For the first time in history, NASA has discovered a total of eight planets orbiting a distant star that is much like our Sun, the space agency announced on Thursday.
The star, Kepler-90, is 2,545 light-years from Earth and located in the Draco constellation. It is the first star known to humans to support just as many planets as the known Solar System, but what is exciting to many is that astronomers believe that this is in fact only the beginning of a long line of discoveries to come out of our latest technological advances.
For a time, researchers had known that a total of seven planets were orbiting Kepler-90, but Google Artificial Intelligence had a hand in discovering the eighth planet when it looked into archival data originally obtained by NASA’s Kepler telescope, designed specifically to look for planets.
With the idea of eventually differentiating among exoplanets, Christopher Shallue, senior software engineer at Google AI in California, and Andrew Vanderburg, astronomer and NASA Sagan postdoctoral fellow at the University of Texas, Austin, trained a computer how to differentiate between images of cats and dogs, refining their approach to identify exoplanets in Kepler data based on the change in light when a planet passed in front of its star. The neural network learned to identify these by using signals that had been vetted and confirmed in Kepler’s planet catalog. Ninety-six percent of the time, it was accurate.
Scientists have discovered seven Earth-sized planets, so tightly packed around a dim star that a year there lasts less than two weeks. The number of planets and the radiation levels they receive from their star, TRAPPIST-1, make these worlds a miniature analogue of our own Solar System.
The excitement surrounding TRAPPIST-1 was so great that the discovery was announced with an article in Nature accompanied by a NASA news conference. In the last two decades, nearly 3,500 planets have been found orbiting stars beyond our Sun, but most don’t make headlines.
How likely are we really to find a blue marble like our Earth among these new worlds?
Earth 2.0?
We still know little about these planets with certainty, but initial clues look enticing.
All seven worlds complete an orbit in between 1.5 and 13 days. So closely are they huddled that a person standing on one planet might see the neighbouring worlds in the sky even larger than our Moon. The short years place the planets closer to their star than any planet sits to the Sun. Happily, they avoid being baked by TRAPPIST-1 because it is incredibly dim.
TRAPPIST-1 is a small ultracool dwarf star with a luminosity roughly 1/1000th that of the Sun. Comparing the two at Wednesday’s news conference, lead author of the Nature paper, Michaël Gillon, said that if the Sun were scaled to the size of a basketball, TRAPPIST-1 would be a puny golf ball. The resulting paltry amount of heat means that three of the seven TRAPPIST-1 planets actually receive similar amounts of radiation as Venus, Earth and Mars.
This alternative Solar System does look like a compact version of our own, but does TRAPPIST-1 include an Earth 2.0?
Artists impression of the seven TRAPPIST-1 worlds, compared to our solar system’s terrestrial planets. NASA/JPL-Caltech
Here’s the good news first.
The seven siblings are all Earth-sized, with radii between three quarters and one times that of our home planet and masses that range from roughly 50% to 150% of Earth’s (the mass of the outermost world remains uncertain).
Because all are smaller than 1.6 times Earth’s radius, the seven TRAPPIST-1 planets are likely to be rocky worlds, not gaseous Neptunes. TRAPPIST-1d, e and f are within the star’s temperate region — aka the “Goldilocks zone” where it’s not too hot and not too cold — where an Earth-like planet could support liquid water on its surface.
The orbits of the six inner planets are nearly resonant, meaning that in the time it takes for the innermost planet to orbit the star eight times, its outer siblings make five, three and two orbits.
Such resonant chains are expected around stars where the planets have moved from where they originally formed. This migration occurs when the planets are still young and embedded in the star’s gaseous planet-forming disc. As the gravity of the young planet and the gas disc pull on one another, the planet’s orbit can change, usually moving towards the star.
If multiple planets are in the system, their gravity also pulls on one another. This nudges the planets into resonant orbits as they migrate through the gas disc. The result is a string of resonant planets close to the star, just like that seen encircling TRAPPIST-1.
