanets.
Frank Drake and the Drake Equation,
UT: You had the opportunity to talk with some of the great minds of our time — of course, Frank Drake is just such an icon of SETI and the potential for finding life out there in the Universe. But I think one of the most amazing things in your book that I’d never heard of before comes in one of the first chapters where you’re talking with Frank Drake and his idea for a spacecraft that uses the Sun as a gravitational lens to be able to see distant planets incredible detail. That’s amazing!
LB: If you use the Sun as a gravitational lens as sort of an ultimate telescope is really fascinating. As Drake said in the book, you can get some mind-boggling, insane data if you used the Sun as a gravitational lens, and align that with another gravitational lens in the Alpha Centauri system and you can send a high bandwidth radio signal in between those two stars with just the power of a cell phone. In visible light, you could possibly see things on a nearby exoplanet like night-time lighting, the boundary between land and sea, clouds, and weather patterns. It just boggles the mind.
There are other techniques out there that could in theory deliver these sorts of similar observations, but there’s just kind of a technical sweetness to the notion that the stars themselves could be the ultimate telescopes that we use to explore the universe and understand our place in it. I think that’s kind of a wild, poetic and elegant idea.
UT: Wow, that is so compelling. And speaking of compelling, can you talk about Sara Seager and the time you were able to spend with her, getting to know her and her work? Her story is quite compelling not to mention heartbreaking.
LB: She’s a remarkable woman and a brilliant scientist and I feel deeply privileged and honored to be able to tell her story — and that she shared so many details of her personal story with me. Really, she’s kind of a microcosm of the field at large. She crossed over from what she originally studied — from cosmology to exoplanetolgy — and her career seems to be defined by the refusal to accept that certain things might be impossible. She is always pushing the envelope and just keeps her eyes on the prize, so to speak, of finding smaller more Earth-like planets that could be habitable and finding ways of determining what they are actually like. There’s a parallel there between her path and astronomy at large, where there is tension between parts of the professional community. A lot of astronomy is concerned with studying how the universe began and the ancient, the distant, the dead. Exoplanetology is more concerned with the nearest stars to Earth and the planets — the new, the nearby and the living. I feel like she represents that shift and embodies some of that tension.
There’s also an element of tragedy, where she suffered a significant loss with the death of her husband, and had to find a way to get through it and get stronger coming out on the other side. I see similarities between that and what has happened in the field at large where we’ve seen big federally funded plans for future, next-generation telescopes like the Terrestrial Planet Finder, be scuttled on the rocky reefs of politics — and other things. It’s complicated why that’s happened, but there’s no denying that is HAS happened. 15 years ago we were talking about launching TPS by 2014 and now here we are, almost to 2014 and the James Webb telescope isn’t even launched and its eating up all the money for everything else. And now the notion of doing these big kinds of life-finding missions have fallen by the wayside. There’s been kind of the death of a dream, and the bright future that was forecast for what was going to happen for exoplanets doesn’t seem like it’s going to be. The community has had to respond to that and rebuild from that, and there doesn’t seem to be a lot of unity on what the best path forward is.
And also, Sara Seager is walking the line between the old way of big, federally funded projects and a new private, philanthropic path that may or may not be sustainable or successful, but it’s different and trying to do science in a new way. So maybe we don’t need to rely on big government or NASA to do this. Maybe we could ask philanthropists or crowd-funding or new enterprises that could help finance the projects in going forward. She’s got her feet in both worlds and is emblematic of the field right now.
Artistic representations of the only known planets around other stars (exoplanets) with any possibility to support life as we know it. Credit: Planetary Habitability Laboratory, University of Puerto Rico, Arecibo.
UT: Yes, as you mention in the book, there is this tragic possibility that we may never find the things that these scientists are searching for — “mirror Earths, alien life, extraterrestrial intelligence, or a future beyond our lonely, isolated planet.” What do you see as the future of the search for exoplanets, in this age of funding cuts?
