# If thousands of exoplanets have been discovered, why haven't we discovered planet 9 yet?

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According to NASA, we've discovered 3,453 exoplanets so far [1]. With some of the farthest being at 13,000 light years away [2]. This would suggest today scientists have a really advanced capability for this kind of discoveries. However, the existence of a ninth planet in the solar system (which has not only been proposed but its characteristics are already predicted with great precision[3]) has not yet been discovered. If a ninth planet existed in our solar system, shouldn't it have already been discovered? How can exoplanets thousands of light years away be discovered weekly, while a giant planet right in our backyard still lays undiscovered?

There are two primary reasons Planet Nine (if it exists) is extremely hard to detect: it's very dim and its position is not well constrained.

• Brightness - We are able to see planets in our solar system because they reflect light from the sun. The amount of light they reflect depends on how large the object is, how reflective it is, and (most importantly) the amount of sunlight that hits it. The hypothesized distance at which Planet Nine is orbiting is so far away that it will receive only a tiny amount of sunlight. At this distance, its apparent magnitude would be greater than 22. In comparison, Pluto's apparent magnitude is about 15. Magnitude is a logarithmic scale, so that means that Planet Nine would be $631$ times dimmer than Pluto, making it incredibly hard to see.

• Unknown Position - The estimates of Planet Nine's characteristics are based on how it gravitationally disrupts the orbits of some trans-Neptunian objects. Based on these observations, scientists can calculate where it is likely to be, but these calculations depend on its size. Estimates of its mass range from 2 to 4 times the mass of the Earth. And all of these are just estimates, giving us a relatively large patch of sky to search to find Planet Nine.

Combining both of these reasons, it is very difficult to search for Planet Nine. We're looking for an incredibly dim object in a large area of the sky.

Now, when we look for exoplanets, things are actually a lot easier. To detect exoplanets we hardly ever look for the planets directly. We've only detected about 20 planets via direct imaging. Instead, we look at the light from the star. The stars are many orders of magnitude brighter than the planets, and are very easy to see. All we have to do is see how the light from the star changes over time. An early method detect shifts in the radial velocity of the star (its motion towards or away from us) using Doppler spectroscopy. The more prevalent method today, the one that the Kepler mission used, is transit photometry. We measure the light output of a star, and look for dips in its intensity, caused by the planet orbiting in front of the star and blocking some of the light.

Because we're looking at a star, we don't have the problems we have when looking for Planet Nine. Stars are very bright, their positions are known very accurately, and we don't have to wait too long to make a detection. Plus, surveys like Kepler are able to scan many, many stars at once.

## There's Basically 'No Chance' for Earth-Like Planets to Form an Atmosphere Around Hot Young Stars

Recent exoplanet surveys suggest that there might be thousands of Earth-like worlds in other solar systems, just waiting to be discovered. It's too bad that their atmospheres — and, with them, any hope of sustaining life — were probably obliterated by their local stars.

That's the ruthless takeaway of a new study published April 19 in the journal Astronomy and Astrophysics, anyway. In the new paper, a team of European researchers created a computer model to simulate atmosphere formation on Earth-like planets orbiting around hot, young stars. Because young suns tend to emit extremely high amounts of X-rays and ultraviolet (UV) radiation, most potentially habitable exoplanets would likely see their atmospheres obliterated within 1 million years of the planet's birth. [9 Scientific Excused For Why We Haven't Found Aliens Yet]

"An Earth-like atmosphere cannot form when the planet is orbiting within the habitable zone of a very active star," the researchers wrote in the study. "Instead, such an atmosphere can only form after the activity of the star has decreased to a much lower level."

When astronomers talk about the activity of a star, they're referring to the amount of radiation emitted. Not unlike humans and puppies, young stars tend to be highly active, then significantly decrease their activity levels as they age. The precise activity levels at different ages depend on the star's mass.

In the case of M-dwarf stars — which are slightly smaller than Earth's sun and believed to be the dominant type of star in nearby solar systems — it can take several billion years before solar activity diminishes to levels comparable to Earth's sun today. In that time, the researchers found, any exoplanet orbiting in the habitable zone around such a star would be bombarded with so much radiation that there would be little chance of an atmosphere surviving the first 100,000 years.

As a result, most Earth-like exoplanets detected around M-dwarf stars in nearby solar systems probably have very thin atmospheres or none at all, the researchers concluded, leaving the surfaces of those planets exposed to the punishing effects of solar radiation. Unfortunately, that means life on even the most habitable-looking planets might be rarer than previously thought.

