The "Goldilocks" zone around a star is where a planet is neither too hot nor too cold to support liquid water. Ilustration by Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley.
How common are planets like Earth? That’s been a question astronomers and dreamers have pondered for decades, and now, thanks to the Kepler spacecraft, they have an answer. One in five Sun-like stars in our galaxy have Earth-sized planets that could host life, according to a recent study of Kepler data.
“What this means is, when you look up at the thousands of stars in the night sky, the nearest sun-like star with an Earth-size planet in its habitable zone is probably only 12 light years away and can be seen with the naked eye. That is amazing,” said UC Berkeley graduate student Erik Petigura, who led the analysis of the Kepler and Keck Observatory data.
The Kepler telescope’s mission was to try and find small rocky planets with the potential for hosting liquid water and perhaps the ingredients needed for biology to take hold. For four years, the space telescope monitored the brightness of more than 150,000 stars, recording a measurement every 30 minutes.
Analysis by UC Berkeley and University of Hawaii astronomers shows that one in five sun-like stars have potentially habitable, Earth-size planets. (Animation by UC Berkeley/UH-Manoa/Illumina Studios)
For a recent focused study, scientists concentrated on 42,000 sun-like stars (G and K type stars), looking for periodic dimmings that occur when a planet transits — or crosses in front of — its host star. A team of scientists from the Kepler mission and the Keck telescope in Hawaii have announced that from that survey, they found 603 planets, 10 of which are Earth size and orbit in the habitable zone, where conditions permit surface liquid water.
Since there are about 200 billion stars in our galaxy, with 40 billion of them like our Sun, noted planet-hunter Geoff Marcy said that gives us about 8.8 billion Earth-size planets in the Milky Way.
But Marcy also cautioned that Earth-size planets in Earth-size orbits are not necessarily hospitable to life, even if they orbit in the habitable zone of a star where the temperature is not too hot and not too cold.
“Some may have thick atmospheres, making it so hot at the surface that DNA-like molecules would not survive. Others may have rocky surfaces that could harbor liquid water suitable for living organisms,” Marcy said. “We don’t know what range of planet types and their environments are suitable for life.”
Analysis of four years of precision measurements from Kepler shows that 22±8% of Sun-like stars have Earth-sized planets in the habitable zone. If these planets are as prevalent locally as they are in Kepler field, then the distance to the nearest one is around 12 light-years.Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley.
All of the potentially habitable planets found in their survey are around K stars, which are cooler and slightly smaller than the sun, Petigura said. But the team’s analysis shows that the result for K stars can be extrapolated to G stars like the sun.
The Kepler spacecraft is now crippled because of faulty gyroscopes, but scientists say had Kepler survived for an extended mission, it would have obtained enough data to directly detect a handful of Earth-size planets in the habitable zones of G-type stars.
If the stars in the Kepler field are representative of stars in the solar neighborhood, then the nearest (Earth-size) planet is expected to orbit a star that is less than 12 light-years from Earth and can be seen by the unaided eye. Future instrumentation to image and take spectra of these Earths need only observe a few dozen nearby stars to detect a sample of Earth-size planets residing in the habitable zones of their host stars.
“For NASA, this number – that every fifth star has a planet somewhat like Earth – is really important, because successor missions to Kepler will try to take an actual picture of a planet, and the size of the telescope they have to build depends on how close the nearest Earth-size planets are,” said Andrew Howard, astronomer with the Institute for Astronomy at the University of Hawaii. “An abundance of planets orbiting nearby stars simplifies such follow-up missions.”
This illustration compares our Earth with the newly confirmed lava planet Kepler-78b. Kepler-78b is about 20 percent larger than Earth, with a diameter of 9,200 miles, and weighs roughly 1.8 times as much as Earth.
David A. Aguilar (CfA)
A newly verified planet found in data from the Kepler mission delivers on the space telescope’s task of finding Earth-size planets around other stars. The new planet, called Kepler-78b, is the first Earth-sized exoplanet discovered that has a rocky composition like that of Earth. Similarities to Earth, however, end there. Kepler-78b whizzes around its host star every 8.5 hours at a distance of about 1.5 million kilometers, making it a blazing inferno and not suitable for life as we know it.
“We’ve been hearing about the sungrazing Comet ISON that will go very close to the Sun next month,” said Andrew Howard, of the University of Hawaii at Manoa’s Institute for Astronomy. “Comet ISON will approach the Sun about the same distance that Kepler-78b orbits its star, so this planet spends its entire life as a sungrazer.”
Howard is the lead author on one of two papers published in Nature that details the discovery of the new planet. He spoke during a media webcast discussing the finding.
“This is a planet that exists but shouldn’t,” added astronomer David Latham of the Harvard-Smithsonian Center for Astrophysics (CfA), also discussing the discovery during the webcast.
Kepler-78b is 1.2 times the size of Earth with a diameter of 14,800 km (9,200 miles) and 1.7 times more massive. As a result, astronomers say it has a density similar to Earth’s, which suggests an Earth-like composition of iron and rock. A handful of planets the size or mass of Earth have been discovered, but Kepler-78b is the first to have both a measured mass and size. With both quantities known, scientists can calculate a density and determine what the planet is made of.
