When we talk about the enormity of the cosmos, it’s easy to toss out big numbers – but far more difficult to wrap our minds around just how large, how far, and how numerous celestial bodies really are.
To get a better sense, for instance, of the true distances to exoplanets – planets around other stars – we might start with the theater in which we find them, the Milky Way galaxy.
What is a galaxy, anyway?
Our galaxy is a gravitationally bound collection of stars, swirling in a spiral through space. Based on the deepest images obtained so far, it’s one of about 2 trillion galaxies in the observable universe. Groups of them are bound into clusters of galaxies, and these into superclusters; the superclusters are arranged in immense sheets stretching across the universe, interspersed with dark voids and lending the whole a kind of spiderweb structure. Our galaxy probably contains 100 to 400 billion stars, and is about 100,000 light-years across. That sounds huge, and it is, at least until we start comparing it to other galaxies. Our neighboring Andromeda galaxy, for example, is some 220,000 light-years wide. Another galaxy, IC 1101, spans as much as 4 million light-years.
Ok, fine, but what the heck is a light-year?
Glad you asked. It’s one of the most commonly used celestial yardsticks, the distance light travels in one year. Light zips along through interstellar space at 186,000 miles (300,000 kilometers) per second (more than 66 trips across the entire United States, in one second). Multiply that by all the seconds in one year, and you get 5.8 trillion miles (9.5 trillion kilometers). Just for reference, Earth is about eight light minutes from the Sun. A trip at light speed to the very edge of our solar system – the farthest reaches of the Oort Cloud, a collection of dormant comets way, way out there – would take about 1.87 years. Keep going to Proxima Centauri, our nearest neighboring star, and plan on arriving in 4.25 years at light speed.
If you could travel at light speed. Which, unless you’re a photon (a particle of light), you can’t, and, by current physics, might never be possible. But I digress.
Can we get back to those…X-planets?
Exoplanets. Let’s toss around some more big numbers. First, how many are there? Based on observations made by NASA’s Kepler space telescope, we can confidently predict that every star you see in the sky probably hosts at least one planet. Realistically, we’re most likely talking about multi-planet systems rather than just single planets. In our galaxy of hundreds of billions of stars, this pushes the number of planets potentially into the trillions. Confirmed exoplanet detections (made by Kepler and other telescopes, both in space and on the ground) now come to more than 3,900 – and that’s from looking at only tiny slices of our galaxy. Many of these are small, rocky worlds that might be at the right temperature for liquid water to pool on their surfaces.
Where is the nearest one of these exoplanets?
It’s a small, probably rocky planet orbiting Proxima Centauri – as mentioned before, the next star over. A little more than four light-years away, or 24 trillion miles as the crow flies. If an airline offered a flight there by jet, it would take 5 million years. Not much is known about this world; its close orbit and the periodic flaring of its star lower its chances of being habitable.
I’d also point you to the TRAPPIST-1 system: seven planets, all roughly in Earth’s size range, orbiting a red dwarf star about 40 light-years away. They are very likely rocky, with four in the “habitable zone” – the orbital distance allowing potential liquid water on the surface. And computer modeling shows some have a good chance of being watery – or icy – worlds. In the next few years, we might learn whether they have atmospheres or oceans, or even signs of habitability.
Ok. Thanks. I need to go.
I understand. You’re short on time. That reminds me: Did you know time slows down in the presence of gravity?
I know it’s slowing down right now.
I guess that’s a discussion for another time.
A trip down the list of exoplanets found so far is a wild ride. These planets beyond our solar system, whether orbiting other stars or floating freely between them, can make the planets closer to home look tame by comparison. “Hot Jupiters” are star-hugging, infernal worlds. “Super Earths” are super mysterious. Frozen planets, gas giants that make Jupiter look puny, or small, rocky planets in Earth’s size range but in tight orbits around red dwarf stars – the catalog keeps growing, and soon, that growth will become exponential.
The more than 3,900 exoplanets confirmed so far are really a tiny sampling of what could amount to trillions in our galaxy. And they likely will be joined by tens of thousands more that are expected to be discovered by NASA’s TESS space telescope (the Transiting Exoplanet Survey Satellite).
