# Detecting a Rogue Planet from Earth

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This is for a novel.

A rogue planet, is, by a one-in-a-zillion freak chance, on its way to collide with Earth. It has been drifting in interstellar space since its original formation (billions of years ago), and thus will be about as cold as it can get. It is approaching at a sharp angle to the ecliptic, and at its nearest point it will pass within the orbit of the Moon.

I'm currently envisioning its mass to be roughly equal to Neptune, but the only real constraint is that even if it does not collide with the Earth, its passage will end life on Earth with near certainty.

My question is: How close can such a planet come and still have a reasonable chance of not being detected from Earth by modern astronomy? Assume that the astronomical community is at its usual level of vigilance. I don't want the failure to detect it earlier to be due to some fluke; the oncoming collision is flukey enough.

I assume that this will depend on its composition, whether predominantly rocky or predominantly gaseous. I'd prefer the answer to be based on whichever type can get closer without being detected.

## Aliens could have spotted Earth cross the sun from more than 1,700 star systems

E.T. could also potentially have detected life on Earth, scientists say.

In the past 5,000 years, more than 1,700 nearby stars could have seen Earth and potentially detected life here, and four of those stars are known to possess rocky planets of their own, a new study finds.

When astronomers hunt for exoplanets, the most productive strategy to date is to look for worlds that "transit," or cross in front of, their stars. "Seventy percent of all known exoplanets have been found by transits so far," study lead author Lisa Kaltenegger, an astrophysicist and director of the Carl Sagan Institute at Cornell University in Ithaca, New York, told Space.com.

This fact led Kaltenegger to speculate what stars could see Earth. Last year she and her colleagues identified more than 1,000 nearby stars that could currently see the pale blue dot of Earth cross in front of the sun. But then she began to wonder how vantage points of Earth might change over time.

Lisa Kaltenegger, Carl Sagan Institute

"I was wondering how long that front-row seat to find Earth through the dip in brightness of the star lasts," Kaltenegger said. "The cosmos is dynamic, so a vantage point is not forever &mdash it is gained and lost."

To see which nearby stars could have seen Earth pass in front of the sun in the past or might do so in the future, the scientists analyzed the European Space Agency's Gaia database. The observations from the mission, which launched in 2013 and continues today, include a map of the positions and motions of more than 331,000 stars within about 325 light-years of the sun.

"Our solar neighborhood is a dynamic place where stars enter and exit that perfect vantage point to see Earth transit the sun at a rapid pace," study co-author Jackie Faherty, an astrophysicist and senior scientist at the American Museum of Natural History, said in a statement. "Gaia has provided us with a precise map of the Milky Way galaxy, allowing us to look backward and forward in time, and to see where stars had been located and where they are going."

The researchers identified 1,715 stars that could have spotted Earth transits since about 5,000 years ago, which is about when civilizations began to bloom on Earth. Although 313 of these can no longer see Earth cross in front of the sun, 1,402 still can. Moreover, another 319 should enter this "Earth transit zone" in the next 5,000 years.

Of those 2,034 total stars in the 10,000-year span examined, the scientists found that 75 lay within 100 light-years of Earth, close enough for human-made radio waves to have swept over the star, since commercial radio stations on Earth began broadcasts about a century ago.

Previous research found that rocky planets are not only common in the Milky Way, but often lie in the habitable or "Goldilocks" zones, where a planet can retain liquid water on its surface. Based off this prior work, the researchers estimated that 29 of these 75 stars may possess rocky worlds in their habitable zones.

Seven of the 2,034 stars are known hosts of exoplanets that have had or will have the chance to detect Earth just as Earth's scientists have detected them. Three of these seven exoplanetary systems &mdash K2-65, K2-155 and K2-240 &mdash can currently see Earth.

"Who knows if life evolved there too, but if it did, and it had a similar technology level that we have, then such nominal alien observers could have spotted or will spot life on our own world," Kaltenegger said. "If someone had found us already I wonder what they would think about us?"

Three of these seven exoplanetary systems &mdash Teegarden's star, GJ 9066 and Trappist-1 &mdash will be able to spot Earth in the coming decades and centuries. Moreover, they will be close enough to detect our radio waves. Intriguingly, the Trappist-1 system, which is about 45 light-years from Earth, is home to seven Earth-size planets, four of them in the habitable zone. Trappist-1 will enter the Earth transit zone in 1,642 years and remain there for 2,371 years.

One of the seven exoplanetary systems, Ross 128, could have seen Earth transit the sun for 2,158 years, starting about 3,057 years ago and ending about 900 years ago. This stellar system includes a red dwarf host star located about 11 light-years away from Earth in the Virgo constellation and is the second-closest planetary system with an Earth-size exoplanet, which is about 1.8 times the size of our planet. But any observers in the Ross 128 system would have missed the chance to detect any of our radio waves. "Would anyone there have concluded that there was intelligent life on Earth 900 years ago?" Kaltenegger wondered.