Its star is slightly smaller and less massive than the sun and is located about 400 light-years from Earth in the constellation Cygnus.
However, the close-in orbit of Kepler-78b poses a challenge to theorists. According to current theories of planet formation, it couldn’t have formed so close to its star, nor could it have moved there. Back when this planetary system was forming, the young star was larger than it is now. As a result, the current orbit of Kepler-78b would have been inside the swollen star.
This diagram illustrates the tight orbit of Kepler-78b, which orbits its star every 8.5 hours at a distance of less than a million miles. It is only 2.7 stellar radii from the center of the star, or 1.7 stellar radii from the star’s surface. David A. Aguilar (CfA)
“It couldn’t have formed in place because you can’t form a planet inside a star,” said team member Dimitar Sasselov, also from CfA. “It couldn’t have formed further out and migrated inward, because it would have migrated all the way into the star. This planet is an enigma.”
One idea, suggested Howard, is that the planet is the remnant core of a former gas giant planet, but that turns out to be a problem as well. “We just don’t know what the origin of this planet is,” Howard said.
However, the two teams of planet hunters feel that its existence bodes well for future discoveries of habitable planets.
The two independent research teams used ground-based telescopes for follow-up observations to confirm and characterize Kepler-78b. The team led by Howard used the W. M. Keck Observatory atop Mauna Kea in Hawaii. The other team led by Francesco Pepe from the University of Geneva, Switzerland, did their ground-based work at the Roque de los Muchachos Observatory on La Palma in the Canary Islands.
To determine the planet’s mass, the teams employed the radial velocity method to measure how much the gravitation tug of an orbiting planet causes its star to wobble. Kepler, on the other hand, determines the size or radius of a planet by the amount of starlight blocked when it passes in front of its host star.
“Determining mass of an Earth-sized planet is technically daunting,” Howard said during the webcast, explaining how they used the HIRES (High Resolution Echelle Spectrometer) on Keck. “We pushed HIRES to its limit. The observations were difficult because the star is young with many more star spots (just like sunspots on our Sun) than our Sun, and we have to remove them from our data. But since this planet orbits every eight and a half hours, we were able to watch an entire orbit in one night. We clearly saw the planet’s signal, and we watched it eight different nights.”
David Aguilar from CfA said both teams knew the other team was studying this star, but they didn’t compare their work until both teams were ready to submit their papers so that they wouldn’t influence each other. “It was very encouraging both teams got the same result,” Aguilar said.
Howard also thought having two separate teams work on the same target was great. “We didn’t have to wait for further confirmation of the planet, because the two teams confirmed each other,” he said. “In science, this is as good as it gets.”
Francesco Pepe from the second team said they benefitted from using a twin of the original HARPS (High Accuracy Radial velocity Planet Searcher) which has found nearly 200 exoplanets. “HARPS North at La Palma has the same precision and efficiency as its twin,” Pepe explained during the webcast, “and we decided to guarantee time to follow up on small exoplanet candidates from Kepler. We optimized our observing strategy and we expect many more confirmations in the coming years from this technique.”
As for Kepler-78b, this is a doomed world. Gravitational tides will continue to pull Kepler-78b even closer to its star. Eventually it will move so close that the star’s gravity will rip the world apart. Theorists predict that the planet will vanish within three billion years. Interestingly, astronomers say, our solar system could have held a planet like Kepler-78b. If it had, the planet would have been destroyed long ago leaving no signs for astronomers today.
“We did not detect additional planets in this system,” said Howard, “but we hope to observe this system more in the future.”
A diagram of the Kepler space telescope. Credit: NASA
In May of this year, the Kepler planet-hunting telescope lost its ability to precisely point toward stars, putting its exoplanet search in jeopardy. Two of the four reaction wheels failed, and Kepler scientists say the spacecraft needs at least three reaction wheels to be able to point precisely enough to continue the mission. In the latest update from Kepler, mission manager Roger Hunter says that the team has made a little headway and had initial success in testing the two failed reaction wheels. But the big test will come later to see how much friction the two wheels generate with continued use.
On Thursday, July 18, 2013 the team initiated recovery tests on the spacecraft’s two failed wheels in order to characterize how the two wheels (Reaction Wheels (RW) 4 and 2) operated and to determine if either could be returned to full use.
RW4 did not spin in the positive (or clockwise) direction but the wheel did spin in the negative (or counterclockwise) direction. Wheel 4 is thought to be the more seriously damaged of the two, Hunter said.
Then, on Monday, July 22 the team tested RW2, and that wheel responded positively to test commands and spun in both directions.
“Over the next two weeks, engineers will review the data from these tests and consider what steps to take next,” Hunter said. “Although both wheels have shown motion, the friction levels will be critical in future considerations. The details of the wheel friction are under analysis.”
Too much friction from the reaction wheels can cause vibration and impact the pointing precision of the telescope.
Kepler has found over 2,700 planetary candidates, with 130 confirmed planets, from the size of Earth’s moon to larger than Jupiter. There are two years of data that has yet to be combed through to detect the faint periodic dimming of distant starlight – the telltale sign of a planet transiting the face of its host star.