Astronomers who analyze data from the last exoplanet survey, by NASA’s Kepler space telescope, can already paint a demographic portrait of what TESS will likely find.
According to NASA’s Exoplanet Archive, of the 3,924 exoplanets confirmed so far, 1,665 can be classed as “Neptune-like” – gaseous worlds around the size of Neptune. The rest of the breakdown:
- 1,213 earn the title of gas giant, like Jupiter or Saturn.
- 878 are classified as super Earths, a reference only to their size – larger than Earth and smaller than Neptune – but not suggesting they are necessarily similar to our home planet. The true nature of these planets remains shrouded in uncertainty because we have nothing like them in our own solar system – and yet, they are among the most common planet types found so far in the galaxy.
- 156 of the confirmed exoplanets are considered terrestrial, that is, rocky planets about the size of Earth; further investigation will determine whether some of them possess atmospheres, oceans, or other signs of habitability.
- 12 are simply classed as “unknown.” In other words, their presence has been detected by one of several indirect methods, but we know little else about them.
More variety is hidden within these broad categories. Hot Jupiters, for instance, were among the first planet types found – gas giants like Jupiter, yes, but orbiting so close to their stars that their temperatures soar into the thousands of degrees (Fahrenheit or Celsius). These large planets make such tight orbits that they cause a pronounced “wobble” in their stars, their gravity tugging them first this way, then that. That made them easier to detect in the early days of planet hunting.
Or consider the rogue planets: worlds hurtling alone through the galaxy, with no companion star. Many of these worlds could have been ejected from their original solar system, amid the gravitational jostling during the early phases of formation. The final “kick” could have come from another planet or even from the star itself.
The galaxy also seems to be home to a great many oddly sized planets, including those super Earths. Are they super-sized, rocky worlds, like scaled-up versions of planets in Earth’s size range? Or are they low-density worlds with puffy atmospheres? Further investigation is needed.
As if that weren’t enough, scientists also have noted what seems to be a strange gap in planet sizes. It’s been dubbed the Fulton gap, after Benjamin Fulton, lead author on a paper describing it. The Kepler data show that planets of a certain size-range are rare – those between 1.5 and 2 times the size of Earth. It’s possible that this represents a critical size in planet formation: Planets that reach this size quickly attract a thick atmosphere of hydrogen and helium gas and balloon up into gaseous planets, while planets smaller than this limit are not large enough to hold such an atmosphere and remain primarily rocky. On the other hand, the smaller planets that orbit close to their stars could be the cores of Neptune-like worlds that had their atmospheres stripped away.
Explaining the Fulton gap will require a far better understanding of how solar systems form.
As is often the case in science, the more we learn about the kaleidoscope of exoplanets, the more questions they provoke – and the more mysterious our universe becomes.
Stars jostling around the galaxy aren’t quite like a cosmic game of pool. But they do have occasional near misses as they speed past each other. Back when spears and stone points were the height of human technology, astronomers say, our solar system had a close encounter of the interstellar kind.
The brief visitor was Scholz’s star, and it might have grazed the outer edge of the solar system’s distant Oort Cloud about 70,000 years ago – carrying its companion, a likely brown dwarf, along for the ride.
It’s unclear whether the near miss was close enough to give objects in the Oort Cloud, our solar system’s halo of dormant comets, a gravitational nudge to fall toward the Sun. But the interstellar trespasser highlights a sometimes-forgotten reality: On long time scales, stars seem to fly around like sparks from a campfire, occasionally coming close enough to disturb each other’s cometary clouds.
Such close passes could have profound implications for exoplanets – planets orbiting other stars – and how they got where they are. At least some of the time, an interloper could become a thief, stripping a star of one or more planets – or vice versa.
Our solar system, too, might have been shaped and sculpted by stellar flybys.
A 2018 study showed that the orbital motions of some of our solar system’s small bodies appear still to bear the imprint of Scholz’s gravitational wake. And some planet-like objects in the Kuiper belt, the collection of rocky and icy bodies past the orbit of Neptune, could have been stolen from another star far earlier – in fact, soon after our Sun was born. Scholz’s flyby could just be the latest in a series.
The discovery of our star-crossed close encounter was almost as random as the event itself.