Ideally, Kaltenegger would like to see which exoplanets could have seen Earth as far back as 2 billion years ago, "when Earth's atmosphere was first transformed due to life &mdash the big oxidation event. Because that is how long anyone looking at Earth could have figured out that there is life on our beautiful pale blue planet." Unfortunately, the Gaia database is not precise enough to allow scientists "to go that far back in time," she noted.

The scientists noted that the ability to see Earth transiting the sun also provides the chance to analyze light streaming through Earth's atmosphere to detect potential chemical signs of life, such as oxygen. This technique is one of the ways that the James Webb Space Telescope, expected to launch later this year, is set to analyze several transiting exoplanets to look for hints of life there. "We are right now on the threshold of finding life in the cosmos," Kaltenegger said.

Perhaps anyone seeing Earth from a distant star might want a closer look at our planet. For instance, the $100 million Breakthrough Starshot project aims to launch microchip-size spacecraft at the closest exoplanet detected around Proxima Centauri 4.2 light-years from Earth to better study that world. "One might imagine that worlds beyond Earth that have already detected us, are making the same plans for our planet and solar system," Faherty said in a statement. In the future, NASA's Transiting Exoplanet Survey Satellite (TESS) and other telescopes will look for exoplanets around the 2,034 stars the researchers identified, "hopefully many in the habitable zone," Kaltenegger said. "Once someone finds such planets, then my team will model them and, assuming they have life, figure out how much time the James Webb Space Telescope would need to find it, and apply for that time." Kaltenegger and Faherty detailed their findings online June 23 in the journal Nature. ## How to Find a Rogue Planet So how does a planet end up all alone? And how is it possible to find a dark world in the middle of nowhere? ### Finding a Star-less Planet The fact that PSO J318.5-22 doesn't orbit a star means there is little light to obscure it from searching telescopes. But it also means there's little light to illuminate the planet. The team that found this rogue planet started by looking for brown dwarfs, or failed stars. Using the Pan-STARRS wide-field telescope atop the Haleakala volcano on Maui, they surveyed huge swaths of space and mined a database of billions of objects. "Pan-STARRS is surveying the sky using a CCD digital camera with 1.4 billion pixels, and it takes images using filters that select certain colors of light," says Katelyn Allers, an astronomy professor at Bucknell University and coauthor of the paper. "We searched the incredibly large Pan-STARRS database to find objects that seemed like they would have cool, planet-like temperatures, which is indicated by red colors." PSO J318.5-22 showed up in those infrared readings as faint and red, but much redder than any brown dwarfs on record. Allers and the rest of the team knew they'd spotted a good candidate for a free-floating planet. "The thing that's really interesting about our new discovery is that it looks just like the young planets that have been directly imaged around other stars," says team leader Michael Liu of the Institute for Astronomy at the University of Hawaii. Once the astronomers caught a glimpse of PSO J318.5-22's infrared signature using Pan-STARRS, the team switched to smaller telescopes to image the planet in more detail. "Once we were able to get a spectrum, we could then estimate the temperature and age and very precisely measure the distance," Liu tells PopMech. ### How It Got There The researchers estimate that PSO J318.5-22 is 12 million years old. But they don't know how it wound up free-floating&mdashthough there are two possibilities that astronomers think are plausible. The first idea is that PSO J318.5-22 formed the same way the Earth and the planets of our solar system did: in the circumstellar disk of a star. Then, Allers says, somehow the planet got separated from its parent star and shot off into space. According to a study in Monthly Notices of the Royal Astronomical Society, this could happen when a passing star exerts its gravitational pull and yanks the planet out of orbit. It's also possible that the parent star, upon ending its hydrogen-burning life and expanding into a red giant, could push a planet out of its system. The second possibility is that the planet has been a loner from the beginning. It might have formed in isolation from a cloud of collapsed matter, one that did not have enough mass to ever reach the stage of fusing hydrogen, at which point it could have become a star. "Some recent work of Viki Joergens [of the Max-Planck Institute for Astronomy in Heidelberg, Germany] has shown evidence that some planetary-mass objects form like stars, and accrete material from their own circumstellar disks," Allers says. "But we'll need to obtain additional data to try and distinguish between these two formation mechanisms for PSO J318.5-22." ### How Many Rogue Planets Are There? Similar free-floating planetary-mass objects have been discovered in the past, but while their infrared signatures are typically associated with brown dwarfs, PSO J318.5-22 most closely resembles a planet. Brown dwarves, like stars, form from collapsing clouds of gas and dust, but do not coalesce into dense enough objects to trigger nuclear fusion. They are too big to be planets, yet too small to be proper stars. Brown dwarves are thought to be as prevalent as stars, and much