Red Dwarf Stars Archives

October 25, 2017October 25, 2017 by Matt Williams

Water Worlds Don’t Stay Wet for Very Long

Artist's depiction of a waterworld. A new study suggests that Earth is in a minority when it comes to planets, and that most habitable planets may be greater than 90% ocean. Credit: David A. Aguilar (CfA)

When hunting for potentially habitable exoplanets, one of the most important things astronomers look for is whether or not exoplanet candidates orbit within their star’s habitable zone. This is necessary for liquid water to exist on a planet’s surface, which in turn is a prerequisite for life as we know it. However, in the course of discovering new exoplanets, scientists have become aware of an extreme case known as “water worlds“.

Water worlds are essentially planets that are up to 50% water in mass, resulting in surface oceans that could be hundreds of kilometers deep. According to a new study by a team of astrophysicists from Princeton, the University of Michigan and Harvard, water worlds may not be able to hang on to their water for very long. These findings could be of immense significance when it comes to the hunt for habitable planets in our neck of the cosmos.

This most recent study, titled “The Dehydration of Water Worlds via Atmospheric Losses“, recently appeared in The Astrophysical Journal Letters. Led by Chuanfei Dong from the Department of Astrophysical Sciences at Princeton University, the team conducted computer simulations that took into account what kind of conditions water worlds would be subject to.

This study was motivated largely by the number of exoplanet discoveries have been made around low-mass, M-type (red dwarf) star systems in recent years. These planets have been found to be comparable in size to Earth – which indicated that they were likely terrestrial (i.e. rocky). In addition, many of these planets – such as Proxima b and three planets within the TRAPPIST-1 system – were found to be orbiting within the stars habitable zones.

However, subsequent studies indicated that Proxima b and other rocky planets orbiting red dwarf stars could in fact be water worlds. This was based on mass estimates obtained by astronomical surveys, and the built-in assumptions that such planets were rocky in nature and did not have massive atmospheres. At the same time, numerous studies have been produced that have cast doubt on whether or not these planets would be able to hold onto their water.

Basically, it all comes down to the type of star and the orbital parameters of the planets. While long-lived, red dwarf stars are known for being variable and unstable compared to our Sun, which results in periodic flares up that would strip a planet’s atmosphere over time. On top of that, planets orbiting within a red dwarf’s habitable zone would likely be tidally-locked, meaning one side of the planet would be constantly exposed to the star’s radiation.

Because of this, scientists are focused on determining just how well exoplanets in different types of star systems could hold onto their atmospheres. As Dr. Dong told Universe Today via email:

“It is fair to say that the presence of an atmosphere is perceived as one of the requirements for the habitability of a planet. Having said that, the concept of habitability is a complex one with myriad factors involved. Thus, an atmosphere by itself will not suffice to guarantee habitability, but it can be regarded as an important ingredient for a planet to be habitable.”

*Illustration showing the possible surface of TRAPPIST-1f, one of the newly discovered planets in the TRAPPIST-1 system. Credits: NASA/JPL-Caltech*

To test whether or not a water world would be able to hold onto its atmosphere, the team conducted computer simulations that took into account a variety of possible scenarios. These included the effects of stellar magnetic fields, coronal mass ejections, and atmospheric ionization and ejection for various types of stars – including G-type stars (like our Sun) and M-type stars (like Proxima Centauri and TRAPPIST-1).

With these effects accounted for, Dr. Dong and his colleagues derived a comprehensive model that simulated how long exoplanet atmospheres would last. As he explained it:

“We developed a new multi-fluid magnetohydrodynamic model. The model simulated both the ionosphere and magnetosphere as a whole. Due to the existence of the dipole magnetic field, the stellar wind cannot sweep away the atmosphere directly (like Mars due to the absence of a global dipole magnetic field), instead, the atmospheric ion loss was caused by the polar wind.

“The electrons are less massive than their parent ions, and as a result, are more easily accelerated up to and beyond the escape velocity of the planet. This charge separation between the escaping, low-mass electrons and significantly heavier, positively-charged ions sets up a polarization electric field. That electric field, in turn, acts to pull the positively charged ions along behind the escaping electrons, out of the atmosphere in the polar caps.”

Artist’s impression of the view from the most distant exoplanet discovered around the red dwarf star TRAPPIST-1. Credit: ESO/M. Kornmesser.