It started when astronomer Eric Mamajek, deputy program scientist for NASA’s Exoplanet Exploration program, was a professor at the University of Rochester. He took yearly excursions to Santiago, Chile, where he made observations with world-class telescopes.
On one such visit in 2013, a fellow astronomer, Valentin Ivanov, showed him a peculiar result: A newly discovered nearby star with a lengthy catalog designation (later nicknamed for its discoverer, Ralf-Dieter Scholz) seemed almost to be sitting still. Most stars move perceptibly across the sky over the course of a year, as measured in a unit called “arc seconds.” In terms of such “sideways” motion, this one hardly moved at all. Yet the star was only 22 light-years away – quite near to us by galactic standards.
Mamajek knew that could mean only one thing. Either the star was heading straight for us, or it was heading directly away. In this case, the astronomers had obtained measurements of the star’s Doppler shift – the reddening of light if a star is moving away, or a shift toward blue if it’s moving toward us.
“It was screaming away at 80 kilometers per second,” Mamajek recalled. And it didn’t take him long to do the math.
“In less than 15 minutes, we figured out that this star had passed within a light-year of the solar system, 70 or 80 thousand years ago,” he said.
The closest stars to our Sun today are the three in the Alpha Centauri system, about four light-years away. But if there were a star one light-year away, that could very well approach or even intersect with the outermost edge of the Oort Cloud.
Mamajek thinks that Scholz’s star, now known as the star that came closest to our solar system, could eventually lose that title. Extremely precise data from the European Space Agency’s new Gaia space probe will likely reveal more near misses, possibly closer ones. And in any case, another close pass by a star known as GJ 710 is due in about 1.3 million years.
For now, however, Scholz’s keeps its prize.
And might those rock-hammering, spear-shaping early humans have caught a glimpse as the star passed by? Turns out, not terribly likely. Scholz’s star is a red dwarf, the smallest and faintest kind of star we know. Even at its nearest point, about 55,000 astronomical units from our Sun (5.1 trillion miles), Scholz’s star would have been 100 times too dim to be seen with the naked eye.
Still, there’s a chance the visitor made itself known. Red dwarfs are known periodically to emit extremely bright flares.
If the star sent up a flare as it was cruising past our solar system, our cave-dwelling ancestors just might have seen it.
This past July I joined a group of geologists, geochemists, microbiologists, and fellow astronomers on a tour of some of the best-preserved evidence for early life.
Entitled the Astrobiology Grand Tour, it was a trip led by Dr. Martin Van Kranendonk, a structural geologist from the University of New South Wales, who had spent more than 25 years surveying Australia’s Pilbara region. Along with his graduate students he had organized a 10-day excursion deep into the outback of Western Australia to visit some of astrobiology’s most renowned sites.
The trip would entail long, hot days of hiking through unmaintained trails on loose surface rocks covered by barb-like bushes called spinifex. As I was to find out, nature was not going to give up its secrets easily. And there were no special privileges allocated to astrophysicists from New Jersey.
The state of Western Australia, almost four times the size of the American state of Texas but with less than a tenth of the population (2.6 million), is the site of many of astrobiology’s most heralded sites. For more than three billion years, it has been one of the most stable geologic regions in the world.
It has been ideal for geological preservation due to its arid conditions, lack of tectonic movement, and remoteness. The rock records have in many places survived and are now able to tell their stories (to those who know how to listen).
Our trip began with what felt like a pilgrimage. We left Western Australia’s largest city Perth and headed north for Shark Bay. It felt a bit like a pilgrimage because the next morning we visited one of modern astrobiology’s highlights – the living stromatolites of Shark Bay.
Stromatolites literally mean “layered rocks”. It’s not the rocks that are alive but rather the community of microbial mats living on top. They are some of the Earth’s earliest ecosystems.
First they rolled in one by one, those newly discovered planets, like billiard balls pushed across a table.
Counting them was easy.
Then they came in handfuls. Still quite manageable; as ground-based observatories began to pile up their discoveries of exoplanets – planets around other stars – in the 1990s and early 2000s, astronomers had no trouble keeping a running tally.
But when discoveries of exoplanets began to flow from space-based telescopes, it was like a pool shark making a big, smashing break. The billiard balls raced across the table in bunches. In just a few years, scientists were racking up new planets by the thousands.
