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 The discovery was withdrawn by the Canadian team that announced it in 1992, after the data backing it up was determined to be too wobbly for astronomers to be sure the planet was real.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.