more abundant than free-floating planets. Still, there are probably plenty of rogue planets. Thousands have been discovered over the past decade, but the majority of these have been too far away to capture by direct imagingio9 reports that there could be billions of free-roaming planets formed by incredibly dense, fast-moving globulettes in the Rosette Nebula, which is 4600 light years from Earth. These small clouds of matter could have collapsed upon themselves, forming planets that have broken away from the nebula. Liu says that while the researchers are unsure of the exact number of potential free-floating planets, now that they've found PSO J318.5-22, they can use it's infrared signature as a template to find other candidates among the catalogues amassed by Pan-STARRS. And the chances are good there are more rogue planets closer to home. Judging from information gathered from star-forming regions, Allers says, it appears that there is about one planetary-mass object for every 10 sun-like stars. "This very roughly corresponds to about a dozen free-floating planets closer than 100 light years to us," she says. "We've definitely got more work to do to find all of them." ## Einstein's magnifying glass Most exoplanets in our galaxy are visible only because of their host stars. In a literal sense, stars provide the light that allows astronomers to directly observe alien worlds. When a planet is too small or too distant to be seen directly, scientists can still detect it from the slight gravitational pull it exerts on its host star (called the radial velocity method) or by the flickering that occurs when a planet passes in front of the star's Earth-facing side (the transit method). Rogue planets, by definition, have no star to light their way - or to light a telescope's way to them. Instead, detecting rogue planets involves a facet of Einstein's theory of general relativity known as gravitational lensing. Through this phenomenon, a planet (or even more massive object) acts as a cosmic magnifying glass that temporarily bends the light of objects behind it from Earth's perspective. "If a massive object passes between an Earth-based observer and a distant source star, its gravity may deflect and focus light from the source," Mroz explained in a statement. "The observer will measure a short brightening of the source star." The smaller that light-bending object is, the briefer the star's perceived brightening will be. While a planet several times the mass of Jupiter might create a brightening effect that lasts a few days, a measly planet the mass of Earth will brighten the source star for only a few hours, or less, the researchers said. This exceptionally rare occurrence is called "microlensing." "Chances of observing microlensing are extremely slim," Mroz added in the statement. "If we observed only one source star, we would have to wait almost a million years to see the source being microlensed." Fortunately, Mroz and his colleagues weren't observing just one star for their study - they were watching hundreds of millions of them. Using observations from the Optical Gravitational Lensing Experiment (OGLE), a star survey based at the University of Warsaw in Poland that has turned up at least 17 exoplanets since 1992, the team stared into the center of the Milky Way, looking for any signs of microlensing. In June 2016, they witnessed the shortest microlensing event ever seen. The star in question, located roughly 27,000 light-years away in the densest part of the galaxy, brightened for just 42 minutes. Calculations showed that the offending object was not bound to any star within 8 astronomical units (AU, or eight times the average distance from Earth to the Sun), suggesting it was almost certainly a tiny planet on the run, ejected from its home solar system after a brush with a much more massive object. Depending on how far away the planet is from the source star (it's impossible to tell with current technology), the rogue world is likely between one-half and one Earth mass. In either case, this roaming world would be the lowest-mass rogue planet ever detected. According to Mroz, that's a "huge milestone" for the science of planet formation. "Theories of planet formation have predicted that the majority of free-floating planets should be of Earth mass or smaller, but this is the first time that we could find such a low-mass planet," Mroz said. "It's really amazing that Einstein's theory allows us to detect a tiny piece of rock floating in the galaxy." Many more tiny pieces of rock may soon follow, study co-author Radek Poleski of the University of Warsaw told Live Science. Future planet-hunting telescopes, like NASA's Nancy Grace Roman Space Telescope (slated to launch in the mid-2020s), will be much more sensitive to the galaxy's teensiest microlensing events than the nearly 30-year-old OGLE experiment is, Poleski said. If orphan planets of roughly Earth's mass are indeed some of the most common denizens of the galaxy, it shouldn't be long before many more of them turn up. This article was originally published by Live Science. Read the original article here. ## Astronomers find weird rogue world with wild auroras Artist’s concept of rogue planet SIMP J01365663+0933473. The free-floating planet has a magnetic field millions of times more powerful than Earth’s and intense auroras. Image via Caltech/Chuck Carter/NRAO/AUI/NSF. Rogue planets are free-floating worlds, drifting in space between the stars. A growing number of these odd objects have been discovered in recent years, not gravitationally tethered to any star. Now, a weird new one has been found and