Counting Our Countless Worlds

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The Milky Way has several hundred billion stars, and many scientists are now convinced it has even more planets and moons. (NASA)
The Milky Way is home to several hundred billion stars, and many scientists are now convinced it has even more planets and moons. (NASA)

Imagine counting all the people who have ever lived on Earth, well over 100 billion of them.

Then imagine counting all the planets now orbiting stars in our Milky Way galaxy , and in particular the ones that are roughly speaking Earth-sized. Not so big that the planet turns into a gas giant, and not so small that it has trouble holding onto an atmosphere.

In the wake of the explosion of discoveries about distant planets and their suns in the last two decades, we can fairly conclude that one number is substantially larger than the other.

Yes, there are many, many billions more planets in our one galaxy than people who have set foot on Earth in all human history. And yes, there are expected to be more planets in distant habitable zones as there are people alive today, a number upwards of 7 billion.

This is for sure a comparison of apples and oranges. But it not only gives a sense of just how commonplace planets are in our galaxy (and no doubt beyond), but also that the population of potentially habitable planets is enormous, too.   “Many Worlds,” indeed.

The populations of exoplanets identified so far, plotted according to the radius of the planet and how many days it takes to orbit. The circles in yellow represent planets found by Kepler, light blue by using ground-based radial velocity, and pink for transiting planets not found by Kepler, and green, purple and red other ground-based methods. (NASA Ames Research Center)
The populations of exoplanets identified so far, plotted according to the radius of the planet and how many days it takes to orbit. The circles in yellow represent planets found by Kepler, light blue by using ground-based radial velocity, and pink for transiting planets not found by Kepler, and green, purple and red other ground-based methods. (NASA Ames Research Center)

It was Ruslan Belikov, an astrophysicist at NASA’s Ames Research Center in Silicon Valley who provided this sense of scale.  The numbers are of great importance to him because he (and others) will be making recommendations about future NASA exoplanet-finding and characterization missions based on the most precise population numbers that NASA and the exoplanet community can provide.

Natalie Batalha, Mission Scientist for the Kepler Space Telescope mission and the person responsible for assessing the planet population out there, sliced it another way. When I asked her if her team and others now expect each star to have a planet orbiting it, she replied: “At least one.”

Kepler-186f was the first rocky planet to be found within the habitable zone -- the region around the host star where the temperature is right for liquid water. This planet is also very close in size to Earth. (NASA Ames/SETI Institute/JPL-Caltech)
Kepler-186f was the first rocky planet to be found within the habitable zone — the region around the host star where the temperature is right for liquid water. This planet is also very close in size to Earth. (NASA Ames/SETI Institute/JPL-Caltech)

I caught up with Belikov, Batalha and several dozen others intimately involved in cataloguing the vast menagerie of exoplanets at a “Hack Event” earlier this month at Ames. The goal of the three-day gathering was to find ways to improve the already high level of reliability and completeness regarding planets identified by Kepler.

It also provided an opportunity to learn more about how, exactly, these scientists can be so confident about the very large numbers of exoplanets and habitable zone exoplanets they describe. After all, the total number of confirmed exoplanets is a bit under 2,000 – a majority found by Kepler but hundreds of others by pioneering astronomers using ground-based telescopes and very different techniques. Kepler has another 3,000 planet candidates that scientists are in the process of analyzing and most likely confirming, but still. Four thousand is minuscule compared with two hundred billion.

Not everyone completely agrees that we’re ready to estimate such large numbers of exoplanets—suggesting that we need more data before making such important estimates — but the community consensus is that their extrapolations from current data are solid and scientific. And here is why:

The Kepler telescope looks out at a very small portion of the sky with a limited number of stars – about 190,000 of them during its four year survey. And it identifies planets based on the tiny dimming of stars when an object (almost always a planet) crosses between the star and the telescope.

