The most common objection I hear about what we call the ‘habitable zone’ is that it specifies conditions only for life as we know it. It leaves out, for example, conceivable biospheres under the ice of gas giant moons, examples of which we possibly have here in the Solar System. But there is another issue with defining habitability in terms of atmospheric pressures that can support liquid water on the surface. As Jason Wright and Noah Tuchow (both at Penn State) point out in a recent paper, the classic habitable zone concept does not take the evolution of both planet and star into account. It’s a solid point. A planet now residing in the habitable zone could have remained habitable since the earliest era of its formation. Or it could have become habitable at a later time. Thus Tuchow and Wright make a distinction between what they refer to as the Continuous Habitable Zone (CHZ) and a class of planets they refer to as ‘belatedly habitable.’ These worlds may benefit from changes in the...

Looking at flare activity in young M-dwarf stars, as we did in the last post, brings out a notable difference between these fast-spinning stars and stars like the Sun. Across stellar classifications from M- to F-, G- and K-class stars, there is commonality in the fusion of hydrogen into helium in the stellar cores. But the Sun has a zone at which energy carried toward the surface as radiative photons is absorbed or scattered by dense matter. At this point, convection begins as colder matter moves downward and hot matter rises. This radiative zone giving way to convection is distinctive -- stars in the M-class range, a third of the mass of the Sun and lower, do not possess a radiative core, but undergo convection throughout their interior. Image: Interior structure of the Sun. Credit: kelvinsong / Wikimedia Commons CC BY-SA 3.0. If we're going to account for magnetic phenomena like starspots, flares and coronal mass ejections, we can come up with a model that fits stars with a...

We can learn a lot about stars by studying magnetic activity like starspots, flares and coronal mass ejections (CMEs). Starspots are particularly significant for scientists using radial velocity methods to detect planets, because they can sometimes mimic the signature of a planet in the data. But the astrobiology angle is also profound: Young M-dwarfs, known for flare activity, could be fatally compromised as hosts for life because strong flares can play havoc with planetary atmospheres. Given the ubiquity of M-dwarfs -- they’re the most common type of star in our galaxy -- we’d like to know whether or not they are candidates for supporting life. A paper from Ekaterina Ilin and team at the Leibniz Institute for Astrophysics in Potsdam digs into the question by looking at the orientation of magnetic activity on young M-dwarfs. The sample is small, though carefully chosen from the processing of over 3000 red dwarf signatures obtained by TESS, the Transiting Exoplanet Survey Satellite...

How planets grow in double-star systems has always held a particular fascination for me. The reason is probably obvious: In my younger days, when no exoplanets had been discovered, the question of what kind of planetary systems were possible around multiple stars was wide open. And there was Alpha Centauri in our southern skies, taunting us by its very presence. Could a life-laden planet be right next door? What Kedron Silsbee and Roman Rafikov have been working on extends well beyond Alpha Centauri, usefully enough, and helps us look into how binaries like Centauri A and B form planets. Says Rafikov (University of Cambridge), "A system like this would be the equivalent of a second Sun where Uranus is, which would have made our own solar system look very different." How true. In fact, imagining how different our system would work if we had a star among the outer planets raises wonderful questions. Could we have a habitable world around each star in such a binary? And if so, wouldn't...

The things we look for around other stars do not necessarily surprise us. I think most astronomers were thinking we'd find planets around a lot of stars when the Kepler mission began its work. The question was how many -- Kepler was to give us a statistical measurement on the planet population within its field of stars, and it succeeded brilliantly. These days it seems clear that we can find planets around most stars, in all kinds of sizes and orbits, as we continue to seek an Earth 2.0.. The continuing news about the star PDS 70, a young T Tauri star about 400 light years away in Centaurus, fits the same mold. Here we're talking not just about planets but their moons. No exomoons have been confirmed, but there seems no reason to assume we won't begin to find them -- surely the process of forming moons is as universal as that of planet formation. The interest is in the observation, how it is made, and what it implies about our ability to move forward in characterizing planetary...