observed more closely, as discussed in a new peer-reviewed paper in The Astrophysical Journal on July 31, 2018. This world may be unbound to a star, but it is still energetic, with a powerful magnetic field four million times stronger than Earth’s and auroras more powerful than Earth’s northern lights. Named SIMP J01365663+0933473 by earthly astronomers, this newly-discovered rogue planet is only 20 light-years from Earth, quite close by, astronomically speaking. Astronomers discovered it using the National Science Foundation’s Karl G. Jansky Very Large Array (VLA). They say it’s the first radio observation of a planetary-mass object beyond our solar system and the first time scientists have measured the magnetic field of one of these isolated planets. Illustration of the magnetic field around Jupiter. The magnetic field of SIMP J01365663+0933473 is thought to behave in a similar way, but is 20,000 times more powerful. Image via Yned/Wikimedia Commons. SIMP J01365663+0933473 was first observed in 2016, but the newest observations were needed to reveal its bizarre nature. It was thought at first to be a brown dwarf – on object smaller than a star but larger than a planet – but additional observations indicated that it’s just small enough to be called a planet, albeit still huge – 12.7 times the mass and 1.2 times the radius of Jupiter. It is also very hot – over 1,500 degrees Fahrenheit (825 Celsius) on its surface. According to Melodie Kao at Arizona State University, who led the new study: This object is right at the boundary between a planet and a brown dwarf, or ‘failed star,’ and is giving us some surprises that can potentially help us understand magnetic processes on both stars and planets. Perhaps the most surprising thing about SIMP J01365663+0933473 is its powerful magnetic field, some four million times stronger than Earth’s. By comparison, Jupiter’s magnetic field, the strongest in the solar system, is 20,000 times more powerful than Earth’s. As might be expected, the magnetic field on the rogue planet also produces stunning auroras. The researchers involved think that this rogue world’s auroras might be more similar to Jupiter’s than Earth’s however SIMP J01365663+0933473 does not have a star bombarding it with solar wind particles like Earth does, so the theory is that this unbound world might have its own moon. Jupiter’s auroras are produced not primarily by the sun, as Earth’s are, but primarily by charged particles coming from its volcanically active moon Io. As Kao noted: This particular object is exciting because studying its magnetic dynamo mechanisms can give us new insights on how the same type of mechanisms can operate in extrasolar planets. We think these mechanisms can work not only in brown dwarfs, but also in both gas giant and terrestrial planets. The northern lights (auroras) on Earth as seen from orbit. SIMP J01365663+0933473 has similar auroral displays, but they are much more intense. Image via NASA/ESA. The new findings about this rogue planet will help astronomers learn more about planetary magnetic fields, exoplanetary ones in particular, but also provide another way of finding exoplanets. As co-author Gregg Hallinan of Caltech said: Detecting SIMP J01365663+0933473 with the VLA through its auroral radio emission also means that we may have a new way of detecting exoplanets, including the elusive rogue ones not orbiting a parent star. It’s now estimated that there might be one Jupiter-mass rogue planet for every four stars in our galaxy – an incredible number, although a bit smaller than previously thought. According to Przemek Mróz, an astronomer at the University of Warsaw Observatory in Poland: Our new microlensing observations are in agreement with theoretical expectations on the frequency of free-floating Jupiters, and with infrared surveys for planetary-mass objects in star-forming regions. We found that Jupiter-mass planets are at least 10 times less frequent than previously thought. One previous estimate, published in a Nature paper in 2011, was that there could be twice as many rogue planets as stars. So far, 17 known or suspected rogue planets have been discovered. Even though rogue planets have no stars from which to receive heat, they could still retain thick hydrogen and helium atmospheres that could trap heat left over from their formation. They could also remain geologically active for long periods of time. Artist’s concept of another rogue planet, CFBDSIR J214947.2-040308.9. It’s now estimated there may be one Jupiter-mass rogue planet for every 4 stars in our galaxy. Image via ESO/L. Calçada/P. Delorme/Nick Risinger/R. Saito/VVV Consortium. Could there even be life on rogue planets? According to David Bennett, a senior NASA research scientist: If you want to understand the possibility of life on other planets, it takes more than just finding one in the same size and orbit as Earth and trying to study it. There are many things that feed into a planet’s habitability – its atmosphere, its history, its water content – and those things can go all the way back to the details of its formation … If we really want to look for life, we need to understand the processes involved in planet formation, some of which can eject planets and make them free-floating. Bottom line: SIMP is a bizarre world, floating freely through space in remote solitude. But far from being quiet, it has a powerful magnetic field, millions of times stronger than Earth’s, and intense auroras similar to Jupiter’s but much more powerful. Its unique nature will help astronomers learn more about magnetic fields on exoplanets and even help in