Being born far from the star offers a couple of potential advantages. Dim stars like TRAPPIST-1 are irritable when young, emitting flares and high radiation that may sterilise the surface of nearby planets. If the TRAPPIST-1 system did indeed form further away and migrate inwards, its worlds may have avoided getting fried.
Originating where temperatures are colder would also mean the planets formed with a large fraction of ice. As the planets migrate inwards, this ice could melt into an ocean. This notion is supported by the estimated densities of the planets, which are low enough to suggest volatile-rich compositions, like water or a thick atmosphere.
Not an Earth?
Since our search for extraterrestrial life focuses on the presence of water, melted icy worlds seem ideal.
But this may actually bode ill for habitability. While 71% of the Earth’s surface is covered by seas, water makes up less than 0.1% of our planet’s mass. A planet with a high fraction of water may become a water world: all ocean and no exposed land.
Deep water could also mean there’s a thick layer of ice on the ocean floor. With the planet’s rocky core separated from both air and sea, no carbon-silicate cycle could form – a process that acts as a thermostat to adjust the level of warming carbon dioxide in the air on Earth.
If the TRAPPIST-1 planets can’t compensate for different levels of radiation from their star, the temperate zone for the planet shrinks to a thin strip. Any little variation, from small ellipicities in the planet’s orbit to variations in the stellar brightness, could turn the world into a snowball or baked desert.
Jupiter’s moon Io, is in resonance with moons Europa and Ganymede, and its tidal heating powers its volcanoes. NASA/JPL/University of Arizona
Even if the oceans were sufficiently shallow to avoid this fate, an icy composition might produce a very strange atmosphere. On the early Earth, air was spewed out in volcanic plumes. If a TRAPPIST-1 planet’s interior is more akin to a giant comet than to a silicate-rich Earth, the air expelled risks being rich in the greenhouse gases of ammonia and methane. Both trap heat at the planet’s surface, meaning the best location for liquid water might actually be in a region cooler than the “Goldilocks zone”.
Finally, the TRAPPIST-1 system’s orbits are problematic. Situated so close to the star, the planets are likely in tidal lock – with one face permanently turned towards the star – resulting in perpetual day on one side and everlasting night on the other.
Not only would this be weird to experience, the associated extremes of temperatures could also evaporate all water and collapse the atmosphere if the planet’s winds are unable to redistribute heat.
Also, even a small ellipticity in the planets’ seemingly circular orbits could power a second kind of warmth, called tidal heating, making the planets into Venus-like hothouses. Slight elongations in the planet’s path around its star would cause the pull from the star’s gravity to strengthen and weaken during its year, flexing the planet like a stress ball and generating tidal heat.
This process occurs on three of Jupiter’s largest moons whose mildly elliptical paths are caused by resonant orbits similar to the TRAPPIST-1 worlds. In Europa and Ganymede, the flexing heat allows subsurface liquid oceans to exist. But Jupiter’s innermost moon, Io, is the most volcanic place in our Solar System.
If the TRAPPIST-1 planets’ orbits are similarly bent, they could turn out to be sweltering.
The view from here
So how will we ever know what the TRAPPIST-1 planets are really like? To investigate the possible scenarios, we need to take a look at the atmosphere of the TRAPPIST-1 siblings.
TRAPPIST-1 was named for the Belgian 60cm TRAnsiting Planets and Planetesimal Small Telescope in Chile that detected the star’s first three planets last year (it also happens to be the name of a type of Belgian beer). As the name suggests, both the original three worlds and four new planetary siblings were discovered using the transit technique; the tiny dip in starlight as the planets passed between the star and the Earth.
Transiting makes the planets excellent candidates for the next generation of telescopes with their ability to identify molecules in the planet’s air as starlight passes through the gas. The next five years may therefore give us the first real look at a rocky planet with a very different history to anything in our Solar System.
Thomas Zurbuchen, associate administer of the Science Mission Directorate at NASA, declared the discovery of TRAPPIST-1 as, “A leap forward to answering ‘are we alone?’”.