And it wasn’t just the number, but the types of planets that had to be accounted for. Hot Jupiters, gas giants, rocky, Earth-sized worlds, “super Earths”; hints of potentially frozen, scalding, lava-choked, icy, steamy or watery planets.
NASA’s Exoplanet Science Institute, keepers of the NASA Exoplanet Archive, set up automated counters of exoplanet discoveries – running, online dashboards tracking the number and variety. The latest totals: some 3,700 confirmed exoplanets in our galaxy, with thousands more candidate planets that remain unconfirmed.
But now, after piling up two decades worth of exoplanet discoveries, NASA scientists have begun a wholesale reshuffling of their counting methods.
At first, this means a drop in the number of “candidate” planets, with roughly half moving to the “confirmed” category. These planets were already confirmed but were being double counted: The previous number on the counter, 4,496, was labeled “candidates,” but critically, it included the combined total of confirmed and unconfirmed exoplanets, and only from NASA’s Kepler space telescope observations from 2009 to 2013.
In the new counter, only “unconfirmed” planets are labeled as “candidates.” The count also pulls in other NASA mission discoveries, including Kepler’s more recent observations and future exoplanet finds.
That means the initial candidate total drops to 2,724.
We’re going to need a bigger counter
But the drop in candidates is temporary. Once the next torrent is unleashed – exoplanet discoveries from the just-launched Transiting Exoplanet Survey Satellite (TESS), likely to begin to flow in early 2019 – planetary candidates are expected to soar into the tens of thousands.
“There could be over 10,000 candidates within the first couple of years,” said Eric Mamajek, the deputy program chief scientist for NASA’s Exoplanet Exploration Program. “Hundreds will be smaller than Neptune – dozens of things smaller than two or three (times Earth’s diameter), within the habitable zone of mostly M-dwarfs (red dwarf stars). There are also going to be thousands upon thousands of Jupiters detected around faint stars. All will initially be unconfirmed, but (some) will need further analysis and observation to follow up.”
And that could be just the beginning. In a kind of echo of “Moore’s law,” the rough doubling of computer processing power each year, Mamajek points out that exoplanet discoveries have doubled roughly every two years over the past three decades. The trend should continue over the next 10 years with data from TESS and future missions, such as the Wide Field Infrared Survey Telescope (WFIRST). That should keep the planet counter clicking.
Other changes reveal the evolving nature of exoplanet science. During the first Kepler mission, the space telescope stared at a patch of sky for four years, watching more than 150,000 stars. For many of those stars, the telescope’s extremely sensitive detectors picked up tiny dips in starlight – the shadow of an orbiting planet passing in front of its star.
Scientists analyzed these dips and published papers, announcing raft after raft of exoplanet candidates and pushing them into the thousands. Follow-up observations and analytical techniques allowed large numbers of these candidates to be confirmed – to make sure they weren’t due to statistical noise or, perhaps, a companion star in a double-star system, masquerading as a planet.
“There’s always more work that needs to be done to confirm them,” Mamajek said. “They don’t come with a big stamp on their head that says, ‘planet.’”
All hands on deck for TESS
Planets from Kepler’s later observations also must be confirmed. These came after the failure of stabilizing components on the Kepler spacecraft ended its initial four-year stare. Clever engineers found a way to use the pressure of sunlight to stabilize the spacecraft, though its observation periods are now much shorter, about 80 days apiece.
But Kepler’s latest discoveries are confirmed using a different approach. The imaging data goes straight out to the astronomical community, rather than first being filtered through a scientific team. Candidate and confirmed planets are then published by the community at large.
“TESS will provide an official list of candidates,” said David Ciardi, a research astronomer and the chief scientist for NASA’s Exoplanet Science Institute at Caltech. “Then a bunch of candidates, the community will also provide. It is going to be super exciting!”
Precision counting and a bigger pool of astronomers: It’s all to make sure that, amid a coming avalanche of exoplanet discoveries, planet counters don’t get left behind the eight ball.
