Astronomy

Has the existence of earth-sized rogue planets been confirmed?

Has the existence of earth-sized rogue planets been confirmed?


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The list of known rogue planets provided by Wikipedia is fairly short. Some are "candidates"; others "may be" red dwarfs. The two confirmed rogue planets are several times the size of Jupiter.

I understand that rogue planets can only be detected if they glow (which immediately brings the phrase "red dwarf" to mind) or if they cross the path of light emanating from a star. This makes detection extremely difficult.

The reason I ask is I'm not quite certain whether objects the size of Jupiter or smaller can even exist in interstellar space. Maybe dark matter grinds them to dust, or maybe they implode in the absence of any meaningful inertial frame - I don't know. Has anything Earth-sized been detected out there?


The list on Wikipedia looks current. It doesn't list planets that have only been detected as one-off microlensing events. This can detect Jupiter sized objects (with considerable uncertainty in mass). But observing a microlensing doesn't allow for study, so a microlensing is not considered to be enough evidence for naming or cataloging a known planet. So the short answer is no Earth sized rogue planets are known.

This is entirely due to the difficulty in detection. An Earth sized rogue planet would cool on its surface to a few degrees above absolute zero, and be undetectable with current telescopes. Microlensing events with small rogue planets would also be smaller and harder to detect above noise.

An Earth sized rogue planet has a perfectly good inertial frame, (it is the bound planets that are in a rotating, non-inertial frame), nor does a frame of reference have anything to do with implosions. Dark matter is doesn't grind anything since the main property of Dark matter is that it doesn't interact with regular matter.

In all likelihood, there are billions of rogue planets in the galaxy, and some are smaller, and some are Earth-sized and "rocky". They could have been formed around a star and then ejected.


To add to the answer from James K, the ESA program "Gaia", amongst many other purposes, intends to capture hints predicting microlensing events. There is much to study in that domain, and the amateur astronomer community can help.

http://www.cosmos.esa.int/web/gaia/IoW_20161027

EDIT : forgot to mention the linked possible microlensing event is bound to happen the two first weeks of November 2016, which makes it quite current.


First ‘Earth Without A Sun’ Discovered: Thousands More To Be Revealed Soon

When a mass passes through the line-of-sight to a distant light source, it bends, magnifies, and . [+] distorts the light. This type of event, known as gravitational microlensing, affords us an opportunity to detect massive objects like planets without stars: objects that emit no detectable light of their own.

Jan Skowron / Astronomical Observatory, University of Warsaw

For countless millennia, planets beyond our Solar System were mere speculation.

Today, we know of over 4,000 confirmed exoplanets, with more than 2,500 of those found in the Kepler . [+] data. These planets range in size from larger than Jupiter to smaller than Earth. However, these methods that are so useful for detecting planets around stars, like the transit or stellar wobble methods, cannot detect rogue planets.

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

Only since the 1990s has science revealed their existence.

The radial velocity (or stellar wobble) method for finding exoplanets relies on measuring the motion . [+] of the parent star, as caused by the gravitational influence of its orbiting planets. Since the planet and star both orbit their mutual center-of-mass, the star won't remain stationary, but will "wobble" in its orbit, with periodic redshifts and blueshifts revealing the mass and period of the orbiting exoplanet.

Today, more than 4,000 exoplanets are known, revealed from their effects on the stars they orbit.

When planets pass in front of their parent star, they block a portion of the star's light: a transit . [+] event. By measuring the magnitude and periodicity of transits, we can infer the orbital parameters and physical sizes of exoplanets. When transit timing varies and is followed (or preceded) by a smaller-magnitude transit, it may indicate an exomoon as well, such as in the system Kepler-1625.

NASA’s Goddard Space Flight Center/SVS/Katrina Jackson

But plenty of planets should have no parent stars at all.

Perhaps surprisingly, these rogue planets should be extraordinarily common.

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Particular configurations over time, or singular gravitational interactions with passing large . [+] masses, can result in the disruption and ejection of large bodies from solar and planetary systems. In the early stages of a solar system, many masses are ejected just from the gravitational interactions arising between protoplanets, leading to the existence of rogue, orphaned planets.