But the real treasure of TRAPPIST-1 is not the possibility that the planets may be just like the one we call home; it’s the exciting thought that we might be looking at something entirely new.
This 10.5-billion-year-old globular cluster, NGC 6496, is home to heavy-metal stars of a celestial kind! The stars comprising this spectacular spherical cluster are enriched with much higher proportions of metals — elements heavier than hydrogen and helium are curiously known as metals in astronomy — than stars found in similar clusters.
A handful of these high-metallicity stars are also variable stars, meaning that their brightness fluctuates over time. NGC 6496 hosts a selection of long-period variables — giant pulsating stars whose brightness can take up to, and even over, a thousand days to change — and short-period eclipsing binaries, which dim when eclipsed by a stellar companion.
The nature of the variability of these stars can reveal important information about their mass, radius, luminosity, temperature, composition, and evolution, providing astronomers with measurements that would be difficult or even impossible to obtain through other methods.
NGC 6496 was discovered in 1826 by Scottish astronomer James Dunlop. The cluster resides at about 35,000 light-years away in the southern constellation of Scorpius (The Scorpion).
Image credit: ESA/Hubble & NASA, Acknowledgement: Judy Schmidt
Text credit: European Space Agency
Cassini orbited in Saturn’s ring plane — around the planet’s equator — for most of 2015. This enabled a season of flybys of the planet’s icy moons, but did not allow for angled views of the rings and the planet’s poles, like this one. But in early 2016, the spacecraft began to increase its orbital inclination, climbing higher over the poles in preparation for the mission’s final spectacular orbits in 2017.
This view looks toward the sunlit side of the rings from about 16 degrees above the ring plane. The image was taken with the Cassini spacecraft wide-angle camera on Feb. 26 2016 using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 752 nanometers.
The view was obtained at a distance of approximately 1.7 million miles (2.8 million kilometers) from Saturn. Image scale is 103 miles (165 kilometers) per pixel.
The Cassini mission is a cooperative project of NASA, ESA (the European Space Agency) and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.
What’s that over Paris? Cirrus. Typically, cirrus clouds appear white or gray when reflecting sunlight, can appear dark at sunset (or sunrise) against a better lit sky. Cirrus are among the highest types of clouds and are usually thin enough to see stars through. Cirrus clouds may form from moisture released above storm clouds and so may herald the arrival of a significant change in weather. Conversely, cirrus clouds have also been seen on Mars, Jupiter, Saturn, Titan, Uranus, and Neptune. The featured image was taken two days ago from a window in District 15, Paris, France, Earth. The brightly lit object on the lower right is, of course, the Eiffel Tower.
These three bright nebulae are often featured in telescopic tours of the constellation Sagittarius and the crowded starfields of the central Milky Way. In fact, 18th century cosmic tourist Charles Messier cataloged two of them; M8, the large nebula left of center, and colorful M20 near the bottom of the frame The third, NGC 6559, is right of M8, separated from the larger nebula by dark dust lanes. All three are stellar nurseries about five thousand light-years or so distant. The expansive M8, over a hundred light-years across, is also known as the Lagoon Nebula. M20’s popular moniker is the Trifid. In the composite image, narrowband data records ionized hydrogen, oxygen, and sulfur atoms radiating at visible wavelengths. The mapping of colors and range of brightness used to compose this cosmic still life were inspired by Van Gogh’s famous Sunflowers. Just right of the Trifid one of Messier’s open star clusters, M21, is also included on the telescopic canvas.
Today the Sun reaches its northernmost point in planet Earth’s sky. Called a solstice, the date astronomically marks a change of seasons — from spring to summer in Earth’s Northern Hemisphere and from fall to winter in Earth’s Southern Hemisphere. The featured image was taken during the week of the 2008 summer solstice at Stonehenge in United Kingdom, and captures a picturesque sunrise involving fog, trees, clouds, stones placed about 4,500 years ago, and a 4.5 billion year old large glowing orb. Even given the precession of the Earth’s rotational axis over the millennia, the Sun continues to rise over Stonehenge in an astronomically significant way.