discovering new exoplanets. ## Mysterious gigantic rogue planet spotted lurking outside our solar system In the first radio-telescope detection of a planetary-mass object beyond our solar system, astronomers have found the strange celestial body has 12.7 times the mass of Jupiter. It doesn&rsquot appear to orbit a parent star, however, and is only 20 light-years away from Earth. &ldquoThis object is right at the boundary between a planet and a brown dwarf, or &lsquofailed star,&rsquo and is giving us some surprises that can potentially help us understand magnetic processes on both stars and planets,&rdquo study lead astronomer Melodie Kao said. A brown dwarf is an object too large to be a planet, but isn&rsquot big enough to sustain the nuclear fusion of hydrogen in its core that is vital to stars. The object, which has been named SIMP J01365663+0933473, was first detected in 2016, but was thought to be a brown dwarf. The latest data reveals it&rsquos younger than first thought at a relatively youthful 200 million years old, and its mass is smaller, so it could be classified as a planet. Its temperature is also far cooler than the sun, at 825 degrees Celsius. It also has a strong magnetic field, 200 times the strength of Jupiter. The researchers were able to pick up on the object&rsquos magnetic activity using a powerful radio astronomy observatory called the Very Large Array, a National Science Foundation facility in New Mexico. The methods used suggest the researchers may have &ldquoa new way of detecting exoplanets, including the elusive rogue ones not orbiting a parent star,&rdquo researcher Gregg Hallinan said. ## Astronomers Identify 29 Planets Where Aliens Could Detect Life on Earth ###### Text By ###### Share this article A pair of astronomers identified 1,715 star systems within roughly 300 light years of Earth’s solar system where alien lifeforms could have spotted life on Earth in the past 5,000 years, according to a study published on Wednesday in Nature. Among those situated in prime positions to view the planet’s transit, 46 star systems are located close enough to Earth for their planets to intercept radio and TV broadcast signals, which started approximately 100 years ago. “One way we find planets is if they block out part of the light from their host star,” said Lisa Kaltenegger, professor of astronomy and director of the Carl Sagan Institute at Cornell University in New York, to The Guardian. “We asked, ‘Who would we be the aliens for if somebody else was looking?’ There is this tiny sliver in the sky where other star systems have a cosmic front seat to find Earth as a transiting planet.” With Jackie Faherty, a senior scientist at the American Museum of Natural History in New York, Kaltenegger estimated that within those star systems, 29 potentially habitable planets are well-positioned to witness Earth’s transit and near enough to detect human-made transmissions. The duo’s findings provide a more distinct target for scientist’s ongoing search for extraterrestrial life, or SETI. While previous studies have identified star systems able to view Earth’s transit in the present-day, Kaltenegger and Faherty’s findings are the first to expand that knowledge across 5,000 years in the past and 5,000 years in the future. “What we showed in our paper is that most stars have this vantage point [to see Earth transits] for at least 1,000 years, and a lot of stars actually have it for more than 10,000 years,” Kaltenegger told VICE. “We couldn’t say anything more than that because our timeline is 10,000 years, but it was interesting that this vantage point holds for generations of astronomers, or generations of alien astronomers” that “could develop technology to find us.” ## Alien Planet-Hunters In Hundreds Of Nearby Star Systems Could Spot Earth An illustrated view of the Earth moving around our sun, and the stars that have the right vantage point to view that transit — if anyone's out there looking. OpenSpace/American Museum of Natural History hide caption An illustrated view of the Earth moving around our sun, and the stars that have the right vantage point to view that transit — if anyone's out there looking. OpenSpace/American Museum of Natural History Right now, a couple of planets about as massive as Earth are orbiting a dim star that's just a dozen light-years away from us. Those planets could be cozy enough to potentially support life. But if any one is living there — and if these life forms have the same kinds of technology that humans do — they wouldn't be able to detect Earth yet. This will change in just 29 years, according to a report published Wednesday in the journal Nature. That's because stars are constantly moving, and this particular star, called Teegarden's Star, will soon slip into the right location to be able to watch our sun and notice the slight dimming that occurs when Earth passes in front of it. "If they have the same technique as we do, and if there is a 'they,' then they wouldn't know yet that we exist," says Lisa Kaltenegger, director of the Carl Sagan Institute at Cornell University in Ithaca, N.Y. "In 29 years, they would be able to see us." She and Jackie Faherty, a senior scientist at the American Museum of Natural History in New York City, have just used a new catalogue of stars and their movements to determine what solar systems could potentially detect Earth in the past, present and future. Their work assumes that alien planet-hunters would rely on the same kinds of technologies that people have used to discover more than 4,400 planets orbiting far away stars. Most of those discoveries have been made by