LB: What seems to be happening is that astronomers and planet hunters are needing to change their baselines and move their goalposts. In the past when people talked about space telescopes and finding signs of life, they were thinking of directly imaging planets around Sun-like stars and finding indications of life through studying the atmosphere and even surface features. The new way that is coming about and will likely happen in the next few decades, is an emphasis on smaller, cooler, less sun-like stars — the Red Dwarf or M-Dwarf stars. And it won’t be about directly imaging planets, but looking at transiting planets because it is easier to look at planets around lower-mass stars and at super-Earths that are easier to find and study. But these are rather alien places and we don’t know much about them, so it’s an exciting frontier.
But while transits are jackpots — in that you get all sorts of information like period, mass, radius, density and measures of the planet’s upper atmosphere — transits are very rare. If you think about the nearest thousand stars and if we are just looking for transits, that sort of search will only yield a fraction of the planets and the planetary diversity that exist. If you’re looking for life and potentially habitable planets, we really need a bigger sample and more than just transits to fill out the census of planets orbiting the stars around us.
I think missions like TESS and James Webb are going to be important, but I don’t think it will be enough. It will only leave us on the cusp of answering these bigger questions. I hope I’m wrong and that the emphasis on M-dwarfs and super-Earths and transits will be far more productive and surprising than anyone could have imagined or that there will be technology developed that are orders of magnitude cheaper, more affordable and better than these big telescopes.
But to answer the big questions more robustly in a way that is more satisfying to the public and data-hungry scientists, we’re probably going to have to make big investments, and invest the blood sweat and tears into building one of these big space telescopes. People in the astronomy community have been kicking and screaming about this because they realize the money just isn’t there.
But as someone once told me, there an economic inevitability to this in terms of how much the public can be engaged by these questions and how much they might hunger and thirst for finding other planets and life beyond our solar system. I feel like there is a strong push that could be made. I feel that the public would offer more support for these types of investments rather than for other projects, such as a a big space-based gravitational wave observatory or a big telescope devoted to studying dark energy.
Of course, we are living in this era of constrained and falling budgets, it’s going to be a really hard sell for any of these investments in astronomy, but pursuing the ancient, distant and dead instead of the new, nearby and living is probably a losing proposition, I do I wish astronomers the best of luck, but I hope they make the smart choice to prioritize the most publicly engaging science.
Artist’s impression of the transiting exoplanet HAT-P-7b. Credit: NASA/ESA
UT: You write about the competition and sometimes the disdain that competing astronomers have for each other. Is this competition good, or should there be more unity in the field?
LB: In the interest of the community at large, I’d have to say unity is better and that some people have to wait their turn or reduce their expectations. I’m biased; I’m an advocate of exoplanet missions and these investments. But this is publicly funded science and I think it’s important for the community to be unified because it’s all too easy for the bean-counters in Washington to hear the discordant cacophony coming from the various astronomer hatchlings in the nest, and that there is no consensus except they are hungry and they want more.
They need to be unified to withstand the anti-scientific trends in funding we are seeing in our federal government right now. On the other hand, competition is important. But when you are doing publicly funded science, the scientists need to do a good job of making their case of why they should be funded.
UT: What was the most memorable experience in writing this book?
LB: That’s a really hard question! One of my great privileges and joys of writing the book was having access to these scientists and their work. But one of the most memorable things was visiting California’s Lick Observatory on Mount Hamilton in 2012 for the Transit of Venus. It was the last transit of Venus in our lifetime and it was amazing to stand there and think that the last time the transit was visible from Mount Hamilton was a century before, and realize all the changes that had happened in astronomy since then. This transit happened slowly over hours and it was amazing to stand there and realize, this is the last time in your life you’re going to see it and to wonder what is going to happen in the intervening years until this event occurs again.
But Lick Observatory was an appropriate place to be since that’s where some of the first exoplanets were found. When the last Transit of Venus took place, we hadn’t walked on the Moon, there were no computers, and we’ve had all these great discoveries in astronomy. I was thinking about what the world will be like in another hundred years or so, and thinking how while that is a long time for us, in the scale of planetary time, it is nothing at all! The Sun won’t have significantly aged and Venus will likely look exactly the same in 2117 for the next transit, but I would guess the Earth will be very different then. It’s kind of indicative of this transitional era we’re in. It was a very poignant moment for me.