Scientists at the Smithsonian have discovered a giant planet with three suns and which is 685 light years from us.
They said that while one of its Sun is about 40 times more intense than our Sun. It was also believed by the scientists that the KELT-4 system, which is home to a “Hot Jupiter” planet known as KELT-4Ab, is a binary system. However, it was recently discovered that it is instead a binary pair. This finding was possible by the use of two telescopes located in Arizona and South Africa. These two telescopes when combined together are known as Kilodegree Extremely Little Telescope (KELT).
The other two stars, KELT-4B and KELT-4C appeared dimmer, and could be compared to the Earth’s moon.
The star KELT -4A is close to us and it is brighter, making it easier to study. The other two suns are comparatively far away, dimmer and not that large. It takes KELT-4Ab just three days to orbit KELT-4A.The temperature on Hot Jupiter is very high, touching around thousands of degrees Fahrenheit.
KELT-4Ab is the fourth exoplanet, which has been discovered to have three suns. This giant planet is much closer to earth compared to other three planets therefore, it is much more convenient to observe it closely.
Jason Eastman and his colleagues at the Harvard-Smithsonian Center for Astrophysics, plan to further study these stars, and they aim to find out how their gravity pushed KELT-4Ab into such a hot spot.
Comparing the planet you live on to the size of other celestial bodies has got to be one the biggest mind-blowing experiences a human can go through. To put yourself into perspective and recognize that we are but a nano-blip on the radar of extraterrestrial life makes it all that much more apparent why we haven’t been visited yet… or have we…?
Our solar system’s shadowy ninth (dwarf) planet was the subject of furious speculation and a frantic search for almost a century before it was finally discovered by Clyde Tombaugh in 1930. And remarkably, Pluto’s reality was deduced using a heady array of reasoning, observation and no small amount of imagination.
The 18th and 19th centuries were thick with astronomical discoveries; not least were the planets Uranus and Neptune. The latter, in particular, was predicted by comparing observed perturbations in the orbit of Uranus to what was expected. This suggested the gravitational influence of another nearby planet.
John Couch Adams and Urbain-Jean-Joseph Le Verrier calculated the orbit of Neptune by comparing these perturbations in Uranus’ orbit to those of the other seven known planets. Neptune was hence discovered in the predicted location in 1846.
Soon after this, French physicist Jacques Babinet proposed the existence of an even more distant planet, which he named Hyperion. Le Verrier wasn’t convinced, stating that there was “absolutely nothing by which one could determine the position of another planet, barring hypotheses in which imagination played too large a part”.
Despite that lack of evidence for perturbations in Neptune’s orbit, many predicted the existence of a ninth planet over the next 80 years. Frenchman Gabriel Dallet called it “Planet X” in 1892 and 1901, and the famed American astronomer William Henry Pickering proposed “Planet O” in 1908.
Comets, the law of vegetable growth and a conspiracy
In addition to the perturbations of known planets there were other hypotheses that foretold unknown bodies beyond Neptune.
In the 19th century, it was understood that many comets had highly elliptical orbits that swung past the outer planets at their farthest points from the sun. It was believed that these planets diverted the comets into their eccentric orbits.
Pluto is not only distant, but it’s small. That makes it very difficult to see from Earth. NASA
In 1879 the French astronomer Camille Flammarion predicted a planet with an orbit 24 times that of Earth’s based on comet measurements. Using the same method, George Forbes, professor of astronomy at Glasgow University, confidently announced in 1880 that “two planets exist beyond the orbit of Neptune, one about 100 times, the other about 300 times the distance of the earth from the sun”.
Depending on how the calculations were done, the results predicted anything from one to four planets.
Other predictions were based on what can be described as numerical curiosities or speculations. One of these was the now-discredited Bode’s law, a sort of Fibonacci sequence for planets. The American mathematician Benjamin Pierce was not a fan, claiming that “fractions which express the law of vegetable growth” were more accurate than Bode’s law.