SHANTANU BASU, EDUARD I. VOROBYOV, AND ALEXANDER L. DESOUZA arXiv:1208.3713

Many young planets get ejected as solar systems form, creating “orphaned” planets.

Although we now believe we understand how the Sun and our solar system formed, this early view is an . [+] illustration only. When it comes to what we see today, all we have left are the survivors. What was around in the early stages was far more plentiful than what survives today.

JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/SOUTHWEST RESEARCH INSTITUTE (JHUAPL/SWRI)

Others formed as members of insufficiently massive, failed solar systems.

This star-forming region is rich in ionizing radiation, which is blowing off the remaining gas . [+] that's attempting to collapse to form stars. The regions that fail to gain enough mass will not become stars, but rather rogue planets and rogue planetary systems: terrestrial or gas giant planets without a parent star of their own.

Altogether, rogue planets should outnumber the stars in our Milky Way.

The candidate rogue planet CFBDSIR2149, as imaged in the infrared, is a gas giant world that emits . [+] infrared light but has no star or other gravitational mass that it orbits. It is one of the only rogue planets known, and was only discoverable because of its large-enough mass to emit its own infrared radiation.

Direct infrared imaging only reveals high-mass rogue planets.

Rogue planets may be numerous in the galaxy, but it surprises most to learn that there are between . [+] 100 and 100,000 rogue planets for every star in our galaxy, putting the total number of planets wandering through the Milky Way at somewhere around a quadrillion.

But another method — gravitational microlensing — has begun to change everything.

Any planet passing between us and a star will gravitationally bend the intervening space.

When any mass or system of masses, whether a star, planet, solar system, or something more complex . [+] passes between a telescope and its line-of-sight towards a distant star, the intervening mass distorts the intervening space, allowing for a gravitational microlensing event to occur.

NASA’s Exoplanet Science Institute / JPL-Caltech / IPAC

This magnifies, distorts, and creates multiple images of the background star.

When a gravitational microlensing event occurs, the background light from a star gets distorted and . [+] magnified as an intervening mass travels across or near the line-of-sight to the star. The effect of the intervening gravity bends the space between the light and our eyes, creating a specific signal that reveals the mass and speed of the planet in question.

Jan Skowron / Astronomical Observatory, University of Warsaw

From physics, we can then infer the rogue planet’s properties.

Rogue planets may have a variety of exotic origins, such as arising from shredded stars or other . [+] material, or from ejected planets from solar systems, but the majority should arise from star-forming nebula, as simply gravitational clumps that never made it to star-sized objects. When a microlensing event occurs, we can use the light to reconstruct the intervening planet's mass.

CHRISTINE PULLIAM / DAVID AGUILAR / CFA

Light curve of the ultrashort microlensing event OGLE-2016-BLG-1928, which was likely caused by a . [+] free-floating, rogue planet no more massive than planet Earth.

Mroz et al. 2020, arXiv:2003.01126

Fast imaging is a necessity: the entire event lasted just 42 minutes.

From the beginning of the event, which includes the brightening of the background star, the . [+] distortion of its position, and the appearance of a second light source, until the end, only 42 minutes elapsed. Imaging the same object repeatedly just minutes or hours apart is essential for catching these extremely rapid microlensing events.

Jan Skowron / Astronomical Observatory, University of Warsaw

NASA's Nancy Grace Roman Space Telescope will make its microlensing observations in the direction of . [+] the center of the Milky Way galaxy. The higher density of stars will yield more microlensing events, including those that reveal exoplanets as small and low-in-mass as planet Earth.

NASA's Goddard Space Flight Center/CI Lab

Multiple observatories can work in tandem to identify rogue planets, orbiting exoplanets, and even . [+] multi-component systems as they create microlensing events based on the bending of space along the line-of-sight to a distant object. Better, faster, higher-quality observations can reveal superior details and lower masses for exoplanets.