## Another Day, Another Exoplanet, and Scientists Just Can't Keep Up

As finding alien worlds has gotten easier, learning every single detail scientists can has become, perhaps surprisingly, a bit of a waste of precious time of instruments and computers alike.

To date, scientists have discovered 4,104 confirmed exoplanets. But for every confirmed planet that astronomers nail down, there are handfuls of maybe-planets in the data, whispers in the data that might come from stars hiccuping or pairs of stars dancing or would-be stars that didn't quite make the cut. And scientists no longer have the resources to analyze every potential planet's identity crisis.

"It's gotten to the point that we have so many to choose from now — there's so many exciting candidates coming in that we actually don't have to look at every single one and confirm every single one," Jessie Christiansen, an astronomer at Caltech and NASA's Exoplanet Science Institute, told Space.com. "You really have to prioritize, you have to look at this list of planets that are coming out and say, 'OK, which one do we really think we're going to learn the most about?'"

Properly confirming an exoplanet is a laborious process that requires scientists to determine both the size and the mass of the object to rule out other phenomena masquerading as a planet. Those observations use instruments that are in high demand from scientists studying a host of phenomena.

And the confirmation process can be time-consuming. In particularly knotty situations, Christiansen said, it can stretch up to a year. "Some of these planet candidates really, if it gets its hooks in you, if it becomes your thing that you're trying to solve," she said, "you can sink all the time into these things."

But there are two different ways to learn about exoplanets. One approach zooms in on individual planets to learn as much as possible — whether it is rocky or gaseous, whether it has an atmosphere and what that atmosphere looks like, how it may have become the way it is. But these questions can only be answered about planets that orbit particularly bright stars otherwise, scientists can't get enough data.

The second approach looks at the diversity of planets across the universe as a population. "The Kepler mission was interested in statistics," Christiansen said. "The point was to get thousands of planets to put in our buckets and say, 'OK, this one is the most common, this one is the next most common and that kind of thing.'"

Which is precisely what the Kepler Space Telescope did between 2009 and 2018 during its two distinct missions, called Kepler and K2, finding more than 2,500 confirmed exoplanets. That bounty of riches prompted a shift in scientists' mindsets, Christiansen said, as individual worlds became less unique.

"If it's the 80th hot Jupiter that's been found and we don't have any reason to believe it's going to be different from the 79 that came before it," she said, "are we really going to scrutinize it in the same way that we scrutinized the first 79?"

And so, as the Kepler discoveries piled up, scientists introduced a new technique of evaluating potential planets, called validation. With more easily acquired observations, astronomers run a statistical model evaluating the probability of non-planet explanations for the data they have acquired below a certain cutoff, that's good enough for scientists focused on surveying populations of exoplanets.

"There was this revolution in the field in our thinking, which was, we don't actually have to confirm every single one, we can validate them," Christiansen said. "So you believe it's a planet, statistically, but you actually haven't measured the mass. That's kind of the cheap — I'm using air quotes — that's kind of a cheap way of confirming planets."

But even validating planets is now too expensive a process for exoplanet scientists to apply to every potential world. And the scarcity of planet-confirmation resources astronomers face is only going to become a starker problem, Christiansen said.

In April 2018, NASA launched its new planet-finder, the Transiting Exoplanet Survey Satellite, or TESS. Scientists expect to confirm about 16,000 planets spotted in its data — but that requires sniffing out somewhere in the vicinity of 100,000 to 300,000 candidate planets and evaluating each.

"Now I have to look at all these candidates and decide which ones I even want to confirm," Christiansen said, who said she's excited by the bounty of exoplanets, despite the strict prioritization it requires. "I've been hunting for planets since 2004 — 15 years — and plenty of scientists have been hunting for even longer. And this is the first time I've really just sat back and gone, 'Wow. It's not worth doing some of this, just in terms of the time.'"

And the embarrassment of riches will only continue, she said. NASA's next exoplanet-finder, the Wide Field Infrared Survey Telescope (WFIRST), may allow scientists to discover 100,000 confirmed exoplanets — which means even more hundreds of thousands of candidates to evaluate.

"Coming up with new statistical ways to deal with this I think will be even more important as a tool going forward," Christiansen said. "We have more planets than we have resources, but that's only going to get worse and much worse, like, exponentially for the next decade."

## Phone home?

"We should be prepared" for aliens, says professor of space science John Zarnecki, from the Open University. Stephen Hawking says aliens almost certainly exist and senior Seti astronomer Seth Shostak has said that the hunt for alien life should take into account alien "sentient machines", almost disregarding the possibility that there's nothing to search for.

But many scientists argue that because humans have been using wave technology for little over a century - compared to the Earth's age of over four billion years - even if anyone is out there, the window of opportunity to have similar technology is incredibly small.

Indeed, the radio wave as we know it for our communication purposes, is already changing from an analogue wave into a digital pulse, a much more complex signal to detect. And similarly, the waves scientists are looking for may not be the right ones. While a larger amount of the wave spectrum is being examined, it is still a small fraction.

The theory goes that no other inhabited planet is likely to be using the same technology at the same time, or at least within distance of making contact. The actual practicalities of ET phoning home would be, they would argue, basically impossible.

## Why Haven't Scientists Found 'Earth 2.0' Yet?

The ideal 'Earth 2.0' will be an Earth-sized, Earth-mass planet at a similar Earth-Sun distance from . [+] a star that's very much like our own. We have yet to find such a world.

NASA Ames/JPL-Caltech/T. Pyle

Over the past 30 years, astronomers have gone from zero known extra-solar planets to thousands.

The radial velocity (or stellar wobble) method for finding exoplanets relies on measuring the motion . [+] of the parent star, as caused by the gravitational influence of its orbiting planets.

Periodic changes in a star's motion or regular brightness dips give them away.

When a properly-aligned planet passes in front of a star relative to our line-of-sight, the overall . [+] brightness dips. When we see the same dip multiple times with a regular period, we can infer the existence of a potential planet.

William Borucki, Kepler Mission principal investigator, NASA / 2010

Thanks to these techniques, we've revealed the masses and radii of worlds nearby and thousands of light years away.

While Kepler has found some Earth-size planets, the vast majority of the ones discovered are larger . [+] than Earth, and have very short orbital periods these are the easiest worlds to detect.

NASA Ames / W. Stenzel Princeton University / T. Morton

Over 200 are Earth-sized, with many residing in the so-called habitable zone around their stars.

The habitable zone is the range of distances from a star where liquid water might pool on the . [+] surface of an orbiting planet. If a planet is too close to its parent star, it will be too hot and water would have evaporated. If a planet is too far from a star it is too cold and water is frozen. Stars come in a wide variety of sizes, masses and temperatures. Stars that are smaller, cooler and lower mass than the Sun (M-dwarfs) have their habitable zone much closer to the star than the Sun (G-dwarf). Stars that are larger, hotter and more massive than the Sun (A-dwarfs) have their habitable zone much farther out from the star.

NASA/Kepler Mission/Dana Berry

Kepler-186f is one of the smallest, most Earth-sized planets found around a star, with a size just . [+] 17% larger than Earth. But it orbits a red dwarf star, meaning it's not going to have Earth-like conditions. This is true as well for Kepler-438b, one of the other smallest, most Earth-sized planets (just 12% larger than Earth).

NASA Ames/JPL-Caltech/T. Pyle

There are three primary reasons for this.

Most of the planets we know of that are comparable to Earth in size have been found around cooler, . [+] smaller stars than the Sun. This makes sense with the limits of our instruments these systems have larger planet-to-star size ratios than our Earth does with respect to the Sun.

1.) Most of the small planets we know of are found around red dwarf stars.

We've classified many worlds outside of our Solar System as being potentially habitable, owing to . [+] their distance from their star, their radius and their temperatures. But many of the worlds we've found are too large to be rocky, and are found orbiting red dwarf stars, making them quite unlike Earth is.

NASA Ames / N. Batalha and W. Stenzel

Red dwarfs are the most common, and offer the largest planet-to-star size and mass ratios, making planets easier to detect.

The assumption that worlds just a little bit larger/more massive than Earth would be rocky may be . [+] erroneous, and may cause us to eliminate a large fraction of what were previously classified as potentially habitable worlds.

2.) Larger planets are easier to find most are too large to be rocky without a giant gas envelope.

Illustration of the planet-finding space telescope, Kepler, from NASA. Kepler has found thousands of . [+] planets around stars in the Milky Way, teaching us about the mass, radius, and distribution of worlds beyond our Solar System. But its primary mission lasted only three years, meaning planets with long periods (on the order of years) could not be robustly detected.

3.) We didn't observe them for long enough to detect planets with

Today, we know of over 3,500 confirmed exoplanets, with more than 2,500 of those found in the Kepler . [+] data. These planets range in size from larger than Jupiter to smaller than Earth. Yet because of the limitations on the size of Kepler and the duration of the mission, there have been zero Earth-sized planets found around Sun-like stars that fall into Earth-like orbits.

NASA/Ames Research Center/Jessie Dotson and Wendy Stenzel missing Earth-like worlds by E. Siegel

If our own Solar System were at the distance of most stars, we wouldn't have discovered Earth.

It will take longer-duration missions with excellent light-gathering power and sensitivity to reveal . [+] the first Earth-like world around a Sun-like star. There are plans in both NASA's and ESA's timelines for such missions.

It's the next generation of planet-finders, like James Webb and PLATO, that will hopefully deliver our first true Earth-like world.

Mostly Mute Monday tells the scientific story of an astronomical object or phenomenon in images, visuals, and no more than 200 words. Talk less smile more.

## Size matters in the detection of exoplanet atmospheres

Artist’s impressions of exoplanetary system. Credit: Alexaldo

A group analysis of 30 exoplanets orbiting distant stars suggests that size, not mass, is a key factor in whether a planet's atmosphere can be detected. The largest population-study of exoplanets to date successfully detected atmospheres around 16 'hot Jupiters', and found that water vapour was present in every case.

The work by a UCL-led team of European researchers has important implications for the comparison and classification of diverse exoplanets. The results will be presented by Angelos Tsiaras at the European Planetary Science Congress (EPSC) 2017 in Riga on Tuesday 19th September.

"More than 3,000 exoplanets have been discovered but, so far, we've studied their atmospheres largely on an individual, case-by-case basis. Here, we've developed tools to assess the significance of atmospheric detections in catalogues of exoplanets," said Angelos Tsiaras, the lead author of the study. "This kind of consistent study is essential for understanding the global population and potential classifications of these foreign worlds."

The researchers used archive data from the ESA/NASA Hubble Space Telescope's Wide Field Camera 3 (WFC3) to retrieve spectral profiles of 30 exoplanets and analyse them for the characteristic fingerprints of gases that might be present. About half had strongly detectable atmospheres.

Results suggest that while atmospheres are most likely to be detected around planets with a large radius, the planet's mass does not appear to be an important factor. This indicates that a planet's gravitational pull only has a minor effect on its atmospheric evolution.

Artist’s impressions of exoplanetary system. Credit: Alexaldo

Most of the atmospheres detected show evidence for clouds. However, the two hottest planets, where temperatures exceed 1,700 degrees Celsius, appear to have clear skies, at least at high altitudes. Results for these two planets indicate that titanium oxide and vanadium oxide are present in addition to the water vapour features found in all 16 of the atmospheres analysed successfully.

"To understand planets and planet formation we need to look at many planets: at UCL we are implementing statistical tools and models to handle the analysis and interpretation of large sample of planetary atmospheres. 30 planets is just the start," said Ingo Waldmann, a co-author of the study.

Artist’s impressions of exoplanetary system. Credit: Alexaldo

"30 exoplanet atmospheres is a great step forward compared to the handful of planets observed years ago, but not yet big-data. We are working at launching dedicated space missions in the next decade to bring this number up to hundreds or even thousands," commented Giovanna Tinetti, also UCL.

Artist’s impressions of exoplanetary system. Credit: Alexaldo

## Pandora mission would expand NASA's capabilities in probing alien worlds

An exoplanet as it is about to cross in front of – or transit – its star. Credit: NASA's Goddard Space Flight Center

In the quest for habitable planets beyond our own, NASA is studying a mission concept called Pandora, which could eventually help decode the atmospheric mysteries of distant worlds in our galaxy. One of four low-cost astrophysics missions selected for further concept development under NASA's new Pioneers program, Pandora would study approximately 20 stars and exoplanets—planets outside of our solar system—to provide precise measurements of exoplanetary atmospheres.

This mission would seek to determine atmospheric compositions by observing planets and their host stars simultaneously in visible and infrared light over long periods. Most notably, Pandora would examine how variations in a host star's light impacts exoplanet measurements. This remains a substantial problem in identifying the atmospheric makeup of planets orbiting stars covered in starspots, which can cause brightness variations as a star rotates.

Pandora is a small satellite mission known as a SmallSat, one of three such orbital missions receiving the green light from NASA to move into the next phase of development in the Pioneers program. SmallSats are low-cost spaceflight missions that enable the agency to advance scientific exploration and increase access to space. Pandora would operate in Sun-synchronous low-Earth orbit, which always keeps the Sun directly behind the satellite. This orbit minimizes light changes on the satellite and allows Pandora to obtain data over extended periods. Of the SmallSat concepts selected for further study, Pandora is the only one focused on exoplanets.

"Exoplanetary science is moving from an era of planet discovery to an era of atmospheric characterization," said Elisa Quintana, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the principal investigator for Pandora. "Pandora is focused on trying to understand how stellar activity affects our measurements of exoplanet atmospheres, which will lay the groundwork for future exoplanet missions aiming to find planets with Earth-like atmospheres."

Maximizing the scientific potential

Pandora concentrates on studying exoplanetary and stellar atmospheres by surveying planets as they cross in front of—or transit—their host stars. To accomplish this, Pandora would take advantage of a proven technique called transit spectroscopy, which involves measuring the amount of starlight filtering through a planet's atmosphere, and splitting it into bands of color known as a spectrum. These colors encode information that helps scientists identify gases present in the planet's atmosphere, and can help determine if a planet is rocky with a thin atmosphere like Earth or if it has a thick gas envelope like Neptune.

This illustration (not to scale) depicts Pandora’s orbital pattern in Sun-synchronous low-Earth orbit, located approximately 435 to 497 miles (700 to 800 kilometers) above Earth’s surface, as it observes its targeted exoplanets and stars. This orbit enables Pandora to obtain multiple observations of exoplanets over long periods and the Earthshine exclusion zone helps avoid reflected light from Earth. Credit: Lawrence Livermore National Laboratory and NASA’s Goddard Space Flight Center

This mission, however, would take transit spectroscopy a step further. Pandora is designed to mitigate one of the technique's most crucial setbacks: stellar contamination. "Stars have atmospheres and changing surface features like spots that affect our measurements," said Jessie Christiansen, the deputy science lead at the NASA Exoplanet Archive at Caltech in Pasadena, California, and a co-investigator for Pandora. "To be sure we're really observing an exoplanet's atmosphere, we need to untangle the planet's variations from those of the star."

Pandora would separate stellar and exoplanetary signals by observing them simultaneously in infrared and visible light. Stellar contamination is easier to detect at the shorter wavelengths of visible light, and so obtaining atmospheric data through both infrared and visible light would allow scientists to better differentiate observations coming from exoplanet atmospheres and stars.

"Stellar contamination is a sticking point that complicates precise observations of exoplanets," said Benjamin Rackham, a 51 Pegasi b Postdoctoral Fellow at the Massachusetts Institute of Technology in Cambridge and a co-investigator for Pandora. "Pandora would help build the necessary tools for disentangling stellar and planetary signals, allowing us to better study the properties of both starspots and exoplanetary atmospheres."

Joining forces with NASA's larger missions, Pandora would operate concurrently with the James Webb Space Telescope, slated for launch later this year. Webb will provide the ability to study the atmospheres of exoplanets as small as Earth with unprecedented precision, and Pandora would seek to expand the telescope's research and findings by observing the host stars of previously identified planets over longer periods.

Missions such as NASA's Transiting Exoplanet Survey Satellite (TESS), Hubble Space Telescope, and the retired Kepler and Spitzer spacecraft have given scientists astonishing glimpses at these distant worlds, and laid a strong foundation in exoplanetary knowledge. These missions, however, have yet to fully address the stellar contamination problem, the magnitude of which is uncertain in previous studies of exoplanetary atmospheres. Pandora seeks to fill these critical gaps in NASA's understanding of planetary atmospheres and increase the capabilities in exoplanet research.

"Pandora is the right mission at the right time because thousands of exoplanets have already been discovered, and we are aware of many that are amenable to atmospheric characterization that orbit small active stars," said Jessie Dotson, an astrophysicist at NASA's Ames Research Center in California's Silicon Valley and the deputy principal investigator for Pandora. "The next frontier is to understand the atmospheres of these planets, and Pandora would play a key role in uncovering how stellar activity impacts our ability to characterize atmospheres. It would be a great complement to Webb's mission."

This illustration depicts Pandora’s use of transit spectroscopy to reliably identify an exoplanet’s atmospheric composition as it passes in front of its host star. Credit: Lawrence Livermore National Laboratory and NASA’s Goddard Space Flight Center

Lawrence Livermore National Laboratory (LLNL), in Livermore, California, is co-leading the Pandora mission with NASA's Goddard Space Flight Center. LLNL will manage the mission and leverage capabilities developed for other government agencies, including a low-cost approach to the telescope design and fabrication that enables this groundbreaking exoplanet science from a SmallSat platform.

NASA's Pioneers program, which consists of SmallSats, payloads attached to the International Space Station, and scientific balloon experiments, fosters innovative space and suborbital experiments for early-to-mid-career researchers through low-cost, small hardware missions. Under this new program, Pandora would operate on a five-year timeline with a budget cap of \$20 million.

Despite tight constraints, the Pioneers program enables Pandora to concentrate on a focused research question while engaging a diverse team of students and early career scientists from more than a dozen of universities and research institutes. This SmallSat platform creates an excellent blueprint for small-scale missions to make an impact in the astrophysics community.

"Pandora's long-duration observations in visible and infrared light are unique and well-suited for SmallSats," said Quintana. "We are excited that Pandora will play a crucial role in NASA's quest for finding other worlds that could potentially be habitable."

## Terra Nova

One of the biggest questions in astronomical research right now is quite simple to ask but extremely difficult to answer: In the depths of space, is there an Earth-like planet somewhere orbiting a Sun-like star?

The answer is rather surprising: almost certainly yes. We haven’t found a precise twin of Earth yet, but we’ve come mighty close. In fact, it’s likely that there are millions, perhaps billions, of planets like ours in the Milky Way alone. But right now, at this moment, we only know of one for sure: ours.

So when will we actually see that blue-green dot in our telescopes?

The search for alien worlds orbiting other stars—exoplanets—has gone on a long time. Quite a few were thought to have been seen, but they were on the thin, hairy edge of what the technology could do and were later shown to be false positives.

Things changed in 1992. Using sophisticated timing techniques, scientists found the very first confirmed planets, which were orbiting a pulsar, the ultradense core of an exploded supernova. That turns out not to be the most hospitable place in the Universe, what with the pulsar spewing out enough X-rays to thoroughly fry surrounding space. Planets they are, Earth-like they are not.

But then in 1995 came the big announcement: A planet had been found orbiting the star 51 Pegasi. The star is similar to the Sun, but the planet was a shock: It had 0.4 times the mass of Jupiter (150 times the Earth’s mass), but it orbited the star a mere 8 million kilometers (5 million miles) from the star! It screamed around the star in just 4.2 days, a far smaller and shorter orbit than had been thought possible.

The method used to find this planet is called the Doppler technique. When a planet orbits a star, its gravity tugs on the star. The planet makes a big circle while the star makes a smaller one. As the star approaches us in that cycle, its light gets compressed a bit, shifting it to shorter wavelengths. When it recedes from us, the opposite happens. This is essentially the same physics that makes a motorcycle make that “EEEEeeeeeeooooooooowwwwwww” sound as it passes you, what scientists call the Doppler effect.

Astronomers had been very carefully looking at many stars for the Doppler effect, but they’d been looking at timescales of months, not days. Once the planet 51 Peg b (as it’s called a planet is given its star’s name followed by a lower case letter starting at b, then c for the second one discovered, and so on) was found, astronomers looked back at their data and quickly found many more.

This method tends to find huge planets orbiting their stars close in—the effect is larger for that type of world—and so they are not Earth-like at all. These “hot Jupiters” are fascinating in their own right, but they would never be mistaken for home.

Many more of these planets have been found this way, but the real revolution was to come just a few years later.

Kepler is an observatory launched into space in 2009. It was designed to stare at 150,000 stars simultaneously, carefully measuring their starlight. If a planet orbits the star, and we see that orbit edge-on, then the planet will cross the face of its star. The starlight will dim periodically, revealing the presence of the exoplanet.

A few planets had been found this way before, but Kepler opened the floodgates: It has found hundreds of confirmed exoplanets, and thousands more candidates are still awaiting confirmation.*

llustration by NASA/Spaceplace

This technique, called the transit method, makes it easier (though by no means actually easy) to find smaller planets. Kepler has found quite a few Earth-sized planets, and more excitingly, quite a few others orbiting their stars at the right distance.

But, hey, wait a sec. What does it mean to be at “the right distance”?

We don’t know what varied forms life can take out in the cosmos. But it’s not a bad idea to look here at home for hints. All life on Earth needs liquid water, so that’s a pretty good criterion to start with. That means a planet can’t be too close to its star or else all the water will boil away. And if it’s too far, the water will be frozen (though there can be exceptions—some icy moons in the outer solar system like Europa and Enceladus are heated by their parent planets enough that they have interior oceans).

But there’s a clement middle ground, what astronomers call the Goldilocks Zone (or more formally the Habitable Zone), where liquid water can exist on the exoplanet’s surface. The zone depends on many factors including how big and bright the star is, and it’s a good place to start.

So the next question is, have we found the right size planets nestled comfortably in the Goldilocks Zone?

Some astronomers went through the Kepler data looking just at stars like the Sun (ranging from a bit cooler to a bit warmer), more than 42,000 stars in total. From that list, 600 or so had planets. The astronomers then looked for just those planets in the liquid water zone, where they would receive no less than a fourth and no more than four times the light the Earth does (a reasonable range). Finally, they culled the list to include exoplanets that were at least as big as Earth, but no more than twice its diameter. Bigger planets can have Earth-like gravity, but it gets tougher to support life the bigger the planet is, and a planet like that will probably have a hugely thick atmosphere, making it uninhabitable.

## What has been your biggest professional challenge and how did you overcome it?

Overall, I've been incredibly fortunate. I've had wonderful mentors and excellent advisers throughout my career.

The one minor hiccup I faced was that my initial plan for my graduate thesis was derailed by the failure of Kepler's second reaction wheel, a device that helped orient the spacecraft. The Kepler mission was designed to measure the frequency of Earth-like planets orbiting Sun-like stars, so many of the M dwarfs in the Kepler field were not selected as target stars. I had worked with my thesis adviser, David Charbonneau, to submit a proposal to observe several thousand of those missed M dwarfs with Kepler, and we had just learned that the proposal had been approved when the primary mission ended. Fortunately, the story has a happy ending because the Kepler spacecraft went on to observe tens of thousands of M dwarfs during its K2 mission. I've been characterizing the planetary systems orbiting those M dwarfs for the last several years.

## The Diversity of Planets

The Solar System contains four rocky planets, two large gaseous planets, and two other giant worlds, along with five dwarf planets and a wealth of moons, comets, asteroids, and icy worlds. The challenges of observing other star systems means we mostly know about planets orbiting close in to their host stars, with very little information so far about planets orbiting farther out — much less moons, asteroids, and so on. However, a combination of theory and observation is bringing us to a fuller picture of the possible planetary systems, what they contain, and how they were formed.

In the early years of exoplanet research, astronomers were happy just to discover any planet. Today, the focus is on classifying all the systems discovered so far, and hunting for smaller planets orbiting farther out from their host stars. In particular, researchers want to find Earth-sized planets in the habitable zones of their stars: the range of distances where liquid water could conceivably exist. Next-generation observatories such as NASA's Transiting Exoplanet Survey Satellite (TESS) are designed for that purpose.

Many identified exoplanets are different from what we see in the Solar System. Planets more massive than Jupiter are common, but most exoplanets fall between Earth and Neptune in size or mass. These “super-Earths” are probably rocky, which raises questions about how they might differ from the inner planets of the Solar System and whether they could support life as we understand it.

Until the exoplanet revolution, our understanding of planet formation was based entirely on the Solar System. With the extra information from other planetary systems, researchers have a clearer picture of the complexities of how planets form and migrate in infant star systems. Researchers combine theoretical simulations with observations of newborn planetary systems to understand how the diverse worlds we see came into existence. That includes the distribution of atoms and molecules making planets, particularly those required for Earth-like life.

Of course, the best understood planetary system is our own Solar System. Comets and asteroids are remnants of the early years of the Solar System’s existence, providing us with a look at the environment before Earth formed. Astronomers have found similar chemical signatures on comets and in distant star systems, indicating some common processes.

Our best observations of atmospheres also come from planets in our Solar System, including the planet we know best: Earth. Understanding the differences between planets in our Solar System allows us to create models for exoplanet atmospheres. In addition, researchers are developing new methods to detect molecules in the atmospheres of distant worlds, particularly those like water or oxygen that are closely linked to life on Earth.