The Transiting Exoplanet Survey Satellite (TESS), scheduled to launch on April 16, is NASA’s next mission to search for exoplanets – planets outside our solar system. It will look for small planets orbiting nearby, bright stars. TESS will rely on the transit method, looking for periodic dips in starlight that could reveal a planet passing in front of its star.
Here’s why TESS is going to be great:
We’ll find planets everywhere
Thanks to the trailblazing of NASA’s Kepler space telescope, scientists now believe there may be at least one planet around every star in the sky. Kepler successfully discovered nearly 2,700 confirmed planets of all sizes and around all types of stars, as part of its census of how common exoplanets were, by observing 5 percent of the sky during it primary and K2 missions. TESS is designed with a larger field of view and will cover more than 85 percent of the night sky in its search of nearby exoplanets.
We’ll find planets close to Earth
Most of Kepler’s planets are hundreds of light-years away, close enough to measure their size and orbit, but too distant to search for any signs of life. TESS will observe stars that are nearby, relatively speaking, and 30 to 100 times brighter than those surveyed by Kepler, and therefore far easier to study with follow-up observations. TESS will usher in a new era, finding planets suitable for NASA’s upcoming James Webb Telescope, set to launch in 2020. The Webb telescope will examine light from these distant planets to learn the makeup of their atmospheres and look for signs of life.
TESS will likely observe your favorite exoplanets
Think of the sky as a giant sphere surrounding Earth, with TESS looking out from the inside. TESS will observe each half of that sphere for a year at a time, beginning in the south. In the first year, about 500 known exoplanets will be visible to TESS. In year two, over 3,000 will be in the TESS field of view – most of them Kepler planets, since TESS will be observing Kepler’s field of view.
We’ll produce a gold mine of data
TESS will collect starlight from over 200,000 stars every two minutes to search for transiting planets. In addition, TESS will also save the full images taken by each 16.8-megapixel camera every 30 minutes. These Full Frame Images (FFIs) will observe over 30 million astrophysical objects and will be available to the public. We now live in a time when anyone can be a planet (or star, or galaxy) hunter.
TESS can operate for a really long time
TESS plans to achieve its primary science goals with a two-year prime mission. But TESS has fuel reserves for more than a decade of operation if the mission is extended. TESS will be in a unique “high-Earth orbit” that has never been used before, and could remain in that orbit for over a hundred years!
Elisa Quintana is an astrophysicist and TESS Mission support scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. George Ricker of MIT’s Kavli Institute for Astrophysics and Space Research serves as principal investigator for the mission. Additional partners include Orbital ATK, NASA’s Ames Research Center, the Harvard-Smithsonian Center for Astrophysics and the Space Telescope Science Institute. More than a dozen universities, research institutes and observatories worldwide are participants in the mission.
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A massive gas giant more weighty than Jupiter, orbiting an orange star some 45 light years away, might be the most important exoplanet you’ve never heard of.
The planet, called Gamma Cephei A b – “Tadmor” for short – achieved its 15 minutes of fame in 1988. At least, among astronomers. It was the first planet to be discovered outside our solar system.
Or it would have been. The discovery was withdrawn by the Canadian team that announced it in 1992, after the data backing it up was determined to be too wobbly for astronomers to be sure the planet was real. Tadmor was added to a growing list of mistaken exoplanet detections that began as far back as the 19th century.
In this case, “wobbly” turns out to be the right word. The astronomers who thought they’d found the first exoplanet had developed a technique that allowed them to track the subtle motions of stars. The amount of “wobble” would reveal the mass of an object orbiting the star, tugging it first this way, then that. The researchers’ major advance was precision measurement – capturing stellar movements as small as 43 feet (13 meters) per second. That kind of precision was needed to pick up the tiny wobbles, back and forth, that a large orbiting planet caused the star to make.
Despite their advance, the research team, Bruce Campbell, Gordon Walker and Stephenson Yang, worried that periodic changes in the star’s magnetic activity might have looked to them like the gravitational tugs of a planet – in other words, that they might have mistaken jitters within the star for a planet in orbit around it.
They bid goodbye to Tadmor.
Riffle forward through the calendar, and stop in 2002. On-again, off-again Tadmor was on again – this time, its presence solidly confirmed. A team of astronomers that included the original discoverers captured strong evidence of the planet. They used four separate data sets from high-precision “wobble” measurements, known as radial velocity, spanning the period from 1981 to 2002.