The NEID spectrograph has passed the Operational Readiness Review necessary for final acceptance and regular operations. Developed by NASA and the National Science Foundation's NN-EXPLORE exoplanet science program, it has been put through a lengthy commissioning process in the five years since the radial velocity planet hunter design was selected. NEID is mounted on the WIYN 3.5m telescope at Kitt Peak National Observatory in Arizona, and we now have word that its scientific mission has begun. Image: Sunset over Kitt Peak National Observatory during NEID commissioning in January 2020. Credit: Paul Robertson. As a radial velocity instrument, NEID is all about the tugs one or more planets exert on the host star, as measured radially -- toward Earth, then away from it -- during the planets' orbits. The Doppler shift in the star's light contains the information. That these are exquisitely tiny measurements should be obvious. Jupiter induces a 13 meter per second wobble on our star, but...

300 light years from Earth in the constellation Musca, the gas giant TYC 8998-760-1 b, along with a companion planet, orbits an infant K-class star about 17 million years old. We're probably looking at a brown dwarf here rather than a gas giant like Jupiter, for TYC 8998-760-1 b is about 14 times Jupiter's mass, nudging into brown dwarf territory, and it appears to be roughly three times as large, unusual for brown dwarfs. The planet's separation from its host star is pegged at 160 AU. An inflated atmosphere due to processes still unknown? We don't know, but both this and the companion planet have been directly imaged. Now TYC 8998-760-1 b resurfaces through work with the European Southern Observatory's Very Large Telescope, as reported in the latest issue of Nature. Led by first author Yapeng Zhang (Leiden University, The Netherlands), the team of astronomers detected carbon isotopes in the object's atmosphere, showing higher than expected carbon-13 content. Here is the image, first...

Demonstrating once again the role amateurs can play in supporting ongoing observations, a new project linking NASA and the American Association of Variable Star Observers is being launched. Exoplanet Watch isn't about discovering new transiting planets (although the potential is there) as much as tightening up the information we already have about planets currently under investigation. The idea is to help professional observers know when to look, which allows them to maximize precious observing time on instruments that are always in high demand. Transit timing is the key, and the fact is that for many known exoplanets, knowing exactly when to look is problematic. Rob Zellem (JPL) is project lead for Exoplanet Watch: "If there's a 15-minute under-estimate of when a transit will occur, that's an extra 15 minutes I have to build into my observing scenario. Time on big telescopes, especially space telescopes, is very, very precious. If you're observing a lot of planets, [15 minutes]...

Many of the planet-hosting stars being identified by TESS, the Transiting Exoplanet Survey Satellite, may actually be binaries. Unless examined closely, a pair of stars can appear as a single object, requiring high resolution instrumentation to separate into its component parts. As it applies to exoplanet research, this is a problem, for TESS operates by the transit method, tracking the change in a star’s light curve as a planet crosses the face of the star. Light curves yield precious information, but the presence of a second star unknown to researchers can obscure smaller, rocky worlds, just the kind of object we’d like to eventually identify as an Earth 2.0. The problem seems to be wider than we have realized, given that about half of all stars exist in binary systems. New work has put some numbers on the problem. Conducted with data from the Gemini Observatory and the WIYN 3.5-meter telescope at Kitt Peak by NASA Ames researchers, the study examined TESS host stars using a...

Nu2 Lupi is a G-class star not all that far away in astronomical terms (48 light years) in the constellation Lupus, its proximity verified by parallax measurements and firmed up by the Hipparcos satellite. This is one of the closest G-class stars to our own, and it’s a fast mover in other ways, with a high radial velocity. Its age is estimated at roughly 12 billion years, making it one of the oldest stars near our system. HARPS spectrograph data pulled up three planets here in 2019, two of them later found to transit. And now we have, unexpectedly, a third transit. The surprising nature of the third relates to the distance of the third planet from the star. The two inner worlds, with masses between Earth’s and Neptune’s, take 12 and 28 days to orbit Nu2 Lupi. The third takes 107 days, far enough out that a transit seemed unlikely. The ratio of the diameter of the star to the diameter of the orbit comes into play in determining the probability of a transit. We have the European Space...

Call it the Earth Transit Zone, that region of space from which putative astronomers on an exoplanet could see the Earth transit the Sun. Lisa Kaltenegger (Cornell University) is director of the Carl Sagan Institute and the author of a 2020 paper with Joshua Pepper (LeHigh University) that examined the stars within the ETZ (see Seeing Earth as a Transiting World). While Kaltengger and Pepper identified 1004 main sequence stars within 100 parsecs that would see Earth as a transiting planet, Kaltenegger reminds us that stars are ever in motion. Given the abundant resources available in the European Space Agency's Gaia eDR3 catalog, why not work out positions and stellar motions to examine the question over time? After all, there are SETI implications here. We study planetary atmospheres using data taken during transits. Are we, in turn, the subject of such study from astronomers elsewhere in the cosmos? Thus Kaltenegger's new paper in Nature, written with Jackie Faherty (American...

The Allende meteorite is the largest carbonaceous chondrite meteorite ever discovered. Falling over Mexico's state of Chihuahua in 1969 and breaking up in the atmosphere, the object yielded over two tons of material that have provided fodder for scientists interested in the early days of the Solar System. The meteorite contains numerous calcium-aluminum-rich inclusions (CAIs), which are considered to be the first kind of solids formed in the system 4.5 billion years ago. Samples of the Allende meteorite are considered 'primitive,' which in this parlance means unaffected by significant alteration since formation. Now a team led by Tom Zega (University of Arizona Lunar and Planetary Laboratory) has gone to work on a dust grain from this object, in order to simulate the conditions under which it formed in the Sun's protoplanetary disk. The grain was drawn from one of several CAIs discovered in the Allende meteorite sample. Analysis of the sample's chemistry and crystal structure...

As we learn more about how planetary systems form, it's becoming accepted that a large number of planets are being ejected from young systems because of their interactions with more massive worlds. I always referred to these as 'rogue planets' in previous articles on the subject, but a new paper from Patricio Javier Ávila (University of Concepción, Chile) and colleagues makes it clear that the term Free Floating Planet (FFP) is now widespread. A new acronym for us to master! There have been searches to try to constrain the number of free floating planets, though the suggested ranges are wide. Microlensing seems the best technique, as it can spot masses we cannot otherwise see through their effect on background starlight. Of these, the estimates come in at around 2 Jupiter-mass planets and 2.5 terrestrial-class rocky worlds per star that have been flung into the darkness. This is a vast number of planets, but we have to be wary of mass uncertainties, as the cut-off between...

The beauty of nearby M-dwarf stars for exoplanet research is the depth of transits. If we are fortunate enough to find a planet crossing the face of the star as seen from our observatory, the star's small size means a larger portion of its light will be attenuated. As you would imagine, this makes planets easier to spot, but the other significant advantage is that we have greater capability at analyzing the planet's atmosphere. TOI-1231b certainly fits the bill, although it's a bit of an anomaly in the TESS universe. The space observatory operates with a built in observational bias because the Science Processing Operations Center (SPOC) pipeline and the Quick Look Pipeline (QLP) that comb through TESS data on a 2-minute and 30 minute cadence respectively have to show two transits for the planet's period to be determined. Factor in that most of the TESS sky coverage is observed for 28 days and you wind up in the majority of cases with detections of planets with orbital periods of less...

You can blame H. G. Wells' The Time Machine for my interest in the Earth's far future. That swollen red Sun at the end of the novel created vivid 'end of the world' scenarios for me as a boy, and later I would learn that outer planets or moons around a G-class star might turn habitable once it became a red giant. But it would only be in the last few years that I learned how robust the investigations into white dwarf systems -- the fate of a red giant -- have become, and now we're finding out not only that such stars can retain planets, but can conceivably create new ones through an emerging disk packed with the pulverized dust of remnant materials like asteroids. Image: This artist's concept shows a white dwarf debris disk. Credit: NASA/JPL-Caltech. Jordan Steckloff (Planetary Science Institute, Tucson) has just published a short paper on the matter, looking at how white dwarf debris disks emerge. The disks seem to form only after ten to twenty million years following the end of the...

Back in the 1990s, when the first exoplanet detections were made, the best possible targets for radial velocity searches were what we now call 'hot Jupiters.' Radial velocity looks at the Doppler shift of light as a star moves first towards us, then away, tugged by the invisible planet. A massive Jupiter in a tight orbit tugged maximally, and quite often, because its orbit could be measured in mere days or weeks. It was purely selection effect, but it seemed that such planets were common, until we began to discover just how many other kinds of worlds were out there. Outer-system Jupiters like ours are a different problem. A gas giant in a multi-year orbit produces a radial velocity signature that is far smaller and dependent upon long analysis. Thus, early numbers on the existence of gas giants in the Jupiter or Saturn class and similarly far from their host star are just beginning to emerge as exoplanet science matures. We'll be learning more -- a lot more -- but tentative findings...

Laboratory work on Earth is, as we saw yesterday, leading to hypotheses about how planets form and the effect of these processes on subsequent life. Whether in our own outer Solar System or orbiting other stars, planets in the 'ice giant' category, like Uranus and Neptune, remain mysterious, with Voyager 2's flybys of the latter the only missions that have gone near them. We also know that sub-Neptune planets are common, many of these doubtless sharing the characteristics of their larger namesake. Thus recent experiments probing ice giant interiors catch my eye this morning. Involving an international team of collaborators, the work looks at the interactions between water and rock that we would expect to find in the extreme conditions inside an ice giant. Planets like Uranus and Neptune are thought to house most of their mass in a deep water layer, a dense fluid overlaying a rocky core, a sharp departure from terrestrial worlds. What happens at that interface is ripe for examination....

While a planet's position in the habitable zone is thought critical for the development of life like ourselves, new work out of Rice University suggests an equally significant factor in planetary growth. Working at a high-pressure laboratory at the university, Damanveer Grewal and Rajdeep Dasgupta have explored how planets capture and retain key volatiles like nitrogen, carbon and water as they form The team used nitrogen as a proxy for volatile distribution in a range of simulated protoplanets. Two processes are under study here, the first being the accretion of material in the circumstellar disk into a protoplanet, and the rate at which it proceeds. The second is differentiation, as the protoplanet separates into layers ranging from a metallic core to a silicate shell and, finally, an atmospheric envelope. The interplay between these processes is found to determine which volatiles the subsequent planet retains. Most of the nitrogen is found to escape into the atmosphere during...

Is there any chance we may one day find technosignatures around the nearest stars? If we were to detect such, on a planet, say, orbiting Alpha Centauri B, that would seem to indicate that civilizations are to be found around a high percentage of G- and K-class stars. Brian Lacki (UC-Berkeley) examined the question from all angles at the recent Breakthrough Discuss, raising some interesting issues about the implications of technosignatures, and the assumptions we bring to the search for them. We’re starting to consider a wide range of technosignatures rather than just focusing on Dysonian shells around entire stars. Other kinds of megastructure are possible, some perhaps so exotic we wouldn’t be sure how they operated or what they were for. Atmospheres could throw technosignatures by revealing industrial activity along with their potential biosignatures. We could conceivably detect power beaming directed at interstellar spacecraft or even an infrastructure within a particular stellar...

In the ranks of exoplanets we can actually see, we can include the gas giant PDS 70b, a young world orbiting the K-dwarf PDS 70. Bear in mind that of the more than 4,000 exoplanets thus far catalogued, only about 15 have been directly imaged, an indication of how tricky this work is and how far we have to go as we contemplate imaging Earth-size planets and taking spectroscopic measurements of their atmospheres. The most recent PDS 70b work was performed with the Hubble instrument, and is to my knowledge the first direct detection of an exoplanet in the ultraviolet. About 370 light years from Earth, PDS 70 (also known as V1032 Centauri) is a T Tauri star, a class of variables less than 10 million years old; this one appears to be no more than 5 million years old, and its largest planet is still in the process of building mass. The star is known to host at least two actively forming planets within its circumstellar disk of gas and dust, although only the larger is apparent in these UV...