As well as these earnest astronomers, the trans-Neptunian planet idea attracted cranks and visionaries. An interesting contribution came in 1875 from Count Oskar Reichenbach, who accused Le Verrier and Adams of conspiring to conceal the locations of two trans-Neptunian planets.
The early photographic searches
Theories and calculations were all well and good, but many hoped to actually see the hitherto invisible planet(s). From the late 1800s new powerful telescopes equipped with the latest dry-plate photographic technologies were employed to search for undiscovered planets.
Pluto isn’t easy to spot. This 10 minute exposure shows the apparent magnitude of Pluto compared to some nearby stars. Kevin Heider, CC BY-SA
Amateur astronomers such Isaac Roberts and William Edwards Wilson used the predictions of George Forbes to search the skies, taking many hundreds of photographic plates in the process. They found no lurking trans-Neptunian planets.
The professionals fared no better. Edward Charles Pickering, director of the Harvard Observatory and William’s brother, spent around ten years from 1900 searching using his own data and those of earlier astronomers such as Dallet, all to no avail.
Lowell’s approach
In 1906 a new approach was introduced by the veteran astronomer Percival Lowell. Although best known to us for his (mistaken) observations of canals on Mars, Lowell bought a new rigour to analysing the orbit of Uranus based on observational data from 1750 to 1903.
With these improved calculations, hope for a visual fix on the elusive planet was renewed. With the aid of the brothers Vesto and Earl Slipher, Lowell spend the rest of his life scanning photographic plates with a hand magnifier and finally with a Zeiss blink comparator.
In September 1919 William Pickering kicked off another search for “Planet O” based on deviations in Neptune’s orbit. Milton L Humason, from the Mount Wilson Observatory in California, started a search based on these new predictions as well as Lowell’s and Pickering’s 1909 predictions. This search again failed to find any new planets. Pickering continued to publish articles on hypothetical planets but by 1928 he had become discouraged.
Zeiss Blink comparator at Lowell Observatory used in the discovery of Pluto by Clyde Tombaugh in 1930 nivium/Wikimedia, CC BY
A planet among 160,000 stars
As part of Lowell’s legacy, the Lowell Observatory built a special astrographic telescope. It was completed in 1929, and under Vesto Slipher’s direction, a young assistant was assigned to take and examine the photographs of the farthest reaches of the solar system. His name was Clyde Tombaugh.
This was grim, unglamorous work. Each plate was exposed for an hour or more, with Tombaugh adjusting the telescope precisely to keep pace with the slowly turning sky. Today a computer would make the comparisons, but in 1929 they were made by eye, manually flicking between two images. Stars would remain motionless while other bodies would seem to jump between views. Some images would have 40,000 stars, others up to 1 million.
Nearly a year had elapsed when, on February 18, 1930, two images fifteen times fainter than Neptune were found among 160,000 stars on the photographic plates. The discovery was confirmed by examining earlier images. On February 20 the planet was observed to be yellowish, rather than bluish like Neptune. The new planet had revealed its true colours at last.
Announcing a discovery
Slipher waited until March 13 to announce the discovery. This was both Lowell’s birthday and the anniversary date of the discovery of Uranus. The announcement set off a worldwide rush to observe and photograph the new planet.
Now that astronomers, amateur and professional alike, knew what they were looking for, it turned out that Pluto had been hiding in plain view. Re-examination of Humanson’s plates showed four images of Pluto from his 1919 survey, and there were many others.
On March 14, an Oxford librarian read the news to his 11-year old granddaughter Venetia Burney, who suggested the name Pluto. It was also suggested independently in a letter by William Henry Pickering.
To complete the circle, some of Clyde Tombaugh’s remains are in a canister attached to the New Horizons spacecraft.
Most people alive today would not remember a universe without Pluto. And from 2015, its patterned surface will enter our visual vocabulary of the planets. Once seen, it can never again be unseen. Planet X, welcome to our world.