The radial velocity method today has notched hundreds of exoplanet discoveries. It’s been overshadowed only by the “transit” method, responsible for thousands, that looks for a tiny dip in the light from a star as a planet passes in front of it.
And although the list of confirmed exoplanets was just beginning to grow in the early 2000s, Tadmor already had been eclipsed. A planet called 51 Pegasi b, discovered by Michel Mayor and Didier Queloz, stole most of the spotlight in 1995. It was the first confirmed exoplanet detection to capture worldwide public attention.
Tadmor, of course, continues to orbit its big orange sun, somewhere in the constellation Cepheus, presumably unaware of its near-fame on a small blue planet dozens of light-years away. Time rolls on. Happy 30th anniversary, Tadmor.
Look deeply enough into the night sky, and you’ll soon see how radically the universe has changed.
You might have to borrow some space-based spyglasses – NASA’s Kepler, Spitzer or Hubble space telescopes – to peer into the cosmic depths and watch the faint shadows of planets cross the faces of their stars. Or measure the stars’ wobble, the gravitational tugs of orbiting planets. But as your eyes adjust, the new reality becomes crystal clear. For the first time since we began huddling around campfires, weaving scattered stars into pictures and stories, we know with certainty that we belong to a galaxy packed with neighboring worlds – whole systems of stars and planets far beyond, and vastly different from, our own solar system.
This is not your parents’ universe. You can take a planet-hopping vacation across the seven Earth-sized worlds of the system known as TRAPPIST-1, for instance, just 40 light-years away. A somewhat longer trip, around 200 light-years, will take you to Kepler-16b, a planet orbiting two stars. The two suns in its sky make it a real-life Tatooine, straight out of “Star Wars.”
Or how about pitch-black WASP-12b, some 1,400 light-years away, orbiting its star so closely it’s being distorted into an egg shape as it is gradually pulled apart?
The count of confirmed exoplanets – planets around other stars – has passed 3,500 since 1995, when the detection of 51 Pegasi b, a roasting giant in a close orbit around a sun-like star, rang in the era of fast-paced exoplanet discovery. Dozens, then hundreds, then thousands began to jump out of telescope data.
The Kepler space telescope reeled in the largest haul, providing a census of planet types and sizes. A planet as light as Styrofoam, another that could be raining glass. Earth-sized worlds by the bushel, but also oddly sized “super Earths” and “sub-Neptunes,” planets larger than Earth but smaller than Neptune. These are the most common types of planets, though we know next to nothing about them: In our solar system they are conspicuously absent.
Reaction wheel failures ended the Kepler telescope’s primary mission in 2013 after four years of exoplanet observation. Some clever commands from ground-based engineers allowed it to continue functioning as K2, an extended mission mapping new star fields that lie within the plane of Earth’s orbit around the Sun. Its observation times are now shorter, but its ability to discover new exoplanets remains intact.
The K2 mission is, in fact, preparing the way ahead for two new, state-of-the-art planet hunters to be launched in the next two years. The Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope will take their cues from K2, which is identifying interesting exoplanets that the new kids on the block can investigate in greater depth. The Webb telescope will capture the light from some of these planets, with the goal of determining which gases are present in their atmospheres.
"All these worlds are yours. . ."
The age of direct imaging – actual pictures – of exoplanets is upon us, even if the first images are no bigger than a pixel. And the techniques pioneered by the Webb telescope could one day allow us to identify oxygen, carbon dioxide and methane in the skies of some far-off, blue and white marble. In other words, signs of life – and just maybe, another Earth-like planet.
For now we can take these journeys to exotic exoplanets only in our imaginations, though helped along by the visions of space artists. Their visualizations, based on known data, are so sharp they look like photographs. Using exoplanet virtual reality and your cell phone, you can stand on the surface of an orange-tinted world, and look back toward Earth through its alien skies.
Welcome to our new exoplanet blog, part of NASA’s Exoplanet Exploration program. Hitch a ride with us as we take interstellar tours, discover new planets, and press ahead in the search for life. A brand-new universe is waiting.