Korea Astronomy and Space Science Institute

Gravitational microlensing is a powerful tool for detecting exoplanets. This illustration shows the . [+] bending of light from a background source by a planetary system in the foreground. Note that peak alignment corresponds to peak magnification: the greatest apparent brightness of the background light source.

NASA Exoplanet Exploration

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less smile more.


Gemini Observatory and W.M. Keck Observatory Joint Press Release

For Embargoed Release on April 17, 2014 at 11am PDT, 2:00pm EDT, and 8:00am HST

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  • Steve Howell
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The first Earth-sized exoplanet orbiting within the habitable zone of another star has been confirmed by observations with both the W. M. Keck Observatory and the Gemini Observatory. The initial discovery, made by NASA's Kepler Space Telescope, is one of a handful of smaller planets found by Kepler and verified using large ground-based telescopes. It also confirms that Earth-sized planets do exist in the habitable zone of other stars.

"What makes this finding particularly compelling is that this Earth-sized planet, one of five orbiting this star, which is cooler than the Sun, resides in a temperate region where water could exist in liquid form," says Elisa Quintana of the SETI Institute and NASA Ames Research Center who led the paper published in the current issue of the journal Science. The region in which this planet orbits its star is called the habitable zone, as it is thought that life would most likely form on planets with liquid water.

Steve Howell, Kepler's Project Scientist and a co-author on the paper, adds that neither Kepler (nor any telescope) is currently able to directly spot an exoplanet of this size and proximity to its host star. "However, what we can do is eliminate essentially all other possibilities so that the validity of these planets is really the only viable option."

With such a small host star, the team employed a technique that eliminated the possibility that either a background star or a stellar companion could be mimicking what Kepler detected. To do this, the team obtained extremely high spatial resolution observations from the eight-meter Gemini North telescope on Mauna Kea in Hawai`i using a technique called speckle imaging, as well as adaptive optics (AO) observations from the ten-meter Keck II telescope, Gemini's neighbor on Mauna Kea. Together, these data allowed the team to rule out sources close enough to the star's line-of-sight to confound the Kepler evidence, and conclude that Kepler's detected signal has to be from a small planet transiting its host star.

"The Keck and Gemini data are two key pieces of this puzzle," says Quintana. "Without these complementary observations we wouldn't have been able to confirm this Earth-sized planet."

The Gemini "speckle" data directly imaged the system to within about 400 million miles (about 4 AU, approximately equal to the orbit of Jupiter in our solar system) of the host star and confirmed that there were no other stellar size objects orbiting within this radius from the star. Augmenting this, the Keck AO observations probed a larger region around the star but to fainter limits. According to Quintana, "These Earth-sized planets are extremely hard to detect and confirm, and now that we've found one, we want to search for more. Gemini and Keck will no doubt play a large role in these endeavors."

The host star, Kepler-186, is an M1-type dwarf star relatively close to our solar system, at about 500 light years and is in the constellation of Cygnus. The star is very dim, being over half a million times fainter than the faintest stars we can see with the naked eye. Five small planets have been found orbiting this star, four of which are in very short-period orbits and are very hot. The planet designated Kepler-186f, however, is earth-sized and orbits within the star's habitable zone. The Kepler evidence for this planetary system comes from the detection of planetary transits. These transits can be thought of as tiny eclipses of the host star by a planet (or planets) as seen from the Earth. When such planets block part of the star's light, its total brightness diminishes. Kepler detects that as a variation in the star's total light output and evidence for planets. So far more than 3,800 possible planets have been detected by this technique with Kepler.

The Gemini data utilized the Differential Speckle Survey Instrument (DSSI) on the Gemini North telescope. DSSI is a visiting instrument developed by a team led by Howell who adds, "DSSI on Gemini Rocks! With this combination, we can probe down into this star system to a distance of about 4 times that between the Earth and the Sun. It's simply remarkable that we can look inside other solar systems." DSSI works on a principle that utilizes multiple short exposures of an object to capture and remove the noise introduced by atmospheric turbulence producing images with extreme detail.