Still, the loss of Kepler would be a blow to the search for planets orbiting other stars. Earlier this year, Kepler team members said if the spacecraft could no longer do planet-hunting, there’s a chance it could do something else , such as asteroid hunting or other astronomical observations…just something that doesn’t need as precise ability for pointing.
Artst concept of the Kepler telescope in orbit. Credit: NASA
The future of NASA’s Kepler space telescope mission is in doubt, NASA announced yesterday, as it suffered a failure of a second reaction wheel, losing its ability to precisely point to look for planets orbiting other stars. Reaction wheels enable the spacecraft to aim in different directions without firing thrusters, and the spacecraft needs at least three of the four wheels working to provide the ability to point precisely enough to continue the mission.
But, as we pointed out in our article yesterday, the Kepler team said there are still possibilities of keeping the spacecraft in working order, or perhaps even finding other opportunities for different science for Kepler, something that doesn’t require such precise pointing abilities.
“We’re not ready to call the mission down and out just yet,” said John Grunsfeld, NASA’s associate administrator for the Science Mission Directorate, “but by any measure it’s been a spectacular mission.”
Space expert Scott Hubbard has provided additional insight on the possible ways that NASA could bring the spacecraft back online, and what planet hunters will do next if that’s not possible. Hubbard is a consulting professor of aeronautics and astronautics at Stanford’s School of Engineering, and served as director of NASA Ames Research Center during much of the building phase of the Kepler space telescope. He also worked on the project alongside William Borucki, the Kepler science principal investigator at Ames and the driving force behind the effort, for the decades leading up to formal approval of the mission.
Q: How big of a loss will it be if the Kepler space telescope can’t be repaired?
Hubbard: The science returns of the Kepler mission have been staggering and have changed our view of the universe, in that we now think there are planets just about everywhere.
It will be very sad if it can’t go on any longer, but the taxpayers did get their money’s worth. Kepler has, so far, detected more than 2,700 candidate exoplanets orbiting distant stars, including many Earth-size planets that are within their star’s habitable zone, where water could exist in liquid form.
Kepler has done what the program managers said it would do, and that is to give us an inventory of extrasolar planets. It completed its primary observation phase, and had entered its extended science phase. We’re already in the gravy train period – there’s still a year and a half’s worth of data in the pipeline that scientists will analyze to identify other candidate planets, and there will continue to be Kepler science discoveries for quite some time.
Kepler space telescope’s field of view. Credit: NASA
Q: How might NASA engineers go about getting Kepler functional again?
Hubbard: There are two possible ways to salvage the spacecraft that I’m aware of. One is that they could try turning back on the reaction wheel that they shut off a year ago. It was putting metal on metal, and the friction was interfering with its operation, so you could see if the lubricant that is in there, having sat quietly, has redistributed itself, and maybe it will work.
The other scheme, and this has never been tried, involves using thrusters and the solar pressure exerted on the solar panels to try and act as a third reaction wheel and provide additional pointing stability. I haven’t investigated it, but my impression is that it would require sending a lot more operational commands to the spacecraft.
Q: If neither of these options works, Kepler is still an amazing space instrument. Could it conduct other types of experiments?
Hubbard: People have asked about using it to find near-Earth objects, or asteroids. Kepler carries a photometer, not a camera, that looks at the brightness of stars, and so its optics deliberately defocus light from stars to create a nice spread of light on the detector, which is not ideal for spotting asteroids.
Whether or not it could function as a detector for asteroids is something that would have to be studied, but since it wasn’t built as a camera, I would say that I’m skeptical. That said, certainly between Ames Research Center and the Jet Propulsion Laboratory, they’ve got the best people in the world working on it.
Visualization of Kepler’s planet candidates shown in transit with their parent stars. Credit: Jason Rowe/Kepler Mission/NASA
Q: What’s next for exoplanet hunters?
Hubbard: As I said earlier, there is still a year and a half’s worth of data in the pipeline to analyze to identify candidate planets, so there are still discoveries to be made.
It’s important to make clear, though, that in the original queue of missions aimed at finding life elsewhere, a mission like Kepler was a survey mission to establish the statistical frequency of whether these planets are rare or common. It lived the length of its prime mission, and was extremely successful during that time at achieving this goal. It has paved the way for additional missions, such as TESS – Transiting Exoplanet Survey Satellite – and TPF – Terrestrial Planet Finder – which will continue the search for Earth-like exoplanets in the near future.
A diagram of the Kepler space telescope. Credit: NASA
NASA’s Kepler telescope has lost its ability to precisely point toward stars, putting its exoplanet search in jeopardy. One of the reaction wheels –devices which enable the spacecraft to aim in different directions without firing thrusters – has failed. This is of grave concern because last year reaction wheel #2 failed, and now #4 has failed. Kepler scientists say the spacecraft needs at least three reaction wheels to be able to point precisely enough to hunt for planets orbiting distant stars.
“We need three wheels in service to give us the pointing precision to enable us to find planets,” said Bill Borucki, Kepler principal investigator, during a press briefing today. “Without three wheels it is unclear whether we could continue to do anything on that order.”
But the Kepler team said there are still possibilities of keeping the spacecraft in working order, or perhaps even finding other opportunities for different science for Kepler, something that doesn’t require such precise pointing abilities.