watching stars and waiting for a tell-tale dip in brightness that means an orbiting planet has briefly moved in front of the star and blocked some of its light. This planet-finding trick only works when everything is lined up just right. "It made me feel very vulnerable, because I started to think how easy we are to detect, in some ways. We're just a dot, this rock that spins around our Sun and blocks the light every 365 days for an amount of time." Jackie Faherty, senior scientist, American Museum of Natural History, NYC ### How curious aliens could spot us The Earth, of course, is going around the sun, which means this same method could potentially be used by curious aliens to find our planet. René Heller, an astrophysicist and expert in planet detection at the Max Planck Institute for Solar System Research in Germany, started thinking about this back in 2009, when he was working with an office mate to create a sky map with stars that might have transiting planets that astronomers could discover. One day, the two of them started joking around about alien scientists making a similar map that would lead them to Earth. "While we were working, we had this fun idea — if someone else out there is trying to make the same with their sky maps, and whether they would be knowing that we were making sky maps of them," he recalls. "We were trying to turn the tables, in our heads." In 2016, he and a colleague published a paper with a list of 82 stars that would have the right viewpoint to make finding Earth possible. "They might have seen us or might be seeing us transiting the sun," says Heller, and might feel compelled to send some kind of message to us. That work was based on a static map of stars. In reality, the stars travel through space, and that means what they can see will change over time. "I wanted to know who can see us now, but also who could have seen us in the past, and who will see us in the future," says Kaltenegger. She knew that new data from a star cataloging mission called Gaia would make that possible. She and Faherty limited themselves to looking at the local cosmic neighborhood within 300 light years of the Sun, which contains more than 300,000 stars. "We wanted to use the closest stars," explains Faherty. "When it comes to exploring worlds, the nearest ones to us are going to be the most exciting." It turns out that only a small fraction — around 1,715 stars — would have had the right vantage point to spot Earth at some time within the last 5,000 years. In the next 5,000 years to come, 319 additional stars will move into the right position to get a good view. Still, surveys by NASA's Kepler Space Telescope have revealed that planets, including small, rocky planets similar in size to Earth and located in a temperate zone around their stars, are common. Kaltenegger estimates that 500 or so such planets could be orbiting the stars identified in this study. In fact, some of these stars are already known to host at least one planet. The famous TRAPPIST-1 star system, for example, has seven Earth-size planets. It will enter the Earth-viewing zone in 1,642 years, and it will remain there for 2,371 years. Meanwhile, the star named Ross-128, which is orbited by an Earth-sized planet, no longer is in the right location to spot Earth, but could have done so from around 3,000 years ago to about 900 years ago. "Would they have figured out that there is intelligent life on the Earth?" wonders Kaltenegger. After all, human-made radio waves have only been leaving the Earth for about the last century. So the researchers checked to see which of the stars on their list were also close enough (within 100 light years) that our radio waves would have washed over them. They found 46 stars that can currently see Earth transiting the Sun while also being near enough to detect radio waves. "It made me feel very vulnerable," says Faherty, "because I started to think how easy we are to detect, in some ways. We're just a dot, this rock that spins around our Sun and blocks the light every 365 days for an amount of time. We're a classic transiting planet that somebody could find, and then we've got this radio signature that we give off." She notes, however, that no one knows if any nearby worlds have intelligent life, and they are all extremely far away. "Maybe none of them have anything like us," says Faherty. Other researchers say that looking at how perspectives change over time adds a key component to searching for others in the universe — because it takes time for light and radio signals to travel from one star system to another through the vastness of space. ### Maybe build a big sign to signal we're here? When the TRAPPIST-1 star system gets into position to see the Earth more than 1,000 years from now, says Heller, we could even potentially try to communicate with any observers there by doing something that would alter the appearance of Earth's transit across the Sun. "We could construct space-based megastructures that betray our presence to them," Heller notes. "We could, I don't know, install a giant square — transiting together with the Earth, or just a few hours later, in front of the Sun. So they could see, 'Wow, that's not only an Earth-like planet with an interesting atmosphere, but there is also a square!'" But maybe sending radio messages would be easier and not as costly, says Heller. For over half a century, researchers have been searching for any messages sent to us by extraterrestrial life. In 2015, a ten-year,$100 million project called Breakthrough Listen got started to dramatically expand the search for any extraterrestrial communication.