Observations with the W.M. Keck Observatory used the Natural Guide Star Adaptive Optics system with the NIRC2 camera on the Keck II telescope. NIRC2 (the Near-Infrared Camera, second generation) works in combination with the Keck II adaptive optics system to obtain very sharp images at near-infrared wavelengths, achieving spatial resolutions comparable to or better than those achieved by the Hubble Space Telescope at optical wavelengths. NIRC2 is probably best known for helping to provide definitive proof of a central massive black hole at the center of our galaxy. Astronomers also use NIRC2 to map surface features of solar system bodies, detect planets orbiting other stars, and study detailed morphology of distant galaxies.

"The observations from Keck and Gemini, combined with other data and numerical calculations, allowed us to be 99.98% confident that Kepler-186f is real," says Thomas Barclay, a Kepler scientist and also a co-author on the paper. "Kepler started this story, and Gemini and Keck helped close it," adds Barclay.


Lonely Rogue Worlds Surprisingly Outnumber Planets with Suns

Astronomers have discovered a whole new class of alien planet: a vast population of Jupiter-mass worlds that float through space without any discernible host star, a new study finds.

While some of these exoplanets could potentially be orbiting a star from very far away, the majority of them most likely have no parent star at all, scientists say.

And these strange worlds aren't mere statistical anomalies. They likely outnumber "normal" alien planets with obvious parent stars by at least 50 percent, and they're nearly twice as common in our galaxy as main-sequence stars, according to the new study. [Photos: The Strangest Alien Planets]

Astronomers have long predicted the existence of free-flying "rogue alien planets." But their apparent huge numbers may surprise many researchers, and could force some to rethink how the planets came to be.

"Previous observations of bound planets tell us only about planets which are surviving in orbits now," said study lead author Takahiro Sumi, of Osaka University in Japan. "However, [these] findings inform us how many planets have formed and scattered out."

Alien worlds under gravitational lens

Sumi and his colleagues made the find using a method called gravitational microlensing, which watches what happens when a massive object passes in front of a star from our perspective on Earth. The nearby object bends and magnifies the light from the distant star, acting like a lens.

This produces a "light curve" &mdash a brightening and fading of the faraway star's light over time &mdash whose characteristics tell astronomers a lot about the foreground object's size. In many cases, this nearby body is a star if it has any orbiting planets, these can generate secondary light curves, alerting researchers to their presence.

Before the current study, astronomers had used the gravitational microlensing technique to discover a dozen or so of the nearly 550 known alien planets. (NASA's Kepler mission has detected 1,235 candidate planets by a different method, but they still need to be confirmed by follow-up observations.)

Sumi and his team looked at two years' worth of data from a telescope in New Zealand, which was monitoring 50 million Milky Way stars for microlensing events. They identified 474 such events, including 10 that lasted less than two days.

The short duration of these 10 events indicated that the foreground object in each case was not a star but a planet roughly the mass of Jupiter. And the signals from their parent stars were nowhere to be found.

Independent observations from a telescope in Chile backed up the finds. Either these 10 planets orbit very far from their host stars &mdash more than 10 times the Earth-sun distance &mdash or they have no host stars at all, researchers said. [Infographic: A Sky Full of Alien Planets]

Common throughout the galaxy

Gravitational microlensing events are rare, because they require the precise alignment of a background star, a massive foreground object and Earth. So the discovery of 10 short-duration events in two years suggests a huge population of these unbound or distantly orbiting Jupiter-mass exoplanets throughout the galaxy, researchers said.

Sumi and his team calculated, in fact, that these planets are probably almost twice as common in our own Milky Way as main-sequence stars. And they likely outnumber "normal" planets with known host stars by more than 50 percent.

Other studies have established that it's probably pretty rare for huge planets to orbit more than 10 Earth-sun distances from a parent star. So the research team argues that most of the Jupiter-mass planets &mdash at least 75 percent of them &mdash are likely true "rogues," floating through space unbound to a star.