“We’re not ready to call the mission down and out just yet,” said John Grunsfeld, NASA’s associate administrator for the Science Mission Directorate, “but by any measure it’s been a spectacular mission.”
Last year, NASA had approved an extended mission for Kepler through 2016, and so a lot is riding on the health of the spacecraft’s reaction wheels.
Yesterday, (May 14, 2013) Kepler went into safe mode, a pre-programmed software mode that if the observatory has trouble with pointing, it puts the spacecraft in a state where the solar panels turn towards the Sun to maintain power to its systems, as well as sending an alert to ground controllers. When engineers looked at telemetry, they saw indication that reaction wheel #4 was not moving, even after they commanded it to speed up.
“Initially, they did see some movement on the wheel,”said Charles Sobeck, Kepler deputy project manager during today’s briefing, “but it quickly went back to zero speed, indicative of internal failure on the wheel. Our next step is to see what we can do to reduce the fuel consumption, as we would like to extend the fuel reserve as long as we can.”
Sobeck said they have a few things to try yet to perhaps get wheel #4 working again, such as “jiggling” it or trying the wheel in reverse.
“We can try jiggling it, like you’d do with any wheel here on Earth, commanding it to move back and forth,” said Sobeck, “so we can try to bring the wheel back in service. Or perhaps since wheel #2 hasn’t been turned on for eight months, it may come back if we turn it on. It will take us awhile to come up with a plan.”
Sobeck explained they are currently using thrusters to stabilize the spacecraft, and in its current mode, the onboard fuel will last for several months. But they hope to soon put the spacecraft into what is called a “Point Rest State,” which would extend the fuel to last a period of several years.
“The Point Rest State is a sort of oasis where we can park the vehicle while we decide what we can do next, or see if there’s another mode we can operate the spacecraft in,” Sobeck said “Once we know how we can operate, we can know what the spacecraft can do in the future.”
The Point Rest State is a loosely-pointed, thruster-controlled state that minimizes fuels usage while providing a continuous X-band communication downlink. Sobeck described it as using the solar pressure from the Sun in conjunction with minimum thruster use to allow for a periodic slow back and forth rocking motion of the vehicle which is very fuel efficient but still keeps the solar arrays pointing towards the Sun and communications antennas pointed towards Earth.
The software to execute that state was loaded to the spacecraft last week, and last night the team completed the upload of the parameters the software will use.
Sobeck also said there is the possibility of the wheel running in the opposite direction, but running the wheel backward would mean they would need to use more thruster fuel. “The reaction wheels try to balance the forces from the solar pressure, that’s what forces a wheel to run,” he told Universe Today. “If you’re running the wheel backward, you don’t balance the forces, but add to it, and spacecraft will start to tip, so you will have to offset that with additional thruster firings.”
Reaction wheels have been a problem with several different missions, and Sobeck said NASA does have a team looking at problems of reaction wheels and trying to find ways to maximize their longevity.
Earlier this year, elevated friction was detected in reaction wheel #4, and so as a precaution for wheel safety, and as a measure to mitigate the friction, the reaction wheels were spun down to zero-speed and the spacecraft was placed in a thruster-controlled safe mode for several days. After that, the wheel was able to be used again and it operated until this week.
But the team stressed that even if the Kepler spacecraft is unable to make more observations, there are still terabytes of data to pore over yet from the mission.
“We have two years of data that has yet to be searched through,” said Borucki, “I’m optimistic that the data we have we’ll be able to accomplish Kepler’s mission of finding another Earth. We believe that in the next couple of years we will have many more exciting discoveries with respect to finding planets.”
Boricki added that while he’s delighted that they have found so many planetary candidates, on the other hand “I would have been even happier if it had continued another four years. That would have been frosting on the cake,” he said, “but we have an excellent cake right now.”
Kepler has found over 2,700 planetary candidates, with 130 confirmed planets, from the size of Earth’s moon to larger than Jupiter.
“We’ll continue to analyze the data to get the science that Kepler was designed to do,” said Paul Hertz, NASA’s astrophysics director. “Even though Kepler is in trouble, it has collected all the data necessary to answer its scientific objectives. Kepler is not the last exoplanet mission, but the first. It has been a great start to our path of exoplanet exploration.”
There’s also the chance that something else could be done with the spacecraft if it no longer can do planet hunting, such as asteroid hunting or other astronomical observations…just something that doesn’t need as precise ability for pointing. If that’s the case, Hertz said they would open up a call for science mission proposals.
A visualization of the “unseen” red dwarfs in the night sky. Credit: D. Aguilar & C. Pulliam (CfA)
As we’ve reported recently, the likelihood of findings habitable Earth-sized worlds just seems to keep getting better and better. But now the latest calculations from a new paper out this week are almost mind-bending. Using what the authors call a “very careful extrapolation” of the rate of small planets observed around M dwarf stars by the Kepler spacecraft, they estimate there could be upwards of 100 billion Earth-sized worlds in the habitable zones of M dwarf or red dwarf stars in our galaxy. And since the population of these stars themselves are estimated to be around 100 billion in the Milky Way, that’s – on average – an Earth-sized world for every red dwarf star in our galaxy.