UT: It’s similar to how Frank Drake talked about how he and his colleagues thought that searching for radio emissions from other civilizations would be so important in the search for extraterrestrial intelligence, but realizing that Earth’s radio emissions from our technology is waning and only lasted a short period of time.
LB: Yeah, perhaps when people look back at my book in the future, they might say, ‘wow, this guy was so blinkered and stupid — he didn’t see this technologies X, Y and Z coming and didn’t see monumental discoveries A, B, and C coming.’ I kind of hope that’s actually the case, because it will mean the search for extraterrestrial life and intelligence will have surpassed my wildest dreams. However, I didn’t try to predict what was going to happen, but just wanted to capture this strange and seemingly unique moment in time in which we are poised on the threshold of these immense discoveries that could totally transform our conception of the Universe and our place in it.
UT: Talking to you today, we can obviously tell how passionate you are about this subject and you were the perfect person to write about it!
An artist's conception of two magnetic fields interacting in a bow shock. Image credit: NASA
Astronomers may soon be able to observe the shockwaves between the magnetic fields of exoplanets and the flow of particles from the stars they orbit.
Magnetic fields are crucial to a planet’s (and as it turns out a moon’s) habitability. They act as protective bubbles, preventing harmful space radiation from stripping away the object’s atmosphere entirely and even reaching the surface.
An extended magnetic field – known as a planetary magnetosphere – is created by the shock between the stellar wind and the intrinsic magnetic field of the planet. It has the potential to be huge. Within our own Solar System, Jupiter’s magnetosphere extends to distances up to 50 times the size of the planet itself, nearly reaching Saturn’s orbit.
When the wind of high-energy particles from the star hits the planetary magnetosphere, it interacts in a bow shock that diverts the wind and compresses the magnetosphere.
Recently a team of astronomers, led by PhD student Joe Llama of the University of St. Andrews, Scotland, have worked out how we might observe planetary magnetospheres and stellar winds via their bow shocks.
Llama took a careful look at the planet HD 189733b, located 63 light years away toward the constellation Vulpecula. From the Earth, the planet is seen to transit its host star every 2.2 days, causing a dip in the overall light from the system.
As a bright star, HD 189733b has been studied extensively by astronomers. Data collected in July 2008 by the Canada-France-Hawaii telescope mapped the star’s magnetic field. While the magnetic field varied, it was on average 30 times greater than that of our Sun – meaning that the stellar wind is much higher than the solar wind.
This allowed the team to carry out extensive simulations of the stellar wind around HD 189733b – characterizing the bow shock created as the planet’s magnetosphere passes through the stellar wind. With this information they were able to simulate the light curves that would result from the planet and the bow shock orbiting the star.
The bow shock leads the planet – causing the light to drop a little earlier than expected. The amount of light blocked by the bow shock, however, will change as the planet moves through a variable stellar wind. If the stellar wind is particularly strong, the resulting bow shock will be strong, and the transit depth will be greater. If the stellar wind is weak, the resulting bow shock will be weak, and the transit depth will be less.
The video below shows the light curve of a bow shock and exoplanet.
“We found that the shockwave between the stellar and planetary magnetic fields will change drastically as activity on the star varies,” Llama told Universe Today. “As the planet passes through very dense regions of the stellar wind, so the shock will become denser, the material in it will block more light and therefore cause a larger dip in the transit making it more detectable.”
While there were no transit observations for this study, this theoretical outlook demonstrates that it will be possible to detect the bow shock, and therefore the magnetic field, of a distant exoplanet. Dr. Llama comments: “This will help us to better identify potentially habitable worlds.”
The paper has been accepted for publication in Monthly Notices of The Royal Astronomical Society and is available for download here.
An artist's conception of a disintegrating planet - creating a trail of dust - around its rocky star.