Theory predicts that such rogues should exist throughout the galaxy, and other researchers have found evidence of unbound objects that may indeed be orphan planets. But those worlds were much bigger, from three to 10 times Jupiter's mass, and there's a lot of uncertainty in the measurements.

Many of the previously detected objects could actually be "failed stars" known as brown dwarfs, Sumi said.

Sumi and his colleagues report their results in the May 19 issue of the journal Nature.

Rethinking planetary formation theories

The newly discovered rogue planets may have formed close to a host star, then been ejected from their solar systems by the gravitational influence of a huge neighbor planet, researchers said. Indeed, such planet-planet interactions are thought to be responsible for the odd, extremely close-in orbits of the giant alien planets known as "hot Jupiters."

But the abundance of the seemingly starless worlds may force astronomers to rethink some of their ideas about planet formation, according to Sumi.

The "current most recognized planetary formation theory (core accretion model) cannot create so many giant planets," Sumi told SPACE.com in an email interview. "So we need a different theory to create [so] many giant planets, such [as the] gravitational instability model."

In the core accretion model, dust coalesces to form a solid core, which later accretes gas around it, creating a planet. The gravitational instability model invokes the rapid collapse of gas, with a core forming later due to sedimentation.

The new study should inspire much follow-up research. One of the next steps could involve training more instruments on the microlensing alien planets, further monitoring them for any signs of a parent star. Such work, which may take years, could eventually reveal how many of these worlds actually do have parent stars, and how many are true rogues.

"The implications of this discovery are profound," astronomer Joachim Wambsganss, of Heidelberg University, wrote in an accompanying essay in the journal Nature. "We have a first glimpse of a new population of planetary-mass objects in our galaxy. Now we need to explore their proper­ties, distribution, dynamic states and history."


Another Kepler milestone: Astronomers find two Earth-sized planets orbiting the same star!

Astronomers have achieved a big milestone in the search for another Earth: the two smallest confirmed planets ever found orbiting another star… and they’re both about the size of Earth!

Artist’s illustration of the Kepler-20 planets with Earth and Venus for size comparison.

The planets are called Kepler-20e and Kepler-20f, and as you can see by the illustration above they are very close to the same size as our home world: 20e is about 11,100 km (6900 miles) in diameter, and 20f about 13,200 km (8200 miles) across. For comparison, Earth has a diameter of 12,760 km (7930 miles). This makes them the smallest confirmed exoplanets seen orbiting another star! The previous record holder was Kepler-10b, which has a diameter about 40% bigger than Earth’s.

To be clear: while these planets are the size of Earth, they are nowhere near Earth-like. The star, Kepler-20, is very much like the Sun, though a bit smaller and cooler (and 950 light years away). However, both planets orbit the star much closer than Earth does 7.6 million km (4.7 million miles) and 16.6 million km (10.3 million miles), respectively. This is so much closer that both planets must have surface temperatures far hotter than ours, 760°C and 430°C (1400°F and 800°F). Even on the “cooler” planet Kepler-20f, it’s hot enough to melt tin and zinc.

So don’t start packing your bags to visit, even if you could spare a few million years to get there via rocket (950 light years is a bit of a hike). I’ll note that we don’t know the masses of these planets either. I’ll explain that in a moment, but given their sizes it’s expected they’ll have masses similar to Earth’s.

So this is very exciting! For one thing, it shows that Kepler can indeed find planets the size of Earth orbiting distant stars. That right away is fantastic that’s the main goal of Kepler in the first place.

For another, it shows that our solar system is not entirely unique. We do know of several other stars hosting solar systems of their own, but those planets tend to be very massive they’re easier for us to find. Since Kepler-20e and f are so close to Earth-sized, this is a big achievement.

And we’re still not done: there are three other planets in the Kepler-20 system! The others are much larger than the Earth: named Kepler-20b, c, and d, they have diameters of 24,000, 40,000, and 35,000 km (15,000, 24,600, and 22,000 miles) smaller than Uranus and Neptune, but still pretty hefty. We do have the masses for them: 8.7, 16.1, and about 20 times the mass of the Earth. Call them “super-Earths” if you like.