Whoa.
And since our solar system is surrounded by red dwarfs – very cool, very dim stars not visible to the naked eye (less than a thousandth the brightness of the Sun) — these worlds could be close by, perhaps as close as 7 light-years away.
With the help of astronomer Darin Ragozzine, a postdoctoral associate at the University of Florida who works with the Kepler mission (see our Hangout interview with him last year), let’s take a look back at the recent findings that brought about this latest stunning projection.
Back in February, we reported on the findings from Courtney Dressing and Dave Charbonneau from the Center for Astrophysics that said about 6% of red dwarf stars could host Earth-size habitable planets. But since then, Dressing and Charbonneau realized they had a bug in their code and they have revised the frequency to 15%, not 6%. That more than doubles the estimates.
Then, just this week we reported how Ravi Kopparapu from at Penn State University and the Virtual Planetary Lab at University of Washington suggested that the habitable zone around planets should be redefined, based on new, more precise data that puts the habitable zones farther away from the stars than previously thought. Applying the new habitable zone to red dwarfs pushes the fraction of red dwarfs having habitable planets closer to 50%.
The graphic shows optimistic and conservative habitable zone boundaries around cool, low mass stars. The numbers indicate the names of known Kepler planet candidates. Yellow color represents candidates with less than 1.4 times Earth-radius. Green color represents planet candidates between 1.4 and 2 Earth radius. Credit: Penn State.
But now, the new paper submitted to arXiv this week, “The Radius Distribution of Small Planets Around Cool Stars” by Tim Morton and Jonathan Swift (a grad student and postdoc from Caltech’s ExoLab) finds there is an additional correction to the numbers by Dressing and Charbonneau numbers.
“This is basically due to the fact that there are more small planets than we thought because Kepler isn’t yet sensitive to a large number that take longer to orbit,” Ragozzine told Universe Today. “Accounting for this effect and enhancing the calculation using some nice new statistical techniques, they estimate that the Dressing and Charbonneau numbers are actually too small by a factor of 2. This puts the number at 30% in the old habitable zone, and now up to about 100% in the new habitable zone.”
Now, it is important to point out a few things about this.
As Morton noted in an email to Universe Today, it’s important to realize that this is not yet a direct measurement of the habitable zone rate, “but it is what I would classify as a very careful extrapolation of the rate of small planets we have observed at shorter periods around M dwarfs.”
And as Ragozzine and Morton confirmed for us, all of these numbers are based on Kepler results only, and so far, while there confirmed planets around M dwarfs, there are none confirmed so far in the habitable zone.
“They do not use any results from Radial Velocity (HARPS, etc.),” Ragozzine said. “As such, these are all candidates and not planets. That is, the numbers are based on an assumption that most/all of the Kepler candidates are true planets. There are varying opinions about what the false positive rate would be, especially for this particular subset of stars, but there’s no question that the numbers may go down because some of these candidates turn out to be something else other than HZ Earth-size planets.”
Other caveats need to be considered, as well.
“Everyone needs to be careful about what “100%” means,” Ragozzine said. “It does not mean that every M dwarf has a HZ Earth-size planet. It means that, on average, there is 1 HZ-Earth size planet for every M dwarf. The difference comes from the fact that these small stars tend to have planets that come in packs of 3-5. If, on average, the number of planets per star is one, and the typical M star has 5 planets, then only 20% of M stars have planetary systems.”
The point is subtle but important. For example, if you want to plan new telescope missions to observe these planets, understanding their distribution is critical, Ragozzine said.
“I’m very interested in understanding what kinds of planetary systems host these planets as this opens a number of interesting scientific questions. Discerning their frequency and distribution are both valuable.”
Additionally, the new definition of the habitable zone from Kopparapu et al. makes a big difference.
As Ragozzine points out:
“This is really starting to point out that the definition of the HZ is based on mostly theoretical arguments that are hard to rigorously justify,” Ragozzine said. “For example, a recent paper came out showing that atmospheric pressure makes a big difference but there’s no way to estimate what the pressure will be on a distant world. (Even in the best cases, we can barely tell that the whole planet isn’t one giant puffy atmosphere.) Work by Kopparapu and others is clearly necessary and, from an astrobiological point of view, we have no choice but to use the best theory and assumptions available. Still, some of us in the field are starting to become really wary of the “H-word” (as Mike Brown calls it), wondering if it is just too speculative. Incidentally, I much, much prefer that these worlds be referred to as potentially habitable, since that’s really what we’re trying to say.”
However, Morten told Universe Today that he feels the biggest difference in their work was the careful extrapolation from short period planets to longer periods. “This is why we get occurrence rates for the smaller planets that are twice as large as Dressing or Kopparapu,” he said via email.
He also thinks the most interesting thing in their paper is not just the overall occurrence rate or the HZ occurrence rate even, but the fact that, for the first time, they’ve identified some interesting structure in the distribution of exoplanet radii.