Solar flares – huge eruptions of charged particles from the Sun – present little threat to Earth. On a few rare occasions these particles may disrupt our communications systems and cause radio blackouts. But they tend to be more aesthetically pleasing than harmful. It’s certainly a sight to be seen as these energetic particles collide with our atmosphere, resulting in a cascade of colorful lights – the aurora borealis.
Fortunately our planet provides the protection necessary from such harmful space radiation. But not all planets are quite so lucky. Take for instance Kepler’s latest object of interest: KIC 12557548b, a super Mercury-size planet candidate. Astronomers have recently found that due to this star’s activity – producing massive stellar flares – the planet itself is evaporating.
Only last year, four different sources published evidence that this rocky planet was disintegrating. Thanks to Kepler, it quickly became clear that the total amount of light from KIC 12557548 as a function of time – the light curve of the system – dropped every 15.7 hours as a planet orbited it. But the amount of light blocked due to the transiting planet varied from 0.2% to more than 1.2%.
The amount of light blocked is dependent on the size of the planet. A Jupiter-size planet will block more light than a Mercury-size planet. The variations here suggest a range for the size of the planet: from a super Mercury-sized planet to a Jupiter-sized planet.
But this wasn’t the planet’s only enigma. It also has an asymmetric light curve. The total light from the star drops steadily as the planet begins its transit, plateaus as the planet fully covers the disk of the star, and then increases as the planet ends its transit. But the rate at which the light drops is much faster than the rate at which it increases. It takes longer for the light curve to return to its original brightness, hinting at a tail of debris that trails the planet, continuing to block light.
The light curves of KIC 12557548b. The left-hand plot represents deep transits, whereas the right-hand plot represents more shallow transits. Both plots show a clear asymmetry. Source: Brogi et al. 2012
It appears that the planet is evaporating – emitting small particles of dust into orbit, which then trails behind it. The varying transit depth reflects the amount of dust currently evaporating.
Recently a team from the University of Tokyo analyzed the system in more detail, attempting to explain why this tiny planet is evaporating. “We found that the transit depth negatively correlates with the modulation of the stellar flux,” Dr. Kawahara, lead author on the study, told Universe Today. “The dust amount increases when the planet is located in front of the star spots.”
The transit depth does not vary randomly, but every 22.83 days. This coincides with the modulation of the stellar flux, or simply the stellar rotation period. Star spots may be indirectly detected by a star’s noticeable decrease in stellar flux. Because these star spots are large (much larger than sunspots) they last for long periods of time, and may be used to deduce the star’s rotation period.
Kawahara et al. found that the transit depth periodically varies with the stellar rotation rate – finding a correlation between stellar activity and the rate at which the planet is evaporating.
“Energy from the star spots increases the amount of dust and atmosphere from the planet,” explains Dr. Kawahara. The extreme heat and wind is enough to speed up the motions of the dust molecules; making them fast enough to escape the planet’s gravitational pull.
Future spectroscopic studies may search for molecules in the evaporating atmosphere of KIC 12557548b. But Dr. Kawahara remarks that due to the planet’s faintness it is unlikely. His best hope is that future studies may instead find a similar object closer to us, that may be more easy to study.
The finding is published in The Astrophysical Journal Letters and is available for download here.
Determining weather patterns in exoplanet atmospheres – hundreds to thousands of light years away – is extremely difficult. However, given that it may be one of our best ways to truly characterize these alien words, it’s a challenge astronomers have accepted willingly.
Most models have a very simple foundation, necessarily eliminating the complex physics that is difficult to incorporate and analyze. Recently, a team led by Dr. Konstantin Batygin of Harvard University, added one more parameter to their models, drastically changing their results.
The punch line is this: the inclusion of magnetic fields significantly changes, and actually simplifies, the atmospheric circulation of hot Jupiters.
Hot Jupiters orbit dangerously close to their host stars, roasting in stellar radiation. But they are also tidally locked to their host stars – one hemisphere continually faces the star, while one continuously faces away – creating a permanent dayside and a permanent nightside.
One would expect the temperature gradient between the dayside and the nightside to be very high. However, various weather patterns play a role in strongly decreasing this temperature gradient. As an example, we now know that clouds may significantly decrease the temperature of the dayside.