It might be lingering bashfully on the icy outer edges of our solar system, hiding in the dark, but subtly pulling strings behind the scenes: stretching out the orbits of distant bodies, perhaps even tilting the entire solar system to one side.
If a planet is there, it’s extremely distant and will stay that way (with no chance – in case you’re wondering – of ever colliding with Earth, or bringing “days of darkness”). It is a possible Planet Nine, a world perhaps 10 times the mass of Earth and 20 times farther from the sun than Neptune. The signs so far are indirect, mainly its gravitational footprints, but that adds up to a compelling case nonetheless.
One of its most dedicated trackers, in fact, says it is now harder to imagine our solar system without a Planet Nine than with one.
“There are now five different lines of observational evidence pointing to the existence of Planet Nine,” said Konstantin Batygin, a planetary astrophysicist at Caltech whose team may be closing in. “If you were to remove this explanation, and imagine Planet Nine does not exist, then you generate more problems than you solve. All of a sudden, you have five different puzzles, and you must come up with five different theories to explain them.”
Batygin and his co-author, Caltech astronomer Mike Brown, described the first three breadcrumbs on Planet Nine’s trail in a January 2016 paper, published in the Astronomical Journal. Six known objects in the distant Kuiper Belt, a region of icy bodies stretching from Neptune outward toward interstellar space, all have elliptical orbits pointing in the same direction. That would be unlikely – and suspicious – enough. But these orbits also are tilted the same way, about 30 degrees “downward” compared to the pancake-like plane within which the planets orbit the sun.
Breadcrumb number three: Computer simulations of the solar system with Planet Nine included show that there should be more objects tilted with respect to the solar plane. In fact, the tilt would be on the order of 90 degrees, as if the plane of the solar system and these objects formed an “X” when viewed edge-on. Sure enough, Brown realized that five such objects already known to astronomers fill the bill.
Two more clues emerged after the original paper. A second article from the team, this time led by Batygin’s graduate student, Elizabeth Bailey, showed that Planet Nine could have tilted the planets of our solar system during the last 4.5 billion years. This could explain a longstanding mystery: Why is the plane in which the planets orbit tilted about 6 degrees compared to the sun's equator?
“Over long periods of time, Planet Nine will make the entire solar-system plane precess or wobble, just like a top on a table,” Batygin said.
The last telltale sign of Planet Nine’s presence involves the solar system’s contrarians: objects from the Kuiper Belt that orbit in the opposite direction from everything else in the solar system. Planet Nine’s orbital influence would explain why these bodies from the distant Kuiper Belt end up “polluting” the inner Kuiper Belt.
“No other model can explain the weirdness of these high-inclination orbits,” Batygin said. “It turns out that Planet Nine provides a natural avenue for their generation. These things have been twisted out of the solar system plane with help from Planet Nine and then scattered inward by Neptune.”
The remaining step is to find Planet Nine itself. Batygin and Brown are using the Subaru Telescope in Hawaii’s Mauna Kea Observatory to try to do just that. The instrument is the “best tool” for picking out dim, extremely distant objects lost in huge swaths of sky, Batygin said.
But where did Planet Nine come from? Batygin says he spends little time ruminating on its origin – whether it is a fugitive from our own solar system or, just maybe, a wandering rogue planet captured by the sun’s gravity.
“I think Planet Nine’s detection will tell us something about its origin,” he said.
Other scientists offer a different possible explanation for the Planet Nine evidence cited by Batygin. A recent analysis based on a sky mapping project called the Outer Solar System Origins Survey, which discovered more than 800 new “trans-Neptunian objects,” or TNOs, suggests that the evidence also could be consistent with a random distribution of such objects. Still, the analysis, from a team led by Cory Shankman of the University of Victoria, could not rule out Planet Nine.
If Planet Nine is found, it will be a homecoming of sorts, or at least a family reunion. Over the past 20 years, surveys of planets around other stars in our galaxy have found the most common types to be “super Earths” and their somewhat larger cousins – bigger than Earth but smaller than Neptune.
Yet these common, garden-variety planets are conspicuously absent from our solar system. Weighing in at roughly 10 times Earth’s mass, the proposed Planet Nine would make a good fit.
Planet Nine could turn out to be our missing super Earth.