The Kepler telescope looked constantly for four years at almost 200,000 stars in the Cygnus constellation. (Carter Roberts)
The Kepler telescope looked constantly for four years at almost 200,000 stars in the Cygnus & Lyra constellations.  Its lens is always open, by design. (Carter Roberts)

By identifying those 4,000-plus confirmed and candidate planets over four years, Kepler infers the existence of many, many more. As Batalha explained, a transit of the planet is only observable when the orbit is aligned with the telescope, and the probability of that alignment is very small. Kepler scientists refer to this as a “bias” in their observations, and it is one that can be quantified. For example, the probability that an Earth-Sun twin will be aligned in a transiting geometry is just 0.5%. For every one that Kepler detects, there are 200 others that didn’t transit simply because of the orientation of their orbits.

Then there’s the question of faintness and reliability. Kepler is looking out at stars hundreds, sometimes thousands of light years away.  The more distant a star, the fainter it is and the more difficult it is to gather measurements of –and especially dips in — brightness. When it comes to potentially habitable, Earth-sized planets, Batalha said that only 10,000 to 15,000 of the stars observed are bright enough for planets to be detectable even if they do transit the disk of their host star.

Here’s why: Detecting an Earth-sized planet would be roughly equivalent to capturing the image of a gnat as it crosses a car headlight shining one mile away. For a Jupiter-size planet, the bug would grow to only the size of a large beetle.

Add this bias to the earlier one, and you can see how the numbers swell so quickly. And since Kepler’s mission has been to provide a survey of planets in one small region – and not a census – this kind of statistical extrapolation is precisely what the mission is supposed to do.

There are numerous other detecting challenges posed by the dynamics of exoplanets, stars and the great distances. But then there are also innumerable challenges associated with the workings of the 95 megapixel CCD array that is collecting light for Kepler.   “Sensitivity dropouts” caused by those cosmic rays, horizontal “rolling bands” on the CCDs caused by temperature changes in the electronics, “optical ghosts” from binary stars that create false signals of transits on nearby stars — they are some of the many instrument artifacts that can be mistaken as a drop in light coming from a planet. Kepler’s data processing pipeline, much of which has been transferred over to the NASA Ames supercomputer, has the job of sorting all this out.

 

After the CCDs on the Kepler telescope record the light from stars in its viewing field, the data is sent back to Earth and goes through numerous steps before possibly delivering a “Kepler object of interest,” and possibly a planet candidate. Pleiades is the Ames supercomputer. (NASA Ames)
After the CCDs on the Kepler telescope record the light from stars in its viewing field, the data is sent back to Earth and goes through numerous steps before possibly delivering a “Kepler object of interest,” and possibly a planet candidate. Pleiades is the Ames supercomputer. (NASA Ames)

Adding to the challenge, said Jon Jenkins, a Kepler co-investigator at Ames and the science lead for the pipeline development, is that the stars viewed by Kepler turned out to be themselves “noisier” than expected. Stars naturally vary in their overall brightness, and the data processing pipeline had to be upgraded to account for that changeability.  But that stellar noise has played a key role in keeping Kepler from seeing some of the small planet transits that the team hoped to detect.

What the Hack event and other parallel efforts are doing is finding ways to, as Jenkins put it, “dig into the noise…to move towards the hairy edge of what our data can show.” The final goal: “To come up with the newest, best washer we can to clean the data and come out with an improved catalog of sparkling planets.”

All the data that will come from the primary Kepler mission, which came to a halt in the summer of 2013, has been collected and analyzed already on a first round. But now the entire pipeline of data is going to be reprocessed with its many improvements so the researchers can dig deeper into data trove. Batalha said they hope to find planets – especially Earth-sized planets – this way.

One of the key techniques to measure the performance of Kepler’s analysis pipeline is to inject fake transit signals into the data and see if it picks up their presence. As Batalha explained, this provides another way to gauge the biases in the system, its efficiency at detecting the planets that it could and should see. “If we inject 100 fake things into the pipeline and find 90 of them, that’s means we’re 90 percent complete.” She said the number would then be worked into the calculations of how many planets are out there, and how many of certain sizes will be caught and missed.

Natalie Batalha is the Chief Scientist for the Kepler mission, while announcing the discovery of Kepler’s first rocky planet, Kepler-10b, in January 2011.

So the Hack Event, which brought together astrophysicists, planetary scientists and computer hakers, was designed to come up with ways to improve Kepler’s completeness (seeing everything there to be seen) and reliability (the likelihood that the signal comes from a planet and not an instrument artifact or non-planetary phenomena in space). By computing both the completeness and reliability, scientists are confident that they can eliminate the observation biases and transform the discovery catalog into a directory of actual planets.

This is one of the key accomplishments of the Kepler mission – making it scientifically possible to say that there are billions and billions of planets out there. What’s more, the increased power of Kepler allowed for the discovery of smaller planets, which are now known to make up the bulk of the exoplanets. And while the number of Earth-sized planets detected in that habitable zone is small – around thirty – that’s still quite a remarkable feat. And remember, Kepler is looking at but one small sliver of the sky.

The twelve exoplanets detected so far closest to Earth in size, lined up with the type of stars they orbit. (NASA Ames)
The twelve exoplanets detected and confirmed so far closest to Earth in size, lined up with the type of stars they orbit. (NASA Ames)

Why does it matter how many exoplanets are out there, how many are rocky and Earth-sized, and how many within habitable zones? The last twenty years of exoplanet hunting, after all, has made clear that there are an essentially infinite number of them in the universe, and untold billions in our galaxy.

The answer lies in the insatiable human desire to know more about the world writ large, and how and why different stars have very different solar systems. But more immediately, there’s the need to know how to best design and operate future planet-finding missions. If the goal is to learn how to characterize exoplanets – identify components of their atmospheres, learn about their weather, their surfaces and maybe their cores – then scientists and engineers need to know a lot more about where planets generally, and some specifically, can be found. And those planet demographics just might open some surprising possibilities.

For instance, Belikov and his Ames colleague Eduardo Bendek have proposed a NASA “small explorer” (under $175 million) mission to launch a 30-to-45 centimeter mirror designed to look for Earth-sized planets only at our nearest stellar neighbor, Alpha Centauri. That’s as small a telescope as you can buy off-the-shelf.

Alpha Centauri is the closest star system to our Solar System at about 4.37 lightyears away. (NASA/Hubble Space Telescope)
Alpha Centauri is the closest star system to our Solar System at about 4.37 lightyears away. (NASA/Hubble Space Telescope)

Alpha Centauri is a two-star system, and until recently researchers doubted that binaries like it would have orbiting planets. But Kepler and other planet hunters have found that planets are relatively common around binaries, making Alpha Centauri a better target than earlier imagined.

To make it a truly viable project, ACESat – the Alpha Centauri Exoplanet Satellite – requires something else: a scientifically sound estimate of the likelihood that any star in our galaxy would have an Earth-sized planet in its system. Estimates so far have ranged from 10 percent to 50 percent, but Belikov said newer data is encouraging.

“If that number becomes more firm and approaches 50 percent, then an Alpha Centauri-only mission makes a great deal of sense,” he said. “For a small investment, we could have a real possibility of detecting a planet very close by.”

Intriguing, and an insight into how new space missions are designed based on the science already completed. Both NASA and the European Space Agency have plans to launch three significant exoplanet missions within the decade, and the powerful James Webb Space Telescope will launch in 2018 with some known and undoubtedly some not yet understood capabilities for exoplanet discovery. And perhaps most important, NASA is about to study how a potential mission in the 2030s could be designed with the specific purpose of directly imaging exoplanets – the gold standard for the field. All are being designed based on current exoplanet understandings, including the abundance calculations enabled by the Kepler mission’s observations.

Almost 2,000 exoplanets have now been identified, more than half by Kepler. Another 3,000 exoplanet candidates await confirmation. (NASA Ames)
Almost 2,000 exoplanets have now been detected and confirmed, more than half by Kepler. Another 3,000 exoplanet candidates await confirmation. (NASA Ames)

Future posts will dig deeper into a fair number of the subjects raised here, but for now this much is clear: Our galaxy has many billions of planets, and the process of detecting them is robust and on-going, the process of characterizing them has begun, and all the signs point towards the presence of enormous numbers of planets in habitable zones that, in the biggest picture at least, could possibly support life.

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The Exoplanet Era

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Many, and perhaps most stars have solar systems with numerous planets, as in this artist rendering of Kepler 11. (NASA)

Throughout the history of science, moments periodically arrive when new fields of knowledge and discovery just explode.

Cosmology was a kind of dream world until Edwin Hubble established that the universe was expanding, and doing so at an ever-faster rate. A far more vibrant and scientific discipline was born. On a more practical level, it was only three decades ago that rudimentary personal computers were still a novelty, and now computer-controlled, self-driving cars are just on the horizon. And not that long ago, genomics and the mapping of the human genome also went into hyperspeed, and turned the mysterious into the well known.

Most frequently, these bursts of scientific energy and progress are the result of technological innovation, coupled with the far-seeing (and often lonely and initially unsupported) labor and insights of men and women who are simply ahead of the curve.

We are at another of those scientific moments right now, and the subject is exoplanets – the billions (or is it billions of billions?) of planets orbiting stars other than our sun.

The 20th anniversary of the breakthrough discovery of the first exoplanet orbiting a sun, 51 Pegasi B, is being celebrated this month with appropriate fanfare. But while exoplanet discovery remains active and planet hunters increasingly skilled and inventive, it is no longer the edgiest frontier.

Now, astronomers, astrophysicists, astrobiologists, planetary scientists, climatologists, heliophysicists and many more are streaming into a field made so enticing, so seemingly fertile by that discovery of the apparent ubiquitiousness of exoplanets.

The new goal: Identifying the most compelling mysteries of some of those distant planets, and gradually but inexorably finding ever-more inventive ways to solve them. This is a thrilling task on its own, but the potential prize makes it into quite an historic quest. Because that prize is the identification of extraterrestrial life.

The presence of life beyond Earth is something that humans have dreamed about forever – with a seemingly intuitive sense that there just had to be other planets out there, and that it made equal sense that some of them supported life. Hollywood was on to this long ago, but now we have the beginning technology and fast-growing knowledge to transform that intuitive sense of life out there into a working science.

The thin gauzy rim of the planet in foreground is an illustration of its atmosphere. (NASA’s Goddard Space Flight Center)
The thin gauzy rim of the planet in foreground is an illustration of its atmosphere. (NASA’s Goddard Space Flight Center)

Already the masses and orbits of several thousand exoplanets have been measured. Some planets have been identified as rocky like Earth (as opposed to gaseous like Jupiter.) Some have been found in what the field calls “habitable zones” – regions around distant suns where liquid water could plausibly run on a surface –as it does on Earth and once did on Mars. And some exoplanets have even been determined to have specific compounds – carbon dioxide, water, methane, even oxygen – in their atmospheres.

This and more is what I will be exploring, describing, hopefully bringing to life through an on-going examination of this emerging field of science and the inventive scientists working to understand planets and solar systems many light-years away. Theirs is a daunting task for sure, and progress may be halting. But many scientists are convinced that the goal is entirely within reach – that based on discoveries already made, the essential dynamics and characteristics of very different kinds of planets and solar systems are knowable. Thus the name of this offering: “Many Worlds.”

Artist rendering of early stages of planet formation in the swirl and debris of the disk of a new star. (NASA/JPL-Caltech)
Artist rendering of early stages of planet formation in the swirl and debris of the disk of a new star. (NASA/JPL-Caltech)

I was first introduced to, and captivated by, this cosmic search in a class for space journalists taught by scientists including Sara Seager, a dynamic young professor of physics and planetary science at M.I.T., a subsequently-selected MacArthur “genius,” and a pioneer in the field not of discovering exoplanets, but of characterizing them and their atmospheres. And based on her theorizing and the observations of many others, she was convinced that this characterizing would lead to the discovery of very distant extraterrestrtial life, or at least to the discovery of planetary signatures that make the presence of life highly probable. Just this week, she predicted the discovery could take place within a decade.

It was in 2010 that she began her book “Exoplanet Atmospheres” with the statement: “A new era in planetary science is upon us.” I would take it further: A new era has arrived in the human drive to understand the universe and our place in it. Exoplanets and their solar systems are a magnet to young scientists, says Paul Hertz, the head of NASA’s Astrophysics Division. Almost a third of the papers presented at astronomy conferences these days involve exoplanets, he said, and “it’s hard to find scientists in our field under thirty not working on exoplanets.” Go to a major geology conference, or a planetary science meeting, and much the same will be true.

And why not? I think of this moment as akin to the time in the 17th century when early microscopes revealed a universe of life never before seen. So many new questions to ask, so many discoveries to make, so much exciting and ultimately world-changing science ahead.

But the challenge of characterizing exoplanets and some day identifying signs of life does not lend itself to the kind of solitary or small group work that characterized microbiology (think the breakthrough NASA Kepler mission and the large team needed to make it reality and to analyze its results.) Not only does it require costly observatories and telescopes and spectrometers, but it also needs the expertise that scientists from different fields can bring to the task – rather like the effort to map the human genome.

That is the organizing logic of astrobiology – the more general hunt for life elsewhere in our solar system and far beyond, alongside the search for clues into how life may have started on our planet. NASA is eager to encourage that same spirit in the more specific but nonetheless equally sprawling exploration of exoplanets, their atmospheres, their physical makeup, their climates, their suns, their neighborhoods.

The Earth alongside “Super-Earth-” sized exoplanets identified with the Kepler Space Telescope. (NASA Ames / JPL-Caltech)
The Earth alongside “Super-Earth-” sized exoplanets identified with the Kepler Space Telescope. (NASA Ames / JPL-Caltech)

The result was the creation this summer of the the Nexus for Exoplanet System Science (NExSS), a group that will be led by 17 teams of scientists from around the country already working on some aspect of the rich exoplanet opportunity. The group was selected from teams that had applied for grants from NASA’s Astrobiology Institute, an arm of its larger NASA Astrobiology Program, as well as other NASA programs in the Planetary Sciences, Astrophysics and Astronomy divisions.

Their mandate is to spark new approaches in the effort to understand exoplanets by identifying areas without consensus in the broader community, and then fostering collaborations here and abroad to address those issues. “Many Worlds” grew out of the NExSS initiative, and will chronicle and explain the efforts of some team members as they explore how exo-plants and exo-creatures might be detected; what can be learned from afar about the surfaces and cores of exoplanets and how both play into the possibility of faraway life; the presence and dynamics of exo-weather, what we can learn about exoplanets from our own planet and solar system, and so much more.

A few of the teams are small, but many are quite large, established and mature – perhaps most especially the Virtual Planetary Laboratory at the University of Washington, and run by Victoria Meadows. Since 2001, the virtual lab has collaborated with researchers representing many disciplines, and from as many as 20 institutions, to understand what factors might best predict whether an exoplanet harbors life, using Earth as a model.

But just as I will be venturing beyond NExSS in my writing about this new era of exploration, so too will NExSS be open to the involvement of other scientists in the field. The original group has been tasked with identifying an agenda of sorts for NASA exoplanet missions and efforts ahead. But its aim is to be inclusive and its conclusions and recommendations will only be as useful and important as the exoplanet community writ large determines them to be.

The Carina Nebula, one of many regions where stars come together and planets later form made out of the surrounding dust, gas and later rock. (NASA, ESA, and the Hubble SM4 ERO Team)
The Carina Nebula, one of many regions where stars come together and planets later form made out of the surrounding dust, gas and later rock. (NASA, ESA, and the Hubble SM4 ERO Team)

This is a moment pregnant with promise. Systematically investigating exoplanets and their environs is an engine for discovery and a pathway into that largest question of whether or not we are alone in the universe.

Will scientists some day find worlds where donkeys talk and pigs can fly (as at least one “everything is possible” philosopher has posited)? Unlikely.

But just as microscopes and the scientists using them led to the science of microbiology and most of modern medicine, so too are our orbiting observatories, Earth-based telescopes and the scientists who analyze their results are regularly opening up a world of myriad and often surprising marvels.

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