"It's really hard to find. And we've been discussing for decades and decades, what magic frequencies, what magic times, what magic places," says Jill Tarter, chair emeritus for SETI Research at the SETI Institute in Mountain View, Calif.

This new research will add to that discussion, says Tarter: "I would suspect that Breakthrough Listen will — if they don't already have these stars on their target list, that they would add them."

If the volume of space that needed to be searched for extraterrestrial broadcasts was equivalent to all the Earth's oceans, she says, so far humanity has searched the equivalent of a hot tub's worth of water.

No one should come to the monumental conclusion that humans are alone in the universe "because we haven't found anything in one hot tub's worth of the world's oceans," says Tarter. "We've hardly begun to search."

## An Earth-sized rogue planet discovered in the Milky Way

Our galaxy may be teeming with rogue planets, gravitationally unbound to any star. An international team of scientists, led by Polish astronomers, has announced the discovery of the smallest Earth-sized free-floating planet found to date.

Over 4,000 extrasolar planets have been discovered to date. Although many of the known exoplanets do not resemble those in our solar system, they have one thing in common—they all orbit a star. However, theories of planet formation and evolution predict the existence of free-floating (rogue) planets, gravitationally unattached to any star. Indeed, a few years ago, Polish astronomers from the OGLE team from the Astronomical Observatory of the University of Warsaw provided the first evidence for the existence of such planets in the Milky Way. Writing in Astrophysical Journal Letters, OGLE astronomers announced the discovery of the smallest rogue planet found to date.

Exoplanets can only rarely be directly observed. Usually, astronomers find planets using observations of the light from the planet’s host star. For example, if a planet crosses in front of its parent star’s disk, then the observed brightness of the star periodically drops by a small amount causing so called transits. Astronomers can also measure the motion of the star caused by the planet.

Free-floating planets emit virtually no radiation and—by definition—they do not orbit any host star, so they cannot be discovered using traditional methods of astrophysical detection. Nevertheless, rogue planets can be spotted using an astronomical phenomenon called gravitational microlensing. Microlensing results from Einstein’s theory of general relativity—a massive object (the lens) may bend the light of a bright background object (the source). The lens’ gravity acts as a huge magnifying glass which bends and magnifies the light of distant stars.

“If a massive object (a star or a planet) passes between an Earth-based observer and a distant source star, its gravity may deflect and focus light from the source. The observer will measure a short brightening of the source star,” explains Dr. Przemek Mroz, a postdoctoral scholar at the California Institute of Technology and a lead author of the study. “Chances of observing microlensing are extremely slim because three objects—source, lens, and observer—must be nearly perfectly aligned. If we observed only one source star, we would have to wait almost a million year to see the source being microlensed,” he adds.

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This is why modern surveys hunting for gravitational microlensing events are monitoring hundreds of millions of stars in the Milky Way center, where the chances of microlensing are highest. The OGLE survey—led by Warsaw University astronomers—carries out one such experiment. OGLE is one of the largest and longest sky surveys, starting operations over 28 years ago. Currently, OGLE astronomers are using a 1.3-meter Warsaw Telescope located at Las Campanas Observatory, Chile. Each clear night, they point their telescope to the central regions of the galaxy and observe hundreds of millions of stars, searching for those which change their brightness.

Gravitational microlensing does not depend on the lens’ brightness, so it enables the study of faint or dark objects such as planets. Duration of microlensing events depends on the mass of the lensing object—the less massive the lens, the shorter the microlensing event. Most of the observed events, which typically last several days, are caused by stars. Microlensing events attributed to free-floating planets have timescales of barely a few hours. By measuring the duration of a microlensing event (and shape of its light curve) we can estimate the mass of the lensing object.

The scientists announced the discovery of the shortest-timescale microlensing event ever found, called OGLE-2016-BLG-1928, which has the timescale of just 42 minutes. “When we first spotted this event, it was clear that it must have been caused by an extremely tiny object,” says Dr. Radoslaw Poleski from the Astronomical Observatory of the University of Warsaw, a co-author of the study.

Indeed, models of the event indicate that the lens must have been less massive than Earth, it was probably a Mars-mass object. Moreover, the lens is likely a rogue planet. “If the lens were orbiting a star, we would detect its presence in the light curve of the event,” adds Dr. Poleski. “We can rule out the planet having a star within about 8 astronomical units (the astronomical unit is the distance between the Earth and the sun).”

OGLE astronomers provided the first evidence for a large population of rogue planets in the Milky Way a few years ago. However, the newly-detected planet is the smallest rogue world ever found. “Our discovery demonstrates that low-mass free-floating planets can be detected and characterized using ground-based telescopes,” says Prof. Andrzej Udalski, the PI of the OGLE project.

Astronomers suspect that free-floating planets actually formed in protoplanetary disks around stars (as “ordinary” planets) and they have been ejected from their parent planetary systems after gravitational interactions with other bodies, for example, with other planets in the system. Theories of planet formation predict that the ejected planets should be typically smaller than Earth. Thus, studying free-floating planets enables us to understand the turbulent past of young planetary systems, such as the solar system.

The search for free-floating planets is one of the science drivers of the Nancy Grace Roman Space Telescope, which is currently being constructed by NASA. The observatory is scheduled to start operations in the mid-2020s.