“For example, we show that it appears that planets of roughly 1 Earth radius are actually the most common size of planet around these cool stars,” Morton said. “This makes some intuitive sense given the rocky bodies in our Solar System—there are two planets about the size of Earth, making it the most common size of small planet in our system too! Also, we find that there are lots and lots of planets around M dwarfs that are just beyond the detection threshold of current ground-based transiting surveys—this means that as more sensitive instruments and surveys are designed, we will just keep finding more and more of these exciting planets!”
But Ragozzine told us that even with all aforementioned caveats, the exciting thing is that the main gist of these new numbers probably won’t change much.
“No one is expecting that the answer will be different by more than a factor of a few – i.e., the true range is almost certainly between 30-300% and very likely between 70-130%,” Ragozzine said. “As the Kepler candidate list improves in quantity (due to new data), purity, and uniformity, the main goal will be to justify these statements and to significantly reduce that range.”
Another fun aspect is that this new work is being done by the young generation of astronomers, grad students and postdocs.
“I’m sure this group and others will continue producing great things… the exciting scientific results are just beginning!” Ragozzine said.
NASA's Kepler mission has discovered a new planetary system that is home to the smallest planet yet found around a star like our sun, approximately 210 light-years away in the constellation Lyra. Credit: NASA/Ames/JPL-Caltech
Scientists have discovered a new planet orbiting a Sun-like star, and the exoplanet is the smallest yet found in data from the Kepler mission. The planet, Kepler-37b, is smaller than Mercury, but slightly larger than Earth’s Moon. The planet’s discovery came from a collaboration between Kepler scientists and a consortium of international researchers who employ asteroseismology — measuring oscillations in the star’s brightness caused by continuous star-quakes, and turning those tiny variations in the star’s light into sounds.
“That’s basically listening to the star by measuring sound waves,” said Steve Kawaler, from Iowa State University in the US, and a member of the research team. “The bigger the star, the lower the frequency, or ‘pitch’ of its song.”
The measurements made by the astroseismologists allowed the Kepler research team to more accurately measure the tiny Kepler-37b, as well as revealing two other planets in the same planetary system: one slightly smaller than Earth and one twice as large.
While Kepler 37b is likely a rocky planet, this would not be a great place for humans to live. It’s likely very hot — with a smoldering surface and no atmosphere.
“Owing to its extremely small size, similar to that of the Earth’s moon, and highly irradiated surface, Kepler-37b is very likely a rocky planet with no atmosphere or water, similar to Mercury,” the team wrote in their paper, which was published this week in Nature. “The detection of such a small planet shows for the first time that stellar systems host planets much smaller as well as much larger than anything we see in our own Solar System.”
The host star, Kepler-37, is about 210 light-years from Earth in the constellation Lyra. All three planets orbit the star at less than the distance Mercury is to the Sun, suggesting they are very hot, inhospitable worlds. Kepler-37b orbits every 13 days at less than one-third Mercury’s distance from the Sun. The estimated surface temperature of this smoldering planet, at more than 800 degrees Fahrenheit (700 Kelvin), would be hot enough to melt the zinc in a penny. Kepler-37c and Kepler-37d, orbit every 21 days and 40 days, respectively.
Artist’s concept of Kepler-37b. The planet is slightly larger than our moon, measuring about one-third the size of Earth. Credit: NASA/Ames/JPL-Caltech
The size of the star must be known in order to measure the planet’s size accurately. To learn more about the properties of the star Kepler-37, scientists examined sound waves generated by the boiling motion beneath the surface of the star.
“The technique for stellar seismology is analogous to how geologists use seismic waves generated by earthquakes to probe the interior structure of Earth,” said Travis Metcalfe, who is part of the Kepler Asteroseismic Science Consortium.
The sound waves travel into the star and bring information back up to the surface. The waves cause oscillations that Kepler observes as a rapid flickering of the star’s brightness. The barely discernible, high-frequency oscillations in the brightness of small stars are the most difficult to measure. This is why most objects previously subjected to asteroseismic analysis are larger than the Sun.
“Studying these oscillations been done for a long time with our own Sun,” Metcalfe told Universe Today, “but the Kepler mission expanded that to hundreds of Sun-like stars. Kepler-37 is the coolest star, as well as the smallest star that has been measured with asterosiesmology.”
Kepler-37 has a radius just three-quarters of the Sun. Metcalfe said the radius of the star is known to 3 percent accuracy, which translates to exceptional accuracy in the planet’s size.
Metcalfe launched a non-profit organization to help raise research funds for the Kepler Asteroseismic Science Consortium. The Pale Blue Dot Project allows people to adopt a star to support asteroseismology, since there is no NASA funding for asteroseismology.
“Much of the expertise for this exists in Europe and not in the US, so as a cost saving measure NASA outsourced this particular research for the Kepler mission,” said Metcalfe, “and NASA can’t fund researchers in other countries.”
The Kepler spacecraft carries a photometer, or light meter, to measure changes in the brightness of the stars it is focusing on in the Cygnus region in the sky.
Kepler Mission Star Field. An image by Carter Roberts of the Eastbay Astronomical Society in Oakland, CA, showing the Milky Way region of the sky where the Kepler spacecraft/photometer is pointing. Each rectangle indicates the specific region of the sky covered by each CCD element of the Kepler photometer. There are a total of 42 CCD elements in pairs, each pair comprising a square. Credit: Carter Roberts / Eastbay Astronomical Society.