All these planets huddle pretty closely to their star the orbit of Kepler-20f, the farthest from the star, would still fit comfortably inside the orbit of Mercury! Oddly, the configuration is very different than our own solar system. While ours has the lower-mass planets close in and the bigger ones farther out, in the Kepler-20 system they alternate, going big-little-big-little-big.

So how do we know all this? The Kepler observatory is in space, staring at one patch of sky all the time. There are 100,000 stars in its field of view, including Kepler-20. If there are planets orbiting a star, and we see the orbit of that planet edge-on, then once per orbit the planet directly passes between us and the star, blocking its light a little bit. This is called a transit, and the bigger the planet, the more light it blocks. That’s how the sizes of the five planets were found.

As these planets orbit their star their gravity tugs on it, and that can be measured by carefully observing the star’s light. As a planet pulls it one way and then another, there is a very small Doppler shift in the starlight, and the amount of that shift tells us how hard the planet is tugging on the star, and that in turn depends on the mass of the planet. Only the three bigger planets in the Kepler-20 system pull hard enough for us to measure, which is why we don’t have the masses of 20e and 20f they’re too small to measure.

Also, to be clear, we don’t have direct images of these planets (those pictures above are drawings). They were found indirectly by how they affected their star. But these methods are now tried-and-true, and the existence of these five planets has been confirmed. They’re real.

This is a fantastic discovery for so many reasons: the smallest planets found orbiting another star, the first Earth-sized planets seen by Kepler, both in the same solar system, and in such an oddly-configured and compact system at that. This means we need to think more about how such planets can form, of course, since it’s so weird… but no matter what, it means we’re that much closer to finding the ultimate goal: an Earth-sized planet orbiting a Sun-like star in that star’s habitable zone, where liquid water can exist.

Every time I hear news like this, I wonder how much longer we’ll be waiting to hear that news… and I strongly suspect it won’t be too much longer.


Eva's main passions in life are astrophysics, quantum mechanics, and walking her cat Bop on a sunny Sunday morning.

Eva Lund

A team of Polish astronomers has spotted a so-called orphan planet, or a rogue planet, which wanders lonely along the Milky Way. Object OGLE-2016-BLG-1928 was found using microlensing.

Microlensing is a technique that allows you to observe how a dim and usually invisible celestial body distorts the light from another object, for example, a star that is nearby.

This time, astronomers paid attention to the light of a star in the Sagittarius constellation. They recorded the shortest microlensing event that lasted 41.5 minutes. It happened back in 2016. All these years, scientists have been observing the object. Finally, they confirmed that this event was caused by the smallest orphan planet known to science. It is similar in size to Earth or Mars.

Previously, all rogue planets found were several times heavier than Jupiter, whose mass is 318 times that of Earth. It is not yet clear how objects the size of our planet end up in interstellar space.

Scientists estimate billions of rogue, free-floating planets to exist in our Galaxy, but only a bunch of candidates have been detected among nearly 4,000 worlds beyond our Solar System.


Earth-sized rogue planet discovered in Milky Way

Our galaxy may be full of rogue planets that are not tied to any star by gravity. An international team of scientists led by Polish astronomers announced the discovery of the smallest such free-floating planet discovered to date. It turned out to be the size of the Earth. The results of the study are published by The Astrophysical Journal.

To date, more than 4,000 extrasolar planets have been discovered. While many of them do not look like exoplanets in our solar system, they all have one thing in common – they all orbit a star. However, theories of planetary formation and evolution predict the existence of free-floating planets that are not gravitationally associated with any star.

Indeed, a few years ago, Polish astronomers from the OGLE group at the Astronomical Observatory of Warsaw University provided the first evidence for the existence of such planets in the Milky Way. In an article in the Astrophysical Journal Letters, OGLE astronomers announced the discovery of the smallest rogue planet discovered to date.

Exoplanets are rarely directly observed. Astronomers usually find planets by observing the light from the planet’s host star. For example, if a planet crosses the disk of its parent star, then its observed brightness periodically drops by a small amount, causing so-called transits.