Dr. Batygin’s team analyzed magnetic effects within atmospheric circulation. “The case of hot Jupiters is quite peculiar,” she told Universe Today. “The atmospheres of hot Jupiters have temperatures that reach up to 2000 Kelvin, which is hot enough to ionize trace Alkali metals such as potassium and sodium. So the air on hot Jupiters is actually a weakly conducting plasma.”
Once the alkali metals have been ionized – stripped of their electrons – the upper atmosphere contains all of those charged particles and becomes a plasma. It is then electrically conductive and magnetic effects must be taken into account.
While the underlying physics is pretty complex (with nearly 40 multi-lined equations in the paper alone), the introduction of magnetic effects actually simplified the model’s outcome.
In the absence of magnetic fields, the upper and lower atmospheres feature two distinct patterns of circulation. The upper atmosphere consists of winds blowing away from the dayside in all directions. And the lower atmosphere consists of zonal flows – the bands of color on Jupiter. The zonal flows move parallel to lines of latitude in an east-west fashion. Each moves in a different direction than the one above and below it.
“Upon introducing magnetic fields, fancy dayside-to-nightside flows are quenched and the entire atmosphere circulates in an exclusively east-west fashion,” explains Dr. Batygin. The upper atmosphere resembles the lower atmosphere – zonal flows dominate.
Throughout these models, Dr. Batygin et al. assumed a magnetic field aligned with the rotation axis of the planet. Future work will include a closer look at the effect of a more complicated geometry. The team also intends to extend these results to hotter atmospheres, where magnetic fields will slow the rate of these zonal flows. According to Dr. Batygin, “this has potentially observable consequences and we hope to elucidate them in the future.”
These results will be published in the astrophysical journal (preprint available here).
This graphic depicts HD 189733b, the first exoplanet caught passing in front of its parent star in X-rays. Credit: X-ray: NASA/CXC/SAO/K.Poppenhaeger et al; Illustration: NASA/CXC/M.Weiss.
In the medical field, X-rays are used for finding and diagnosing all sorts of ailments hidden inside the body; in astronomy X-rays can also be used to study obscured objects like pulsars and black holes. Now, for the first time, X-rays have been used to study another object in space that tends to be difficult to spot: an extra solar planet. The Chandra X-ray Observatory and the XMM Newton Observatory combined their X-ray super powers to look at an exoplanet passing in front of its parent star.
This is not a new detection of an exoplanet – this same exoplanet, named HD 189733b has been one of the most-observed planets orbiting another star, and was recently in the news for Hubble confirming the planet’s ocean-blue atmosphere and the likelihood of having glass raining down on the planet.
But being able to see the exoplanet in X-rays is good news for future studies and perhaps even detections of planets around other stars.
“Thousands of planet candidates have been seen to transit in only optical light,” said Katja Poppenhaeger of Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., who led the new study, which will be published in the Aug. 10 edition of The Astrophysical Journal. “Finally being able to study one in X-rays is important because it reveals new information about the properties of an exoplanet.”
Artist’s impression of the deep blue planet HD 189733b, based on observations from the Hubble Space Telescope. Credit: NASA/ESA.
HD 189733b is a Jupiter-sized extrasolar planet orbiting a yellow dwarf star that is in a binary system called HD 189733 in the constellation of Vulpecula, near the Dumbell Nebula, approximately 62 light years from Earth.
This huge gas giant orbits very close to its host star and gets blasted with X-rays from its star — tens of thousands of times stronger than the Earth receives from the Sun — and endures wild temperature swings, reaching scorching temperatures of over 1,000 degrees Celsius. Astronomers say it likely rains glass (silicates) – sideways — in howling 7,000 kilometer-per-hour winds.
But it is relatively close to Earth, and so it has been oft-studied by many other space and ground-based telescopes.
In a blog post, Poppenhaeger said she was inspired by the launch of the Kepler telescope, and wondered if exoplanets could be seen in X-rays. She was excited when she found archived data from XMM Newton showing a fifteen hour long observation of the star HD 189733 and the “Hot Jupiter” HD 189733b was crossing in front of the star during that observation.