Metcalfe said this discovery took a long time to verify, as the signature of this very small exoplanet was hard to confirm, to make sure the signature wasn’t coming from other sources such as an eclipsing binary star.
Kawaler said Kepler is sending astronomers photometry data that’s “probably the best we’ll see in our lifetimes,” he said, adding that this latest discovery shows “we have a proven technology for finding small planets around other stars.”
“We uncovered a planet smaller than any in our solar system orbiting one of the few stars that is both bright and quiet, where signal detection was possible,” said Thomas Barclay, lead author of Nature paper. “This discovery shows close-in planets can be smaller, as well as much larger, than planets orbiting our sun.”
And are there more small planets like this out there, just waiting to be found?
As the team wrote in their paper, “While a sample of only one planet is too small to use for determination of occurrence rates it does lend weight to the belief that planet occurrence increases exponentially with decreasing planet size.”
A new study estimates that fewer than 1% of transiting exoplanet systems host civilizations technologically advanced enough to send out radio transmissions that could be detected by our current SETI searches.
That equates to less than one in a million stars in the Milky Way Galaxy that would have intelligent life we could possibly communicate with. But even with those odds, there could be millions of advanced ET’s in the galaxy that we could phone, researchers say.
A group of astronomers, including Jill Tarter from the SETI Institute and scientists at the University of California, Berkeley used the Green Bank Telescope in West Virginia to look for intelligent radio signals from planets around 86 of stars where the Kepler mission has found transiting exoplanets. These specific targets were chosen because they had exoplanets in the habitable zone around the star and there were either five or more exoplanets in the system, or there was super-Earths with relatively long orbits.
The search came up empty in detecting any signals.
“We didn’t find ET, but we were able to use this statistical sample to, for the first time, put rather explicit limits on the presence of intelligent civilizations transmitting in the radio band where we searched,” said Andrew Siemion from UC Berkeley.
The team looked for signals in the 1-2 GHz range which is the region we use here on Earth for our cell phones and television transmissions. Narrowing it down, the team looked for signals that cover no more than 5Hz of the spectrum since there is no known natural mechanism for producing such narrow band signals.
“Emission no more than a few Hz in spectral width is, as far as we know, an unmistakable indicator of engineering by an intelligent civilization,” the team said in their paper.
The telescope spent 12 hours collecting five minutes of radio emissions from each star. Most of the stars were more than 1,000 light-years away, so only signals intentionally aimed in our direction would have been detected. The scientists say that in the future, more sensitive radio telescopes, such as the Square Kilometer Array, should be able to detect much weaker radiation, perhaps even unintentional leakage radiation, from civilizations like our own.
The researchers said these results allows them to put limits on the likelihood of Kardashev Type II civilizations. The Karashev scale is a method of measuring a civilization’s level of technological advancement, based on the amount of energy a civilization is able to utilize. The team said that finding no signals implies that the number of these civilizations that are “noisy” in the 1-2GHz range must less than one in a million per sun-like star.
The team plans more observations with the Green Bank Telescope, focusing on multi-planet systems in which two of the planets occasionally align relative to Earth, potentially allowing them to eavesdrop on communications between the planets.
“This work illustrates the power of leveraging our latest understanding of exoplanets in SETI searches,” said UC Berkeley physicist Dan Werthimer, who heads the world’s longest running SETI project at the Arecibo Telescope in Puerto Rico. “We no longer have to guess about whether we are targeting Earth-like environments, we know it with certainty.”
A new analysis examined the frequencies of planets of different sizes based on findings from NASA's Kepler spacecraft, correcting for both incompleteness and false positives. The results show that one in six stars has an Earth-sized planet in a tight orbit. Credit: F. Fressin (CfA)
The latest analysis of data from the Kepler planet-hunting spacecraft reveals that almost all stars have planets, and about 17 percent of stars have an Earth-sized planet in an orbit closer than Mercury. Since the Milky Way has about 100 billion stars, there are at least 17 billion Earth-sized worlds out there, according to Francois Fressin of the Harvard-Smithsonian Center for Astrophysics (CfA), who presented new findings today in a press conference at the American Astronomical Society meeting in Long Beach, California. Moreover, he said, almost all Sun-like stars have planetary systems.
The holy grail of planet-hunting is finding a twin of Earth – a planet of about the same size and in the habitable zone around similar star. The odds of finding such a planet is becoming more likely Fressin said, as the latest analysis shows that small planets are equally common around small and large stars.
While the list of Kepler planetary candidates contains majority of the knowledge we have about exoplanets, Fressin said the catalog is not yet complete, and the catalog is not pure. “There are false positives from events such as eclipsing binaries and other astrophysical configurations that can mimic planet signals,” Fressin said.
By doing a simulation of the Kepler survey and focusing on the false positives, they can only account for 9.5% of the huge number of Kepler candidates. The rest are bona-fide planets.
This artist's illustration represents the variety of planets being detected by NASA's Kepler spacecraft. Credit: C. Pulliam & D. Aguilar (CfA)
Altogether, the researchers found that 50 percent of stars have a planet of Earth-size or larger in a close orbit. By adding larger planets, which have been detected in wider orbits up to the orbital distance of the Earth, this number reaches 70 percent.