Free-floating planets emit virtually no radiation and – by definition – do not orbit a parent star, so they cannot be detected using traditional astrophysical detection methods. However, rogue planets can be detected using gravitational microlensing. Microlensing is the result of Einstein’s general theory of relativity. A massive object (lens) can deflect light from a bright background object (source). The lens’s gravity acts like a huge magnifying glass that bends and magnifies the light of distant stars.

This is why current research on gravitational microlensing phenomena is tracking hundreds of millions of stars in the center of the Milky Way. The OGLE survey conducted by astronomers at the University of Warsaw is conducting one such experiment. OGLE is one of the largest and longest-running surveys of the sky, begun over 28 years ago. OGLE astronomers are currently using the 1.3-meter Warsaw Telescope located at the Las Campanas Observatory, Chile. Every clear night, they point their telescope at the central regions of the galaxy and observe hundreds of millions of stars in search of those that change their brightness.

And today, scientists announced the discovery of the shortest microlensing event ever discovered, called OGLE-2016-BLG-1928, which takes only 42 minutes. “When we first noticed this event, it was clear that it must have been caused by an extremely tiny object,” explains Dr. Radoslaw Polesski of the Astronomical Observatory at Warsaw University, co-author of the study.

Several years ago, OGLE astronomers provided the first evidence of a large number of rogue planets in the Milky Way. However, the newly discovered planet is the smallest rogue ever discovered. “Our discovery demonstrates that low-mass, free-floating planets can be detected and characterized by ground-based telescopes,” says Professor Andrzej Udalski, OGLE project leader.

Astronomers suspect that free-floating planets actually formed in protoplanetary disks around stars (like “normal” planets) and were later ejected from their parent planetary systems after gravitational interactions with other bodies, such as other planets in the system. Planet formation theories predict that ejected planets should usually be smaller than Earth. Thus, the study of free-floating planets allows scientists to understand the turbulent past of young planetary systems, such as the solar system.

Finding free-floating planets is one of the tasks of the Roman Space Telescope, which is being built by NASA. The observatory is slated to begin operation in the mid-2020s.

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Pleiades Supercomputer Essential to Planet Searches

"The Kepler Mission has already transformed our understanding of how planetary systems work," said Todd Klaus, lead software engineer in the Kepler Science Operations Center (SOC) at NASA's Ames Research Center, Mountain View, Calif. NASA's Pleiades supercomputer, operated by the NASA Advanced Supercomputing (NAS) Division at Ames, is an "essential component to make these planet searches possible," Klaus said.

As of February 27, 2012, the Kepler team has found 2,321 planet candidates&mdashso called because they have not yet been confirmed as true planets&mdashorbiting 1,790 stars. Of these potential 61 have been confirmed as bona fide planets.

The team has identified well over 200 Earth-size planet candidates and more than 900 that are smaller than twice Earth-size (super Earths). Of the 46 potential "sister planets" found in the habitable zone, the region in the planetary system where liquid water could exist, ten of these are near-Earth-size.

In September 2011, the team discovered the extraordinary existence of a planet orbiting two stars, called a circumbinary planet. Then, in January 2012, the team announced the discovery of two more double-star worlds, Kepler-34b and Kepler-35b. With the discovery of these three worlds, Kepler has also established a new class of planetary system.

NASA's Pleiades supercomputer is an essential component to make [Kepler] planet searches possible. Todd Klaus, NASA/Ames

Since then, the Kepler mission has discovered the first Earth-size planets (Kepler-20e and Kepler-20f ) orbiting a sun-like star outside our solar system. While they are too close to their star to be in the habitable zone, they are the smallest exoplanets ever confirmed around a star like our sun. (See sidebar at right, "NAS Speeds Kepler Code.") And just last month, astronomers using Kepler mission data discovered the three smallest planets yet detected orbiting a single star (called KOI-961) beyond our sun. The smallest of these planets is about the size of Mars.

Groundbreaking findings such as these are made possible by a 95-megapixel digital camera residing in the Kepler photometer&mdashthe largest digital camera to ever fly in space. Pixels are downloaded once a month and transferred to the SOC, where they are calibrated, combined to form light curves, corrected for systematic errors introduced in the photometer, and then searched for the signatures of transiting planets. When a planet passes (or transits) in front of its host star, it blocks a small fraction of the light from that star that appears as tiny, repeating pulse or beat. "By measuring the frequency of these beats and the amount of light blocked, we can detect the planets and calculate their size and orbital distance," Klaus explained.


'Superb opportunity'

The Kepler space telescope was designed to look at a fixed swathe of the night sky, staring intently at about 150,000 stars. The telescope is sensitive enough to see when a planet passes in front of its host star, dimming the star's light by a minuscule amount.

Kepler identifies these slight changes in starlight as candidate planets, which are then confirmed by further observations by Kepler and other telescopes in orbit and on Earth.

Kepler 22-b lies 15% closer to its sun than the Earth is to our Sun, and its year takes about 290 days. However, the planet's host star puts out about 25% less light, keeping the planet at its balmy temperature that would support the existence of liquid water.

The Kepler team had to wait for three passes of the planet before upping its status from "candidate" to "confirmed".

"Fortune smiled upon us with the detection of this planet," said William Borucki, Kepler principal investigator at Nasa's Ames Research Center.

"The first transit was captured just three days after we declared the spacecraft operationally ready. We witnessed the defining third transit over the 2010 holiday season."

The results were announced at the Kepler telescope's first science conference, alongside the staggering number of new candidate planets. The total number of candidates spotted by the telescope is now 2,326 - of which 207 are approximately Earth-sized.

In total, the results suggest that planets ranging from Earth-sized to about four times Earth's size - so-called "super-Earths" - may be more common than previously thought.

As candidates for planets similar to Earth are confirmed, the Search for Extraterrestrial Intelligence (Seti) has a narrower focus for its ongoing hunt.

"This is a superb opportunity for Seti observations," said Jill Tarter, the director of the Center for Seti Research at the Seti Institute.

"For the first time, we can point our telescopes at stars, and know that those stars actually host planetary systems - including at least one that begins to approximate an Earth analogue in the habitable zone around its host star."


A few years ago, Polish astronomers at the University of Warsaw identified the first “rogue” planets in the Milky Way. These planets don’t orbit around any particular star and are unbound by gravity. The existence of these rogue planets is in accordance with theories of planetary formation, which predict that some very small planets will be ejected from their parent planetary system. Now, an international team led by the Polish astronomers has described the smallest rogue planet found to date.

Because they don’t orbit around a host star, rogue planets can’t be identified using conventional astronomical methods. To find this one, the researchers used a method called gravitational microlensing. When a large object like a planet passes close to a light source like a star, the planet acts as a “lens,” and its gravity can deflect and magnify the light. As a result, the brightness of the star will increase while they are in alignment. By studying the duration of the event and the shape of the light curve, researchers can determine the mass of the planet. This new rogue planet was the shortest microlensing event ever observed, with a time scale of only 42 minutes. The models developed by the scientists predict that this planet is even smaller than Earth, and close in size to Mars.

Microlensing events are extremely rare, since they require exact alignment of a light source, a lens, and the telescope. The Optical Gravitational Lensing Experiment (OGLE) has been running for over 28 years, and involves tracking the brightness of hundreds of millions of stars in the Milky Way center. This new discovery is the first evidence that low-mass rogue planets can be identified using this technique. Studying these free-floating planets can help us understand how planetary systems evolved, and how they might change in the future.

Lead author Przemek Mroz is a postdoctoral scholar at the California Institute of Technology. Dr. Radoslaw Poleski, a co-author, is a scientist at the University of Warsaw Astronomical Observatory. Professor Andrzej Udalski, also of the University of Warsaw, is the P.I. of the OGLE project. This work was complemented by additional observations by the Korea Microlensing Telescope Network.

Managing Correspondent: Jaclyn Long

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