Because of the brevity of the event, additional observations collected by the Korea Microlensing Telescope Network (KMTNet) were needed to characterize the event. KMTNet operates a network of three telescopes—in Chile, Australia, and South Africa.

Provided by: University of Warsaw

More information: Przemek Mróz et al. A Terrestrial-mass Rogue Planet Candidate Detected in the Shortest-timescale Microlensing Event. The Astrophysical Journal (2020). DOI: 10.3847/2041-8213/abbfad

Image: An Earth-sized rogue planet discovered in the Milky Way

## Needle in a Haystack: Astronomers Discover Rogue Planet With Earth-like Low Mass in Our Galaxy

A planet that drifts away from its solar system and floats around in space forever is termed as a rogue planet. They leave the gravitational pull of their host star and are no longer bound to any star. In a significant breakthrough, a group of astronomers have discovered one such planet in our Milky Way, which has the same mass as that of Mars or Earth.

The new planet, named "OGLE-2016-BLG-1928", has no stars nearby and the planet’s distance from the Earth is also not confirmed yet. The research is significant as it shows the effectiveness of the microlensing technique in detecting rogue planets in space.

Discovering a celestial body is like finding a needle in a haystack. Now imagine spotting a floating object in space which does not emit any light of its own! OGLE (Optical Gravitational Lensing Experiment) collaboration and the KMTN (Korean Microlensing Telescope Network) collaboration are the two international teams that have taken up this extremely challenging task of discovering dark matter and other celestial bodies with no radiation. Around 30 researchers from both these organizations have now discovered this rogue planet.

### Microlensing to detect low-mass planets

In the early years of any solar system, some planets with a low mass can theoretically escape away from the star’s gravitational grip. "According to planet-formation theories, such as the core accretion theory, typical masses of ejected planets should be between 0.3 and 1.0 Earth masses," states the research paper.

Discovery of such low-mass bodies that have been separated from its solar system is done using a technique called microlensing. In the process two objects are needed: a light source, which could be a bright star at a distance and a celestial object with enough mass to bend the light coming out of this light source.

The planet that has low mass acts as a lens that bends the light. The astronomers analyse the variation in the radiation due to the object to determine. If the object at the front has a low mass, then the amount of light bent remains less and the duration of the bending is also relatively less.

According to the study, this discovery has the shortest time scale microlens to this date.

### Tracing the rogue

In the last couple of years, due to many pieces of research and discoveries, our knowledge about exoplanets has expanded, and now we know that there are innumerable rogue planets that are floating around in space. Theoretically, the numbers go up to trillions in just our Milky Way Galaxy.

Scientists attribute the drifting of low-mass planets to the following causes: the scattering of planets, interactions between stars in a star system, passing through of stellar objects, solid interactions between bigger planets that affect smaller planets, and the evolution of the host star beyond the main sequence.

Finding low-mass rogue planets is hard as the microlensing process occurs for a short period if these planets are of small size. The microlensing event that led to the discovery of this new planet lasted only for 41.5 minutes. This is a very short period when one has to get a good amount of data. That is the reason, crucial details like the distance of the planet from Earth are still unknown.

Previously, four other tiny rogue planets have been discovered in microlensing events of short periods. All these discoveries together provide powerful evidence for the existence of many such planets in the Milky way, say the researchers.

### A rogue planet or a stellar object?

Another major challenge for researchers was determining whether the detected object was a rogue planet or a stellar object. When the microlensing event is of a short period, the presence of a stellar substance is a possible occurrence and hence the confirmation becomes necessary. Researchers ruled out the existence of stellar contemporaries up to a distance of 8 Astronomical Units, but some planets circle their stars at a greater distance than this.

The paper says that this planet was found at the edge of current limits of detecting short-timescale microlensing events. The detection of the event was conducted with 15 data points—11 were from OGLE and 4 were from KMTN—which is considered to be relatively less.

The study explains that fewer data points mean that "the declining part of the light curve is not fully covered with observations." These numbers lead to some confusions and uncertainties around the event’s characteristics and the object being a planet. Some of this confusion arrives from the star emitting the light itself. However, the researchers concluded that the properties of this event rule out the possibility of flares coming from a background star.

Even though there is some ambiguity about the mass of the object, most of the properties of the event point to an object with earth-like mass with no sign of a stellar friend up to 8 AU.

The potential in rogue planets for the existence of life is null. Yet, the study of such planets becomes important to understand the intricacies of space.

In the next five years, the Nancy Grace Roman Space Telescope—dubbed to be the Monther of Hubble—will be looking out for such planets. The extremely powerful telescope will also image exoplanets, look into dark matter and obtain spectra of exoplanets’ atmosphere in its mission. This mission, again, will help astronomers to better understand the solar systems and their evolution.

The study is yet to be published in a peer-reviewed journal, but the pre-print can be accessed here.

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