What they found was that their computer simulations were consistent with the current Earth-Sun system. However, in some extreme possibilities – such as exoplanets around M-type stars – the situation is very different and the escape rates could be one thousand times greater or more. The result means that even a water world, if it orbits an red dwarf star, could lose its atmosphere after about a gigayear (Gyr), one billion years.

Considering that life as we know it took around 4.5 billion years to evolve, one billion years is a relatively brief window. In fact, as Dr. Dong explained, these results indicate that planets that orbit M-type stars would be hard pressed to develop life:

“Our results indicate that the ocean planets (orbiting a Sun-like star) will retain their atmospheres much longer than the Gyr timescale as the ion escape rates are far too low, therefore, it allows a longer duration for life to originate on these planets and evolve in terms of complexity. In contrast, for exoplanets orbiting M-dwarfs, they could have their oceans depleted over the Gyr timescale due to the more intense particle and radiation environments that exoplanets experience in close-in habitable zones. If the atmosphere were to be depleted over the timescale less than Gyr, this could prove to be problematic for the origin of life (abiogenesis) on the planet.”

Once again, these results cast doubt on the potential habitability of red dwarf star systems. In the past, researchers have indicated that the longevity of red dwarf stars, which can remain in their main sequence for up to 10 trillion years or longer, make them the best candidate for finding habitable exoplanets. However, the stability of these stars and the way in which they are likely to strip planets of their atmospheres seems to indicate otherwise.

*An artist’s depiction of planets transiting a red dwarf star in the TRAPPIST-1 System. Credit: NASA/ESA/STScl*

Studies such as this one are therefore highly significant in that they help to address just how long a potentially habitable planet around a red dwarf star could remain potentially habitable. As Dr. Dong indicated:

“Given the importance of atmospheric loss on planetary habitability, there has been a great deal of interest in using telescopes such as the upcoming James Webb Space Telescope (JWST) to determine whether these planets have atmospheres and, if so, what their composition are like. It is expected that the JWST should be capable of characterizing these atmospheres (if present), but quantifying the escape rates accurately requires a much higher degree of precision and may not be feasible in the near-future.”

The study is also significant as far as our understanding of the Solar System and its evolution is concerned. At one time, scientists have ventured that both Earth and Venus may have been water worlds. How they made the transition from being very watery to what they are today – in the case of Venus, dry and hellish; and in the case of Earth, having multiple continents – is an all-important question.

In the future, more detailed surveys are anticipated that could help shed light on these competing theories. When the James Webb Space Telescope (JWST) is deployed in Spring of 2018, it will use its powerful infrared capabilities to study planets around nearby red dwarfs, Proxima b being one of them. What we learn about this and other distant exoplanets will go a long way towards informing our understanding of how our own Solar System evolved as well.

Further Reading: CfA, The Astrophysical Journal Letters

September 8, 2017June 20, 2020 by Matt Williams

New Study Claims that TRAPPIST-1 Could Also Have Gas Giants

Most exoplanets orbit red dwarf stars because they're the most plentiful stars. This is an artist's illustration of what the TRAPPIST-1 system might look like from a vantage point near planet TRAPPIST-1f (at right). Credits: NASA/JPL-Caltech

In February of 2017, NASA scientists announced the existence of seven terrestrial (i.e. rocky) planets within the TRAPPIST-1 star system. Since that time, the system has been the focal point of intense research to determine whether or not any of these planets could be habitable. At the same time, astronomers have been wondering if all of the system’s planets are actually accounted for.

For instance, could this system have gas giants lurking in its outer reaches, as many other systems with rocky planets (for instance, ours) do? That was the question that a team of scientists, led by researchers from the Carnegie Institute of Science, sought to address in a recent study. According to their findings, TRAPPIST-1 may be orbited by gas giants at a much-greater distance than its seven rocky planets.

Continue reading “New Study Claims that TRAPPIST-1 Could Also Have Gas Giants”

September 1, 2017September 9, 2017 by Matt Williams

Ultraviolet Light Could Point the Way To Life Throughout the Universe

Artist's impression of how the surface of a planet orbiting a red dwarf star may appear. The planet is in the habitable zone so liquid water exists. However, low levels of ultraviolet radiation from the star have prevented or severely impeded chemical processes thought to be required for life to emerge. This causes the planet to be devoid of life. Credit: M. Weiss/CfA

Ultraviolet light is what you might call a controversial type of radiation. On the one hand, overexposure can lead to sunburn, an increased risk of skin cancer, and damage to a person’s eyesight and immune system. On the other hand, it also has some tremendous health benefits, which includes promoting stress relief and stimulating the body’s natural production of vitamin D, seratonin, and melanin.

And according to a new study from a team from Harvard University and the Harvard-Smithsonian Center for Astrophysics (CfA), ultraviolet radiation may even have played a critical role in the emergence of life here on Earth. As such, determining how much UV radiation is produced by other types of stars could be one of the keys to finding evidence of life any planets that orbit them.

The study, titled “The Surface UV Environment on Planets Orbiting M Dwarfs: Implications for Prebiotic Chemistry and the Need for Experimental Follow-up“, recently appeared in The Astrophysical Journal. Led by Sukrit Ranjan, a visiting postdoctoral researcher at the CfA, the team focused on M-type (red dwarf) stars to determine if this class of star produces enough UV radiation to kick-start the biological processes necessary for life to emerge.

Artist’s impression of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri. The double star Alpha Centauri AB is visible to the upper right of Proxima itself. Credit: ESO

Recent studies have indicated that UV radiation may be necessary for the formation of ribonucleic acid (RNA), which is necessary for all forms of life as we know it. And given the rate at which rocky planets have been discovered around red dwarf stars of late (exampled include Proxima b, LHS 1140b, and the seven planets of the TRAPPIST-1 system), how much UV radiation red dwarfs give off could be central to determining exoplanet habitability.

As Dr. Ranjan explained in a CfA press release:

“It would be like having a pile of wood and kindling and wanting to light a fire, but not having a match. Our research shows that the right amount of UV light might be one of the matches that gets life as we know it to ignite.”

For the sake of their study, the team created radiative transfer models of red dwarf stars. They then sought to determine if the UV environment on prebiotic Earth-analog planets which orbited them would be sufficient to stimulate the photoprocesses that would lead to the formation of RNA. From this, they calculated that planets orbiting M-dwarf stars would have access to 100–1000 times less bioactive UV radiation than a young Earth.

As a result, the chemistry that depends on UV light to turn chemical elements and prebiotic conditions into biological organisms would likely shut down. Alternately, the team estimated that even if this chemistry was able to proceed under a diminished level of UV radiation, it would operate at a much slower rate than it did on Earth billions of years ago.

As Robin Wordsworth – an assistant professor at the Harvard School of Engineering and Applied Science and a co-author on the study – explained, this is not necessarily bad news as far as questions of habitability go. “It may be a matter of finding the sweet spot,” he said. “There needs to be enough ultraviolet light to trigger the formation of life, but not so much that it erodes and removes the planet’s atmosphere.”

Previous studies have shown that even calm red dwarfs experience dramatic flares that periodically bombard their planets with bursts UV energy. While this was considered to be something hazardous, which could strip orbiting planets of their atmospheres and irradiate life, it is possible that such flares could compensate for the lower levels of UV being steadily produced by the star.

This news also comes on the heels of a study that indicated how the outer planets of the TRAPPIST-1 system (including the three located within its habitable zone) might still have plenty of water of their surfaces. Here too, the key was UV radiation, where the team responsible for the study monitored the TRAPPIST-1 planets for signs of hydrogen loss from their atmospheres (a sign of photodissociation).

This research also calls to mind a recent study led by Professor Avi Loeb, the Chair of the astronomy department at Harvard University, Director of the Institute for Theory and Computation, and also a member of the CfA. Titled, “Relative Likelihood for Life as a Function of Cosmic Time“, Loeb and his team concluded that red dwarf stars are the most likely to give rise to life because of their low mass and extreme longevity.

*Artist’s impression of a sunset seen from the surface of an Earth-like exoplanet. Credit: ESO/L. Calçada*

Compared to higher-mass stars that have shorter life spans, red dwarf stars are likely to remain in their main sequence for as long as six to twelve trillion years. Hence, red dwarf stars would certainly be around long enough to accommodate even a vastly decelerated rate of organic evolution. In this respect, this latest study might even be considered a possible resolution for the Fermi Paradox – Where are all the aliens? They’re still evolving!

But as Dimitar Sasselov – the Phillips Professor of Astronomy at Harvard, the Director of the Origins of Life Initiative and a co-author on the paper – indicated, there are still many unanswered questions:

“We still have a lot of work to do in the laboratory and elsewhere to determine how factors, including UV, play into the question of life. Also, we need to determine whether life can form at much lower UV levels than we experience here on Earth.”

As always, scientists are forced to work with a limited frame of reference when it comes to assessing the habitability of other planets. To our knowledge, life exists on only on planet (i.e. Earth), which naturally influences our understanding of where and under what conditions life can thrive. And despite ongoing research, the question of how life emerged on Earth is still something of a mystery.

If life should be found on a planet orbiting a red dwarf, or in extreme environments we thought were uninhabitable, it would suggest that life can emerge and evolve in conditions that are very different from those of Earth. In the coming years, next-generation missions like the James Webb Space Telescope are the Giant Magellan Telescope are expected to reveal more about distant stars and their systems of planets.

The payoff of this research is likely to include new insights into where life can emerge and the conditions under which it can thrive.

Further Reading: CfA, The Astrophysical Journal

August 15, 2017 by Matt Williams

Potentially Habitable, Tidally-Locked Exoplanets May be Very Common, say New Study

Artist's impression of a system of exoplanets orbiting a low mass, red dwarf star. Credit: NASA/JPL

Studies of low-mass, ultra-cool and ultra-dim red dwarf stars have turned up a wealth of extra-solar planets lately. These include the discoveries of a rocky planet orbiting the closest star to the Solar System (Proxima b) and a seven-planet system just 40 light years away (TRAPPIST-1). In the past few years, astronomers have also detected candidates orbiting the stars Gliese 581, Innes Star, Kepler 42, Gliese 832, Gliese 667, Gliese 3293, and others.

The majority of these planets have been terrestrial (i.e. rocky) in nature, and many were found to orbit within their star’s habitable zone (aka. “goldilocks zone”). However the question whether or not these planets are tidally-locked, where one face is constantly facing towards their star has been an ongoing one. And according to a new study from the University of Washington, tidally-locked planets may be more common than previously thought.

The study – which is available online under the title “Tidal Locking of Habitable Exoplanets” – was led by Rory Barnes, an assistant professor of astronomy and astrobiology at the University of Washington. Also a theorist with the Virtual Planetary Laboratory, his research is focused on the formation and evolution of planets that orbit in and around the “habitable zones” of low-mass stars.

*Tidal locking results in the Moon rotating about its axis in about the same time it takes to orbit the Earth (left side). Credit: Wikipedia*

For modern astronomers, tidal-locking is a well-understood phenomena. It occurs as a result of their being no net transfer of angular momentum between an astronomical body and the body it orbits. In other words, the orbiting body’s orbital period matches its rotational period, ensuring that the same side of this body is always facing towards the planet or star it orbits.

Consider Earth’s only satellite – the Moon. In addition to taking 27.32 days to orbit Earth, the Moon also takes 27.32 days to rotate once on its axis. This is why the Moon always presents the same “face” towards Earth, while the side that faces away is known as the “dark side”. Astronomers believe this became the case after a Mars-sized object (Theia) collided with Earth some 4.5 billion years ago.

Aside from throwing up debris that would eventually form the Moon, the impact is believed to have struck Earth at such an angle that it gave our planet an initial rotation period of 12 hours. In the past, researchers have used this 12-hour estimation of Earth’s rotation as a model for exoplanet behavior. However, prior to Barnes’ study, no systematic examinations had ever been conducted.

Looking to address this, Barnes chose to address the long-held assumption that only smaller, dimmer stars could host orbiting planets that were tidally locked. He also considered other possibilities, which included slower or faster initial rotation periods as well as variations in planet size and the eccentricity of their orbits. What he found was that previous studies had been rather limited and only made allowances for one outcome.

As he explained in a University of Washington press statement:

“Planetary formation models, however, suggest the initial rotation of a planet could be much larger than several hours, perhaps even several weeks. And so when you explore that range, what you find is that there’s a possibility for a lot more exoplanets to be tidally locked. For example, if Earth formed with no Moon and with an initial ‘day’ that was four days long, one model predicts Earth would be tidally locked to the sun by now.”

From this, he found that potentially-habitable planets that orbit very late M-type (red dwarf) stars are likely to attain highly-circular orbits about 1 billion years after their formation. Furthermore, he found that for the majority, their orbits would be synchronized with their rotation – aka. they would be tidally-locked with their star. These findings could have significant implications for the study of exoplanets formation and evolution, not to mention habitability.

In the past, tidally-locked planets were thought to have extremes climates, thus eliminating any possibility of life. As an example, the planet Mercury experiences a 3:2 spin-orbit resonance, meaning it rotates three times on its axis for every two orbits it completes of the Sun. Because of this, a single day on Mercury lasts as long as 176 Earth days, and temperature range from 100 (-173 °C; -279 °F) to 700 K (427 °C; 800 °F) between the day side and the night side.

For a tidally-locked planets that orbit close to their stars, it was believed this situation would be even worse. However, astronomers have since come to speculate that the presence of an atmosphere around these planets could redistribute temperature across their surfaces. Unlike Mercury, which has no atmosphere and experiences no wind, these planets could maintain temperatures that would be supportive to life.

In any case, this study is one of many that is putting constraints on recent exoplanet discoveries. This is especially important given that the detection and study of extra-solar planets is still in its infancy, and limited to largely indirect methods. In other words, astronomers make estimates of a planet’s size, composition and whether or not it has an atmosphere based on transits and the influence these planets have on their stars.

In the coming years, next-generations missions like the James Web Space Telescope and the Transiting Exoplanet Survey Satellites (TESS) are expected to improve this situation drastically. In addition to conducting more detailed observations on existing discoveries, they are also expected to uncover a wealth of more planets. If Barnes’ study is correct, the majority of those found will be tidally-locked, but that need not mean they are uninhabitable.

Prof. Barnes paper was accepted for publication by the journal Celestial Mechanics and Dynamical Astronomy. The research was funded by a NASA grant through the Virtual Planetary Laboratory.

Further Reading: University of Washington, arXiv

August 4, 2017February 7, 2021 by Matt Williams

Bad News For Proxima b: An Earth-Like Atmosphere Might Not Survive There

Artist’s impression of Proxima b, which was discovered using the Radial Velocity method. Credit: ESO/M. Kornmesser

Back in of August of 2016, the existence of an Earth-like planet right next door to our Solar System was confirmed. To make matters even more exciting, it was confirmed that this planet orbits within its star’s habitable zone too. Since that time, astronomers and exoplanet-hunters have been busy trying to determine all they can about this rocky planet, known as Proxima b. Foremost on everyone’s mind has been just how likely it is to be habitable.

However, numerous studies have emerged since that time that indicate that Proxima b, given the fact that it orbits an M-type (red dwarf), would have a hard time supporting life. This was certainly the conclusion reached in a new study led by researchers from NASA’s Goddard Space Flight Center. As they showed, a planet like Proxima b would not be able to retain an Earth-like atmosphere for very long.

Red dwarf stars are the most common in the Universe, accounting for an estimated 70% of stars in our galaxy alone. As such, astronomers are naturally interested in knowing just how likely they are at supporting habitable planets. And given the distance between our Solar System and Proxima Centauri – 4.246 light years – Proxima b is considered ideal for studying the habitability of red dwarf star systems.

*This infographic compares the orbit of the planet around Proxima Centauri (Proxima b) with the same region of the Solar System. Credit: Pale Red Dot*

On top of all that, the fact that Proxima b is believed to be similar in size and composition to Earth makes it an especially appealing target for research. The study was led by Dr. Katherine Garcia-Sage of NASA’s Goddard Space Flight Center and the Catholic University of America in Washington, DC. As she told Universe Today via email:

“So far, not many Earth-sized exoplanets have been found orbiting in the temperate zone of their star. That doesn’t mean they don’t exist – larger planets are found more often because they are easier to detect – but Proxima b is of interest because it’s not only Earth-sized and at the right distance from its star, but it’s also orbiting the closest star to our Solar System.”

For the sake of determining if Proxima b could be habitable, the research team sought to address the chief concerns facing rocky planets that orbit red dwarf stars. These include the planet’s distance from its stars, the variability of red dwarfs, and the presence (or absence) of magnetic fields. Distance is of particular importance since habitable zones (aka. temperate zones) around red dwarfs are much closer and tighter.