But the light curve was disappointing, she said. “The star is magnetically active, meaning that its corona is bright and flickering, so its X-ray light curve showed lots of scatter. Looking for a transit signal in this light curve was like trying to hear a whisper in a noisy pub,” Poppenhaeger wrote.
She knew with more data, the transit signal would be clearer, so she applied for – and got – time on Chandra to observe this exoplanet.
She combined the data from all the observations and was finally successful. “I could detect the transit of the planet in X-rays,” Poppenhaeger said. “What surprised me was how deep the transit was: The planet swallowed about 6-8% of the X-ray light from the star, while it only blocked 2.4% of the starlight at optical wavelengths. That means that the planet’s atmosphere blocks X-rays at altitudes of more than 60,000 km above its optical radius – a 75% larger radius in X-rays!”
That means that the outer atmosphere has to be heated up to about 20,000 K to sustain itself at such high altitudes. Additionally, the planet loses its atmosphere about 40% faster than thought before.
Poppenhaeger said she and her colleagues will test more X-ray observations of other similar planets such as CoRoT-2b to learn more about how stars can affect a planet’s atmosphere.
An artist's concept of a rocky world orbiting a red dwarf star. (Credit: NASA/D. Aguilar/Harvard-Smithsonian center for Astrophysics).
Hunters of alien life may have a new and unsuspected niche to scout out.
A recent paper submitted by Associate Professor of Astronomy at Columbia University Kristen Menou to the Astrophysical Journal suggests that tidally-locked planets in close orbits to M-class red dwarf stars may host a very unique hydrological cycle. And in some extreme cases, that cycle may cause a curious dichotomy, with ice collecting on the farside hemisphere of the world, leaving a parched sunward side. Life sprouting up in such conditions would be a challenge, experts say, but it is — enticingly — conceivable.
The possibility of life around red dwarf stars has tantalized researchers before. M-type dwarfs are only 0.075 to 0.6 times as massive as our Sun, and are much more common in the universe. The life span of these miserly stars can be measured in the trillions of years for the low end of the mass scale. For comparison, the Universe has only been around for 13.8 billion years. This is another plus in the game of giving biological life a chance to get underway. And while the habitable zone, or the “Goldilocks” region where water would remain liquid is closer in to a host star for a planet orbiting a red dwarf, it is also more extensive than what we inhabit in our own solar system.
Gliese 581- an example of a potential habitable zone around a red dwarf star contrasted with our own solar system. (Credit: ESO/Henrykus under a Wikimedia Creative Commons Attribution 3.0 Unported license).
But such a scenario isn’t without its drawbacks. Red dwarfs are turbulent stars, unleashing radiation storms that would render any nearby planets sterile for life as we know it.
But the model Professor Menou proposes paints a unique and compelling picture. While water on the permanent daytime side of a terrestrial-sized world tidally locked in orbit around an M-dwarf star would quickly evaporate, it would be transported by atmospheric convection and freeze out and accumulate on the permanent nighttime side. This ice would only slowly migrate back to the scorching daytime side and the process would continue.
Could these types of “water-locked worlds” be more common than our own?
The type of tidal locking referred to is the same as has occurred between the Earth and its Moon. The Moon keeps one face eternally turned towards the Earth, completing one revolution every 29.5 day synodic period. We also see this same phenomenon in the satellites for Jupiter and Saturn, and such behavior is most likely common in the realm of exoplanets closely orbiting their host stars.
The study used a dynamical model known as PlanetSimulator created at the University of Hamburg in Germany. The worlds modeled by the author suggest that planets with less than a quarter of the water present in the Earth’s oceans and subject to a similar insolation as Earth from its host star would eventually trap most of their water as ice on the planet’s night side.
Kepler data results suggest that planets in close orbits around M-dwarf stars may be relatively common. The author also notes that such an ice-trap on a water-deficient world orbiting an M-dwarf star would have a profound effect of the climate, dependent on the amount of volatiles available. This includes the possibility of impacts on the process of erosion, weathering, and CO2 cycling which are also crucial to life as we know it on Earth.
Thus far, there is yet to be a true “short list” of discovered exoplanets that may fit the bill. “Any planet in the habitable zone of an M-dwarf star is a potential water-trapped world, though probably not if we know the planet possesses a thick atmosphere.” Professor Menou told UniverseToday. “But as more such planets are discovered, there should be many more potential candidates.”
Hard times in harsh climes-an artist’s conception of the daytime side of a world orbiting a red dwarf star. (Credit: NASA/JPL-Caltech).
Being that red dwarf stars are relatively common, could this ice-trap scenario be widespread as well?
“In short, yes,” Professor Menou said to Universe Today. “It also depends on the frequency of planets around such stars (indications suggest it is high) and on the total amount of water at the surface of the planet, which some formation models suggest should indeed be small, which would make this scenario more likely/relevant. It could, in principle, be the norm rather than the exception, although it remains to be seen.”
Of course, life under such conditions would face the unique challenges. The daytime side of the world would be subject to the tempestuous whims of its red dwarf host sun in the form of frequent radiation storms. The cold nighttime side would offer some respite from this, but finding a reliable source of energy on the permanently shrouded night side of such as world would be difficult, perhaps relying on chemosynthesis instead of solar-powered photosynthesis.
On Earth, life situated near “black smokers” or volcanic vents deep on the ocean floor where the Sun never shines do just that. One could also perhaps imagine life that finds a niche in the twilight regions of such a world, feeding on the detritus that circulates by.
Some of the closest red dwarf stars to our own solar system include Promixa Centauri, Barnard’s Star and Luyten’s Flare Star. Barnard’s star has been the target of searches for exoplanets for over a century due to its high proper motion, which have so far turned up naught.
The closest M-dwarf star with exoplanets discovered thus far is Gliese 674, at 14.8 light years distant. The current tally of extrasolar worlds as per the Extrasolar Planet Encyclopedia stands at 919.
Searching for and identifying ice-trapped worlds may prove to be a challenge. Such planets would exhibit a contrast in albedo, or brightness from one hemisphere to the other, but we would always see the ice-covered nighttime side in darkness. Still, exoplanet-hunting scientists have been able to tease out an amazing amount of information from the data available before- perhaps we’ll soon know if such planetary oases exist far inside the “snowline” orbiting around red dwarf stars.
Read the paper on Water-Trapped Worlds at the following link.
Artist’s impression of the deep blue planet HD 189733b, based on observations from the Hubble Space Telescope. Credit: NASA/ESA.
Since its discovery in 2005, exoplanet HD 189733b has been one of the most-observed planets orbiting another star, as its size, compact orbit, and proximity to Earth has made it a relatively easy target — as extrasolar planets go. From previous studies, astronomers thought the planet may have an enticing blue-sky atmosphere. Now, further examinations with the Hubble Space Telescope have confirmed this planet really does harbor an azure blue atmosphere, very similar to Earth’s ocean blue color.
But this is no ‘pale blue dot’ ocean world. It is a huge gas giant orbiting very close to its host star. It gets blasted with X-rays from its star — tens of thousands of times stronger than the Earth receives from the Sun — and endures wild temperature swings, reaching scorching temperatures of over 1,000 degrees Celsius. Astronomers say it likely rains glass – sideways — in howling 7,000 kilometer-per-hour winds.
Nope, not a place you’d want to visit.
But the new Hubble observations of its color are the first time an exoplanet’s color has been measured and confirmed. The astronomers measured how much light was reflected off the surface of HD 189733b — a property known as albedo.
“This planet has been studied well in the past, both by ourselves and other teams,” says Frédéric Pont of the University of Exeter, UK, co-author of a new paper. “But measuring its colour is a real first — we can actually imagine what this planet would look like if we were able to look at it directly.”
HD 189733b is a Jupiter-sized extrasolar planet orbiting a yellow dwarf star that is in a binary system called HD 189733 in the constellation of Vulpecula, near the Dumbell Nebula, approximately 62 light years from Earth.
The planet’s blue atmosphere does not come from the reflection of a warm ocean, but is due to a hazy, turbulent atmosphere thought to be laced with silicate particles, which scatter blue light. Earlier observations using different methods have reported evidence for scattering of blue light on the planet, but these most recent Hubble observations give robust confirming evidence, the researchers said.
To make their measurements, the team used Hubble’s Space Telescope Imaging Spectrograph (STIS) to look at the system before, during, and after the planet passed behind its host star as it orbited. As it slipped behind its star, the light reflected from the planet was temporarily blocked from view, and the amount of light observed from the system dropped – not by much, about one part in 10,000 — but this was enough for STIS to determine the albedo.
“We saw the brightness of the whole system drop in the blue part of the spectrum when the planet passed behind its star,” explains Tom Evans of the University of Oxford, UK, first author of the paper. “From this, we can gather that the planet is blue, because the signal remained constant at the other colours we measured.”
Albedo is a measure of how much incident radiation is reflected. The greater the albedo, the greater the amount of light reflected. This value ranges from 0 to 1, with 1 being perfect reflectivity and 0 being a completely black surface. The Earth has an albedo of around 0.4.
According to the team’s paper, HD 189733b has an albedo of 0.4 ± 0.12.
The team says this determination will help in future studies of the atmospheres of other extra solar planets, as well as continuing the studies of one of the most-examined planets orbiting another star.
“It’s difficult to know exactly what causes the colour of a planet’s atmosphere, even for planets in the Solar System,” says Pont [5]. “But these new observations add another piece to the puzzle over the nature and atmosphere of HD 189733b. We are slowly painting a more complete picture of this exotic planet.”
An artist's conception of how common exoplanets are throughout the Milky Way Galaxy. Image Credit: Wikipedia
A new study suggests that the number of habitable exoplanets within the Milky Way alone may reach 60 billion.
Previous research performed by a team at Harvard University suggested that there is one Earth-sized planet in the habitable zone of each red dwarf star. But researchers at the University of Chicago and Northwestern University have now extended the habitable zone and doubled this estimate.
The research team, lead by Dr. Jun Yang considered one more variable in their calculations: cloud cover. Most exoplanets are tidally locked to their host stars – one hemisphere continually faces the star, while one continuously faces away. These tidally locked planets have a permanent dayside and a permanent nightside.
One would expect the temperature gradient between the two to be very high, as the dayside is continuously receiving stellar flux, while the nightside is always in darkness. Computer simulations that take into account cloud cover show that this is not the case.
The dayside is covered by clouds, which lead to a “stabilizing cloud feedback” on climate. It has a higher cloud albedo (more light is reflected off the clouds) and a lower greenhouse effect. The presence of clouds actually causes the dayside to be much cooler than expected.
“Tidally locked planets have low enough surface temperatures to be habitable,” explains Jang in his recently published paper. Cloud cover is so effective it even extends the habitable zone to twice the stellar flux. Planets twice as close to their host star are still cool enough to be habitable.
But these new statistics do not apply to just a few stars. Red dwarfs “represent about ¾ of the stars in the galaxy, so it applies to a huge number of planets,” Dr. Abbot, co-author on the paper, told Universe Today. It doubles the number of planets previously thought habitable throughout the entire galaxy.
Not only is the habitable zone around red dwarfs much larger, red dwarfs also live for much longer periods of time. In fact, the Universe is not old enough for any of these long-living stars to have died yet. This gives life the amount of time necessary to form. After all, it took human beings 4.5 billions years to appear on Earth.
Another study we reported on earlier also revised and extrapolated the habitable zone around red dwarf stars.
Future observations will verify this model by measuring the cloud temperatures. On the dayside, we will only be able to see the high cool clouds. A planet resembling this model will therefore look very cold on the dayside. In fact, “a planet that does show the cloud feedback will look hotter on the nightside than the dayside,” explains Abbot.
This effect will be testable with the James Webb Space Telescope. All in all, the Milky Way is likely to be teeming with life.