Extrapolating from Kepler’s currently ongoing observations and results from other detection techniques, it looks like practically all Sun-like stars have planets.
The team then grouped planets into five different sizes. They found that 17 percent of stars have a planet 0.8 – 1.25 times the size of Earth in an orbit of 85 days or less. About one-fourth of stars have a super-Earth (1.25 – 2 times the size of Earth) in an orbit of 150 days or less. (Larger planets can be detected at greater distances more easily.) The same fraction of stars has a mini-Neptune (2 – 4 times Earth) in orbits up to 250 days long.
Larger planets are much less common. Only about 3 percent of stars have a large Neptune (4 – 6 times Earth), and only 5 percent of stars have a gas giant (6 – 22 times Earth) in an orbit of 400 days or less.
The researchers also asked whether certain sizes of planets are more or less common around certain types of stars. They found that for every planet size except gas giants, the type of star doesn’t matter. Neptunes are found just as frequently around red dwarfs as they are around sun-like stars. The same is true for smaller worlds. This contradicts previous findings.
“Earths and super-Earths aren’t picky. We’re finding them in all kinds of neighborhoods,” says co-author Guillermo Torres of the CfA.
Planets closer to their stars are easier to find because they transit more frequently. As more data are gathered, planets in larger orbits will come to light. In particular, Kepler’s extended mission should allow it to spot Earth-sized planets at greater distances, including Earth-like orbits in the habitable zone.
Kepler detects planetary candidates using the transit method, watching for a planet to cross its star and create a mini-eclipse that dims the star slightly.
Artist concept of a previous multi-planet solar system found by the Kepler spacecraft. Credit: NASA/Tim Pyle
Most planetary systems found by astronomers so far are quite different than our own. Many have giant planets whizzing around in a compact configuration, very close to their star. An extreme case in point is a newly found solar system that was announced on October 15, 2012 which packs five — count ‘em — five planets into a region less than one-twelve the size of Earth’s orbit!
“This is an extreme example of a compact solar system,” said researcher Darin Ragozzine from the University of Florida, speaking at a press conference at the American Astronomical Society’s Division for Planetary Sciences meeting. “If we can understand this one, hopefully we can understand how these types of systems form and why most known planetary systems appear different from our own solar system.”
This new system, currently named KOI-500, was found with data from the Kepler planet-finding spacecraft, and Ragozzine said astronomers have now uncovered a new realm of exo-planetary systems.
“The real exciting thing is that Kepler has found hundreds of stars with multiple transiting planets,” he said. “These are the most information-rich systems, as they can tell you not only about the planets, but also the architecture of how solar systems are put together.”
The fact that almost all solar systems found so far are vastly different than our own has astronomers wondering if we are, in fact, the oddballs. A study from 2010 concluded that only about 10 – 15 percent of stars in the Universe host systems of planets like our own, with terrestrial planets nearer the star and several gas giant planets in the outer part of the solar system.
Part of the reason our dataset of exoplanets is skewed with planets that are close to the star is because currently, that is all we are capable of detecting.
But the surprising new population of planetary systems discovered in the Kepler data that contain several planets packed in a tiny space around their host stars does give credence to the thinking that our solar system may be somewhat unique.
However, perhaps KOI-500 used to be more like our solar system.
“From the architecture of this planetary system, we infer that these planets did not form at their current locations,” Ragozzine said. “The planets were originally more spread out and have ‘migrated’ into the ultra-compact configuration we see today.”
There are several theories about the formation of the large planets in our outer solar system which involves the planets moving and migrating inward and outward during the formation process. But why didn’t the inner planets, including Earth, move in closer, too?
“We don’t know why this didn’t happen in our solar system,” Ragozzine said, but added that KOI-500 will “become a touchstone for future theories that will attempt to describe how compact planetary systems form. Learning about these systems will inspire a new generation of theories to explain why our solar system turned out so differently.”
A few notes of interest about KOI-500:
The five planets have “years” that are only 1.0, 3.1, 4.6, 7.1, and 9.5 days.
“All five planets zip around their star within a region 150 times smaller in area than the Earth’s orbit, despite containing more material than several Earths (the planets range from 1.3 to 2.6 times the size of the Earth). At this rate, you could easily pack in 10 more planets, and they would still all fit comfortably inside the Earth’s orbit,” Ragozzine noted. KOI-500 is approximately 1,100 light-years away in the constellation Lyra, the harp.
Four of the planets orbiting KOI-500 follow synchronized orbits around their host star in a completely unique way — no other known system contains a similar configuration. Work by Ragozzine and his colleagues suggests that planetary migration helped to synchronize the planets.
“KOI” stands for Kepler Object of Interest, and Ragozzine’s findings on this system have not yet been published, and so the system has yet to officially be considered a confirmed planetary system. “Every time we find something like this we give it a license-plate-like number starting with KOI,” Ragozzine said.
When does a KOI become an official planet? Ragozzine said the process is by confirming and validating the data. “Basically you need to prove statistically or by getting a specific measurement that it is not some other astronomical signal,” he said.
This infographic from Space.com supplies more visual details:
