Space and astronomy news
Here’s a fun trip through the galaxy, put together by PhD student Tom Hands at the University of Leicester: In the above video, you can fly to of all the known exoplanets (around single stars only), ordered roughly by semi-major axis of largest orbit. Hands said the video is designed to give the viewer an overview of the current distribution of exoplanets.
Hands used data from the Open Exoplanet Catalogue.
Host: Fraser Cain
Guests: Casey Dreier, Brian Koberlein, Jason Major, Sondy Springmann, David Dickinson
Continue reading “Weekly Space Hangout – April 18, 2014: Another Earth, Dragon Launch, LADEE Impact”
It’s truly a “eureka” moment for Kepler scientists: the first rocky Earth-sized world has been found in a star’s habitable “Goldilocks” zone, the narrow belt where liquid water could readily exist on a planet’s surface without freezing solid or boiling away. And while it’s much too soon to tell if this really is a “twin Earth,” we can now be fairly confident that they do in fact exist.
The newly-confirmed extrasolar planet has been dubbed Kepler-186f. It is the fifth and outermost planet discovered orbiting the red dwarf star Kepler-186, located 490 light-years away. Kepler-186f completes one orbit around its star every 130 days, just within the outer edge of the system’s habitable zone.
The findings were made public today, April 17, during a teleconference hosted by NASA.
“This is the first definitive Earth-sized planet found in the habitable zone around another star,” says lead author Elisa Quintana of the SETI Institute at NASA Ames Research Center. “Finding such planets is a primary goal of the Kepler space telescope. The star is a main-sequence M-dwarf, a very common type. More than 70 percent of the hundreds of billions of stars in our galaxy are M-dwarfs.”
Unlike our Sun, which is a G-type yellow dwarf, M-dwarf stars (aka red dwarfs) are much smaller and dimmer. As a result their habitable zones are much more confined. But, being cooler stars, M-dwarfs have long lifespans, offering planets in their habitable zones — like Kepler-186f — potentially plenty of time to develop favorable conditions for life.
In addition, M-dwarfs are the most abundant stars in our galaxy; 7 out of 10 stars in the Milky Way are M-dwarfs, although most can’t be seen by the naked eye. Finding an Earth-sized planet orbiting one relatively nearby has enormous implications in the hunt for extraterrestrial life.
“M dwarfs are the most numerous stars,” said Quintana. “The first signs of other life in the galaxy may well come from planets orbiting an M dwarf.”
Read more: Earthlike Exoplanets Are All Around Us
Still, there are many more conditions on a planet that must be met for it to be actually habitable. But size, composition, and orbital radius are very important first steps.
“Some people call these habitable planets, which of course we have no idea if they are,” said Stephen Kane, an assistant professor of physics and astronomy at San Francisco State University in California. “We simply know that they are in the habitable zone, and that is the best place to start looking for habitable planets.”
As far as the planetary system’s age is concerned — which relates to how long life could have potentially had to evolve on Kepler-186f’s surface — that’s hard to determine… especially with M-dwarf stars. Because they are so stable and long-lived, once they’re formed M-dwarfs essentially stay the same throughout their lifetimes.
“We know it’s probably older than a few billion years, but after that it’s very difficult to tell,” BAERI/Ames scientist Tom Barclay told Universe Today. “That’s the problem with M-dwarfs.”
The exoplanet was discovered via the transit method used by NASA’s Kepler spacecraft, whereby stars’ brightnesses are continually monitored within a certain field of view. Any dips in luminance reveal the likely presence of a passing planet.
Because of its small size — just slightly over 1 Earth radius — and close proximity to its star, Kepler-186f can’t be observed directly with current telescope technology.
“However, what we can do is eliminate essentially all other possibilities so that the validity of these planets is really the only viable option,” said Steve Howell, Kepler project scientist and a co-author on the paper.
Using the latest advanced imaging capabilities of the Gemini North and Keck II observatories located atop Mauna Kea in Hawaii, astronomers were able to determine that the signals detected by Kepler were from a small orbiting planet and not something else, such as a background or companion star.
“The Keck and Gemini data are two key pieces of this puzzle,” Quintana said. “Without these complementary observations we wouldn’t have been able to confirm this Earth-sized planet.”
Kepler-186f joins the other 20 extrasolar worlds currently listed in the Habitable Exoplanets Catalog, maintained by the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo. To date 961 exoplanets have been confirmed through Kepler observations, with 1,696 total confirmed altogether. (Source)
Read more: Mega Discovery! 715 Alien Planets Confirmed Using a New Trick on Old Kepler Data
Whether Kepler-186f actually resembles Earth or not, this discovery provides more information on the incredible variety of planetary systems to be found even in our little corner of the galaxy.
“The diversity of these exoplanets is one of the most exciting things about the field,” Kane said. “We’re trying to understand how common our solar system is, and the more diversity we see, the more it helps us to understand what the answer to that question really is.”
The SETI Institute’s Allen Telescope Array has surveyed the Kepler-186 system for any potential signals but so far none has been detected. Further observations are planned.
“Kepler-186f is special because we already know that a planet of its size and distance is capable of supporting life.”
– Elisa Quintana, research scientist, SETI Institute
The team’s paper, “An Earth-sized Planet in the Habitable Zone of a Cool Star” by Elisa V. Quintana et al., will be published in the April 18 issue of Science.
Learn more about the Kepler mission here, and read more about this discovery in NASA’s news release here and on the W.M. Keck website here.
Watch some video excerpts of team interviews and data renderings below:
Also, you can download the slides used in the NASA teleconference here.
Sources: San Francisco State University, Gemini Observatory, W.M. Keck Observatory, and SETI news releases
When astronomers detect new exoplanets they typically do so using one of two techniques. First, there’s the famous transit technique, which looks for slight dips in light as a planet passes in front of its host star, and second is the radial velocity technique, which senses the motion of a star due to the gravitational pull of its planet.
But then there is gravitational microlensing, the chance magnification of the light from a distant star by the mass of a foreground star and its planets due to the distortion in the fabric of spacetime. While this technique sounds almost improbable, it is so accurate that every detection skips nominating planets as candidates and immediately verifies them as bona-fide worlds.
But without follow-up observations, the microlensing technique struggles with characterizing the incredibly faint host star. Now, a team of international astronomers led by PhD candidate Jennifer Yee from Ohio State University has detected the first microlensing signature, lovingly called MOA-2013-BLG-220Lb, that looks like a confirmed planet orbiting a candidate brown dwarf — an object so faint because it isn’t massive enough to kick-off nuclear fusion in its core.
Matter — no matter how great or small — curves the fabric of spacetime. It can ultimately acts like a lens by curving the background light around it and therefore magnifying the background source. In microlensing, the intervening matter is simply a faint star or perhaps a planetary system.
“As the ‘lens system’ passes in front of a distant, background star, the magnification of that background star changes as a function of time,” Yee told Universe Today. “By measuring the changing magnification of the background star, we can learn about the lensing star and perhaps whether or not it has a planet.”
In a planetary system, the light from the background star will be magnified when the foreground star passes in front of it. If there is a cirlcing planet, there will be an additional cusp in brightness (to a lesser extent but still a tell-tale detection nonetheless).
At the moment the planetary system transits in front of the background star (and for many years after) we can’t separate the two objects. While the light of the background star may be greatly magnified, its image is distorted because its light merges with the planetary system.
So the microlensing signature cannot tell astronomers anything about the lens system’s star. “It’s out of the ordinary,” Andrew Gould, Yee’s PhD advisor and coauthor on the paper, told Universe Today. “In other techniques people have definitely detected a star and they’re struggling to detect the planet. But microlensing is just the opposite. We detect the planet very clearly, but we can’t detect the host star.”
However, the microlensing signature does give away the lens system’s proper motion — the apparent change in distance over time — as it passes in front of the background star. MOA-2013-BLG-220Lb’s proper motion is extremely high, clocking in at 12.5 milliarcseconds (a distance on the sky that is 2400 times smaller than the size of the full moon) per year. This is roughly three times higher than average.
A high proper motion may be caused by an object that is very close by and is moving slowly or a very distant object moving rapidly. As most stars tend not to move at high speeds, the team assumes the object is relatively close, placing it at a distance of 6,000 light-years.
With a distance fixed, the team is also able to assume a mass for the object. It weighs in below the hydrogen-burning limit and is therefore considered the best brown dwarf candidate microlensing has detected.
“The double-edged sword of microlensing is that no light from the lens star is required,” Yee told Universe Today. “On the one hand, microlensing can find planets around dark or faint objects like brown dwarfs. The flip side is that it’s very difficult to characterize the lens star if its light is not detected.”
Astronomers will have to wait until 2021 to take a second look at the lens system. This time frame is how long we expect it to take before the candidate brown dwarf separates appreciably on the sky from the background star. Once it has done so astronomers will be able to verify whether or not the candidate is truly a brown dwarf.
The paper is available for download here.
Measuring the atmospheric pressure of a distant exoplanet may seem like a daunting task but astronomers at the University of Washington have now developed a new technique to do just that.
When exoplanet discoveries first started rolling in, astronomers laid emphasis in finding planets within the habitable zone — the band around a star where water neither freezes nor boils. But characterizing the environment and habitability of an exoplanet doesn’t depend on the planet’s surface temperature alone.
Atmospheric pressure is just as important in gauging whether or not the surface of an exoplanet may likely hold liquid water. Anyone familiar with camping at high-altitude should have a good understanding of how pressure affects water’s boiling point.
The method developed by Amit Misra, a PhD candidate, involves isolating “dimers” — bonded pairs of molecules that tend to form at high pressures and densities in a planet’s atmosphere — not to be confused with “monomers,” which are simply free-floating molecules. While there are many types of dimers, the research team focused exclusively on oxygen molecules, which are temporarily bound to each other through hydrogen bonding.
We may indirectly detect dimers in an exoplanet’s atmosphere when the exoplanet transits in front of its host star. As the star’s light passes through a thin layer of the planet’s atmosphere the dimers absorb certain wavelengths of it. Once the starlight reaches Earth it’s imprinted with the chemical fingerprints of the dimers.
Dimers absorb light in a distinctive pattern, which typically has four peaks due to the rotational motion of the molecules. But the amount of absorption may change depending on the atmospheric pressure and density. This difference is much more pronounced in dimers than in monomers, allowing astronomers to gain additional information about the atmospheric pressure based on the ratio of these two signatures.
While water dimers were detected in the Earth’s atmosphere as early as last year, powerful telescopes soon to come online may enable astronomers to use this method in observing distant exoplanets. The team analyzed the likelihood of using the James Webb Space Telescope to make such a detection and found it challenging but possible.
Detecting dimers in an exoplanet’s atmosphere would not only help us evaluate the atmospheric pressure, and therefore the state of water on the surface, but other biosignature markers as well. Oxygen is directly tied to photosynthesis, and will most likely not be abundant in an exoplanet’s atmosphere unless it is regularly produced by algae or other plants.
“So if we find a good target planet, and you could detect these dimer molecules — which might be possible within the next 10 to 15 years — that would not only tell you something about pressure, but actually tell you that there’s life on that planet,” said Misra in a press release.
The paper has been published in the February issue of Astrobiology and is available for download here.
Host: Fraser Cain
Astrojournalists: Morgan Rehnberg, David Dickinson, Elizabeth Howell, Jason Major, Casey Dreier, Mike Simmons
Continue reading “Weekly Space Hangout – February 28, 2014: Good NASA News and 715 Confirmed Planets!”
A recent find announced by astronomers may go a long ways towards understanding a crucial “missing link” between planets and stars.
The team, led by Friemann Assistant Professor of Physics at the University of Notre Dame’s Justin R. Crepp, recently released an image of a brown dwarf companion to a star 98 light years or 30 parsecs distant. This discovery marks the first time that a T-dwarf orbiting a Sun-like star with known radial velocity acceleration measurement has been directly imaged.
Located in the constellation Eridanus, the object weighs in at about 52 Jupiter masses, and orbits a 0.95 Sol mass star 51 Astronomical Units (AUs) distant once every 320-1900 years. Note that this wide discrepancy stems from the fact that even though we’ve been following the object for some 17 years since 1996, we’ve yet to ascertain whether we’ve caught it near apastron or periastron yet: we just haven’t been watching it long enough.
The T-dwarf, known as HD 19467 B, may become a benchmark in the study of sub-stellar mass objects that span the often murky bridge between true stars shining via nuclear fusion and ordinary high mass planets.
Brown dwarfs are classified as spectral classes M, L, T, and Y and are generally quoted as having a mass of between 13 to 80 Jupiters. Brown dwarfs utilize a portion of the proton-proton chain fusion reaction to create energy, known as deuterium burning. Low mass red dwarf stars have a mass range of 80 to 628 Jupiters or 0.75% to 60% the mass of our Sun. The Sun has just over 1,000 times Jupiter’s mass.
Researchers used data from the TaRgeting bENchmark-objects with Doppler Spectroscopy (TRENDS) high-contrast imaging survey, and backed it up with more precise measurements courtesy of the Keck observatory’s High-Resolution Echelle Spectrometer or HIRES instrument.
TRENDS uses adaptive optics, which relies on precise flexing the telescope mirror several thousands of times a second to compensate for the blurring effects of the atmosphere. Brown dwarfs shine mainly in the infrared, and objects such as HD 19467 B are hard to discern due to their close proximity to their host star. In this particular instance, for example, HD 19467 B was over 10,000 times fainter than its primary star, and located only a little over an arc second away.
“This object is old and cold and will ultimately garner much attention as one of the most well-studied and scrutinized brown dwarfs detected to date,” Crepp said in a recent Keck observatory press release. “With continued follow-up observations, we can use it as a laboratory to test theoretical atmospheric models. Eventually we want to directly image and acquire the spectrum of Earth-like planets. Then, from the spectrum, we should be able to tell what the planet is made of, what its mass is, radius, age, etc… basically all of its relevant properties.
Discovery of an Earth-sized exoplanet orbiting in a star’s habitable zone is currently the “holy grail” of exoplanet science. Direct observation also allows us to pin down those key factors, as well as obtain a spectrum of an exoplanet, where detection techniques such as radial velocity analysis only allow us to peg an upper mass limit on the unseen companion object.
This also means that several exoplanet candidates in the current tally of 1074 known worlds beyond our solar system also push into the lower end of the mass limit for substellar objects, and may in fact be low mass brown dwarfs as well.
Another key player in the discovery was the Near-Infrared Camera (second generation) or NIRC2. This camera works in concert with the adaptive optics system on the Keck II telescope to achieve images in the near infrared with a better resolution than Hubble at optical wavelengths, perfect for brown dwarf hunting. NIRC2 is most well known for its analysis of stellar regions near the supermassive black hole at the core of our galaxy, and has obtained some outstanding images of objects in our solar system as well.
What is the significance of the find? Free floating “rogue” brown dwarfs have been directly imaged before, such as the pair named WISE J104915.57-531906 which are 6.5 light years distant and were spotted last year. A lone 6.5 Jupiter mass exoplanet PSO J318.5-22 was also found last year by the PanSTARRS survey searching for brown dwarfs.
“This is the first directly imaged T-dwarf (very cold brown dwarf) for which we have dynamical information independent of its brightness and spectrum,” team lead researcher Justin Crepp told Universe Today.
Analysis of brown dwarfs is significant to exoplanet science as well.
“They serve as an essential link between our understanding of stars and planets,” Mr. Crepp said. “The colder, the better.”
And just as there has been a controversy over the past decade concerning “planethood” at the low end of the mass scale, we could easily see the debate applied to the higher end range, as objects are discovered that blur the line… perhaps, by the 23rd century, we’ll finally have a Star Trek-esque classifications scheme in place so that we can make statements such as “Captain, we’ve entered orbit around an M-class planet…”
Something that’s always been fascinating in terms of red and brown dwarf stars is also the possibility that a solitary brown dwarf closer to our solar system than Alpha Centauri could have thus far escaped detection. And no, Nibiru conspiracy theorists need not apply. Mr. Crepp notes that while possible, such an object is unlikely to have escaped detection by infrared surveys such as WISE. But what a discovery that’d be!
In 2012 astronomers announced the discovery of an Earth-like planet circling our nearest neighbor, Alpha Centauri B, a mere 4.3 light-years away. But with such a discovery comes heated debate. A second group of astronomers was unable to confirm the exoplanet’s presence, keeping the argument unresolved to date.
But not to worry. One need only look 2.3 light-years further to see tantalizing — although yet unconfirmed — evidence of an exoplanet circling a pair of brown dwarfs: objects that aren’t massive enough to kick-off nuclear fusion in their cores. There just may be an exoplanet in the third closest system to our Sun.
Astronomers only discovered the system last year when the brown dwarfs were spotted in data from NASA’s Wide-field Infrared Explorer (WISE). Check out a past Universe Today article on the discovery here. They escaped detection for so long because they are located in the galactic plane, an area densely populated by stars, which are far brighter than the brown dwarfs.
Henri Boffin at the European Southern Observatory led a team of astronomers on a mission to learn more about these newly found dim neighbors. The group used ESO’s Very Large Telescope (VLT) at Paranal in Chile to perform astrometry, a technique used to measure the position of the objects precisely. This crucial data would allow them to make a better estimate of the distance to the objects as well as their orbital period.
Boffin’s team was first able to constrain their masses, finding that one brown dwarf weighs in at 30 times the mass of Jupiter and the other weighs in at 50 times the mass of Jupiter. These light-weight objects orbit each other slowly, taking about 20 years.
But their orbits didn’t map out perfectly — there were slight disturbances, suggesting that something was tugging on these two brown dwarfs. The likely culprit? An exoplanet — at three times the weight of Jupiter — orbiting one or even both of the objects.
“The fact that we potentially found a planetary-mass companion around such a very nearby and binary system was a surprise,” Boffin told Universe Today.
The next step will be to monitor the system closely in order to verify the existence of a planetary-mass companion. With a full year’s worth of data it will be relatively straightforward to remove the signal caused by the exoplanet.
So far only eight exoplanets have been discovered around brown dwarfs. If confirmed, this planet will be the first to be discovered using astrometry.
“Once the companion is confirmed, this will be an ideal target to image using the upcoming SPHERE instrument on the VLT,” Boffin said. This instrument will allow astronomers to directly image planets close to their host star — a difficult technique worth the challenge as it reveals a wealth of information about the planet.
Once confirmed, this planet will stand as the closest exoplanet to the Sun, until the debate regarding Alpha Centauri Bb is resolved.
The paper has been accepted for publication as an Astronomy & Astrophysics Letter and is available for download here. For more information on Alpha Centauri Bb please read a paper available here and published in the Astrophysical Journal.
So far, just a handful of planets have been found orbiting stars in star clusters – and actually, astronomers weren’t too surprised about that. Star clusters can be pretty harsh places with hordes of stars huddling close together, with strong radiation and harsh stellar winds stripping planet-forming materials from the region.
But it turns out that perhaps astronomers are beginning to think differently about star clusters as being a homey place for exoplanets.
Scientists using several different telescopes, including the HARPS planet hunter in Chile have now discovered three planets orbiting stars in the cluster Messier 67.
“These new results show that planets in open star clusters are about as common as they are around isolated stars — but they are not easy to detect,” said Luca Pasquini from ESO, who is a co-author of a new paper about these planets. “The new results are in contrast to earlier work that failed to find cluster planets, but agrees with some other more recent observations. We are continuing to observe this cluster to find how stars with and without planets differ in mass and chemical makeup.”
The astronomers are pretty excited about one of these planets in particular, as it orbits a star that is a rare solar twin — a star that is almost identical to our Sun in all respects. This is the first “solar twin” in a cluster that has been found to have a planet.
“In the Messier 67 star cluster the stars are all about the same age and composition as the Sun,” said Anna Brucalassi from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany and lead author of the new paper on these planets. “This makes it a perfect laboratory to study how many planets form in such a crowded environment, and whether they form mostly around more massive or less massive stars.”
This cluster lies about 2,500 light-years away in the constellation of Cancer and contains about 500 stars. Many of the cluster stars are fainter than those normally targeted for exoplanet searches and trying to detect the weak signal from possible planets pushed HARPS to the limit, the team said.
They carefully monitored 88 selected stars in Messier 67 over a period of six years to look for the tiny telltale “wobbling” motions of the stars that reveal the presence of orbiting planets.
Three planets were discovered, two orbiting stars similar to the Sun and one orbiting a more massive and evolved red giant star. Two of the three planets are “hot Jupiters” — planets comparable to Jupiter in size, but much closer to their parent stars and therefore not in the habitable zone where liquid water could exist.
The first two planets both have about one third the mass of Jupiter and orbit their host stars in seven and five days respectively. The third planet takes 122 days to orbit its host and is more massive than Jupiter.
Star clusters come in two main types: open and globular. Open clusters are groups of stars that have formed together from a single cloud of gas and dust in the recent past, and are mainly found in the spiral arms of a galaxy like the Milky Way. Globular clusters are much bigger spherical collections of much older stars that orbit around the center of a galaxy. Despite careful searches, no planets have been found in a globular cluster and less than six in open clusters.
Another study last year from a team using the Kepler telescope found two planets in a dense open star cluster and the team stated that how planets can form in the hostile environments of dense star clusters is “not well understood, either observationally or theoretically.”
Exoplanets have been found in some amazing environments, and astronomers will continue to hunt for planets in these clusters of stars to try and learn more about how and why — and how many — exoplanets exist in star clusters.
ESOcast 62: Three planets found in star cluster from ESO Observatory on Vimeo.
Source: ESO
If the dataset from the Kepler mission is any indication, the most common type of exoplanets in our galaxy aren’t Earth-sized rocky worlds or hot Jupiters. In fact, the most common type of exoplanet isn’t one that we see in our own neighborhood at all.
“Perhaps the most remarkable discovery by Kepler is the amount of planets between the size of Earth to four times the size of Earth,” said Geoff Marcy, professor of astronomy at University of California, speaking at the American Astronomical Society meeting this week in Washington D.C. “This is a size range that dominates the planet inventory from Kepler and it a size range not represented in our own Solar System. We don’t know for sure what these planets are made of and we don’t know how they form.”
These “mini-Neptunes” as Marcy called them, represent a huge sample in the Kepler data; about 75% of the planets found by Kepler vary in size between the Earth and Neptune, and for four years since the Kepler data have been rolling in, scientists have been trying to understand these planets.
“There’s been an enormous amount of measurements and quantitative work by the NASA Ames Kepler team,” Marcy said.
While masses and planet densities emerged from the work, astronomers still aren’t certain how they form or if they are made of rock, water or gas.
The team focused on about 42 of these planets. Two planets highlighted by Marcy in his presentation are thought to be rocky, and are named Kepler-99b and Kepler-406b. Both are forty percent larger in size than Earth and have a density similar to lead. The planets orbit their host stars in less than five and three days respectively, making these worlds too hot for life as we know it.
The team used Doppler measurements of the planets’ host stars to measure the reflex wobble of the host star, caused by the gravitational tug on the star exerted by the orbiting planet. The measured wobble reveals the mass of the planet: the higher the mass of the planet, the greater the gravitational tug on the star and hence the greater the wobble.
They also the measured transit timing variations (TTV) to determine how much neighboring planets can tug on one another causing one planet to accelerate and another planet to decelerate along its orbit.
These measurements allow for computing mass and densities of the planets, as well as figuring out the possible chemical composition of these worlds. The majority of the measurements suggest that the mini-Neptunes have a rocky core but some may have a gaseous outer shell of hydrogen or helium. Some might just be rocky with no outer envelope at all.
“What we think is happening is that some of these planets may have water on top of a rocky core,” Marcy said. “Larger planets might have the same rocky core with added gas. That’s how you get planets measuring from 1 to 4 earth radii. The planets with lower densities imply increasing amounts of gas on top of a rocky core.”
“Kepler’s primary objective is to determine the prevalence of planets of varying sizes and orbits. Of particular interest to the search for life is the prevalence of Earth-sized planets in the habitable zone,” said Natalie Batalha, Kepler mission scientist at NASA’s Ames Research Center. “But the question in the back of our minds is: are all planets the size of Earth rocky? Might some be scaled-down versions of icy Neptunes or steamy water worlds? What fraction are recognizable as kin of our rocky, terrestrial globe?”
The team said that the mass measurements produced by Doppler and TTV will help to answer these questions. The results hint that a large fraction of planets smaller than 1.5 times the radius of Earth may be comprised of the silicates, iron, nickel and magnesium that are found in the terrestrial planets here in the Solar System.
Armed with this type of information, scientists will be able to turn the fraction of stars harboring Earth-sizes planets into the fraction of stars harboring bona-fide rocky planets. And that’s a step closer to finding a habitable environment beyond the Solar System.
Marcy added later in the discussion that there’s one type of telescope that would most helpful: a Terrestrial Planet Finder type mission that would measure the temperature, size, and the orbital parameters of planets as small as our Earth in the habitable zones of distant solar systems. Alas, TPF was canceled.
Read more about the study of mini-Neptunes here.