“Red dwarfs are cooler than our own Sun, so the temperate zone is closer to the star than Earth is to the Sun,” said Dr. Garcia-Sage. “But these stars may be very magnetically active, and being so close to a magnetically active star means that these planets are in a very different space environment than what the Earth experiences. At those distances from the star, the ultraviolet and x-ray radiation may be quite large. The stellar wind may be stronger. There could be stellar flares and energetic particles from the star that ionize and heat the upper atmosphere.”

*At one time, Mars had a magnetic field similar to Earth, which prevented its atmosphere from being stripped away. Credit: NASA*

In addition, red dwarf stars are known for being unstable and variable in nature when compared to our Sun. As such, planets orbiting in close proximity would have to contend with flare ups and intense solar wind, which could gradually strip away their atmospheres. This raises another important aspect of exoplanet habitability research, which is the presence of magnetic fields.

To put it simply, Earth’s atmosphere is protected by a magnetic field that is driven by a dynamo effect in its outer core. This “magnetosphere” has prevented solar wind from stripping our atmosphere away, thus giving life a chance to emerge and evolve. In contrast, Mars lost its magnetosphere roughly 4.2 billion years ago, which led to its atmosphere being depleted and its surface becoming the cold, desiccated place it is today.

To test Proxima b’s potential habitability and capacity to retain liquid surface water, the team therefore assumed the presence of an Earth-like atmosphere and a magnetic field around the planet. They then accounted for the enhanced radiation coming from Proxima b. This was provided by the Harvard Smithsonian Center for Astrophysics (CfA), where researchers determined the ultraviolet and x-ray spectrum of Proxima Centauri for this project.

From all of this, they constructed models that began to calculate the rate of atmospheric loss, using Earth’s atmosphere as a template. As Dr. Garcia-Sage explained:

“At Earth, the upper atmosphere is ionized and heated by ultraviolet and x-ray radiation from the Sun. Some of these ions and electrons escape from the upper atmosphere at the north and south poles. We have a model that calculates how fast the upper atmosphere is lost through these processes (it’s not very fast at Earth)… We then used that radiation as the input for our model and calculated a range of possible escape rates for Proxima Centauri b, based on varying levels of magnetic activity.”

What they found was not very encouraging. In essence, Proxima b would not be able to retain an Earth-like atmosphere when subjected to Proxima Centauri’s intense radiation, even with the presence of a magnetic field. This means that unless Proxima b has had a very different kind of atmospheric history than Earth, it is most likely a lifeless ball of rock.

However, as Dr. Garcia-Sage put it, there are other factors to consider which their study simply can’t account for:

“We found that atmospheric losses are much stronger than they are at Earth, and the for high levels of magnetic activity that we expect at Proxima b, the escape rate was fast enough that an entire Earth-like atmosphere could be lost to space. That doesn’t take into account other things like volcanic activity or impacts with comets that might be able to replenish the atmosphere, but it does mean that when we’re trying to understand what processes shaped the atmosphere of Proxima b, we have to take into account the magnetic activity of the star. And understanding the atmosphere is an important part of understanding whether liquid water could exist on the surface of the planet and whether life could have evolved.”

So it’s not all bad news, but it doesn’t inspire a lot of confidence either. Unless Proxima b is a volcanically-active planet and subject to a lot of cometary impacts, it is not likely be temperate, water-bearing world. Most likely, its climate will be analogous to Mars – cold, dry, and with water existing mostly in the form of ice. And as for indigenous life emerging there, that’s not too likely either.

These and other recent studies have painted a rather bleak picture about the habitability of red dwarf star systems. Given that these are the most common types of stars in the known Universe, the statistical likelihood of finding a habitable planet beyond our Solar System appears to be dropping. Not exactly good news at all for those hoping that life will be found out there within their lifetimes!

But it is important to remember that what we can say definitely at this point about extra-solar planets is limited. In the coming years and decades, next-generation missions – like the James Webb Space Telescope (JWST) and the Transiting Exoplanet Survey Satellite (TESS) – are sure to paint a more detailed picture. In the meantime, there’s still plenty of stars in the Universe, even if most of them are extremely far away!

Further Reading: The Astrophysical Journal Letters

July 21, 2017 by Matt Williams

Earth-Sized Planet Takes Just Four Hours to Orbit its Star

Using data obtained by Kepler and numerous observatories around the world, an international team has found a Super-Earth that orbits its orange dwarf star in just 14 hours. Credit: M. Weiss/CfA

The Kepler space observatory has made some interesting finds since it began its mission back in March of 2009. Even after the mission suffered the loss of two reaction wheels, it has continued to make discoveries as part of its K2 mission. All told, the Kepler and K2 missions have detected a total of 5,106 planetary candidates, and confirmed the existence of 2,493 planets.

One of the latest finds made using Kepler is EPIC 228813918 b, a terrestrial (i.e. rocky) planet that orbits a red dwarf star some 264 to 355 light years from Earth. This discovery raises some interesting questions, as it is the second time that a planet with an ultra-short orbital period – it completes a single orbit in just 4 hours and 20 minutes – has been found orbiting a red dwarf star.

The study, which was recently published online, was conducted by an international team of scientists who hail from institutions ranging from the Massachusetts Institute of Technology (MIT), the California Institute of Technology (Caltech), the Tokyo Institute of Technology, and the Institute of Astrophysics of the Canary Islands (IAC) to observatories and universities from all around the world.

*NASA’s Kepler space telescope was the first agency mission capable of detecting Earth-size planets. Credit: NASA/Wendy Stenzel*

As the team indicated in their study, the detection of this exoplanet was made thanks to data collected by numerous instruments. This included spectrographic data from the 8.2-m Subaru telescope and the 10-m Keck I telescope (both of which are located on Mauna Kea, Hawaii) and the Nordic Optical Telescope (NOT) at the Roque de los Muchachos Observatory in La Palma, Spain.

This was combined with speckle imaging from the 3.5-m WIYN telescope at the Kitt Peak National Observatory in Arizona, photometry from the NASA’s K2 mission, and archival information of the star that goes back over 60 years. After eliminating any other possible explanations – such as an eclipsing binary (EB) – they not only confirmed the orbital period of the planet, but also provided constrains on its mass and size. As they wrote:

“Using a combination of archival images, AO imaging, RV measurements, and light curve modelling, we show that no plausible eclipsing binary scenario can explain the K2 light curve, and thus confirm the planetary nature of the system. The planet, whose radius we determine to be 0.89 ± 0.09 [Earth radii], and which must have a iron mass fraction greater than 0.45, orbits a star of mass 0.463 ± 0.052 M and radius 0.442 ± 0.044 R.”

This orbital period – four hours and 20 minutes – is the second shortest of any exoplanet discovered to date, being just 4 minutes longer than that of KOI 1843.03, which also orbits an M-type (red dwarf) star. It is also the latest in a long line of recently-discovered exoplanets that complete a single orbit of their stars in less than a day. Planets belonging to this group are known as ultra-short-period (USP) planets, of which Kepler has found a total of 106.

*Archival images of the star EPIC 228813918, demonstrating its proper motion over nearly six decades – from (i) 1954, (ii) 1992, and (iii) 2012. Credit: Smith et al.*

However, what is perhaps most surprising about this find is just how massive it is. Though they didn’t measure the planet’s mass directly, their constraints indicate that the exoplanet has an upper mass limit of 0.7 Jupiter masses – which works out to over 222 Earth masses. And yet, the planet manages to pack this gas giant-like mass into a radius that is 0.80 to 0.98 times that of Earth.

The reason for this, they indicate, has to do with the planet’s apparent composition, which is particularly metal-rich:

“This leads to a constraint on the composition, assuming an iron core and a silicate mantle. We determine the minimum iron mass fraction to be 0.525 ± 0.075 (cf. 0.7 for KOI 1843.03), which is greater than that of Earth, Venus or Mars, but smaller than that of Mercury (approximately 0.38, 0.35, 0.26, and 0.68, respectively; Reynolds & Summers 1969).”

Ultimately, the discovery of this planet is significant for a number of reasons. On the one hand, the team indicated that the constraints their study placed on the planet’s composition could prove useful in helping to understand how our own Solar planets came to be.

“Discovering and characterizing extreme systems, such as USP planets like EPIC 228813918 b, is important as they offer constraints for planet formation theories,” they conclude. “Furthermore, they allow us to begin to constrain their interior structure – and potentially that of longer-period planets too, if they are shown to be a single population of objects.”

*An artist’s depiction of extra-solar planets transiting an M-type (red dwarf) star. Credit: NASA/ESA/STScl*

On the other hand, the study raises some interesting questions about USP planets – for instance, why the two shortest-period planets were both found orbiting red dwarf stars. A possible explanations, they claim, is that short-period planets could have longer lifetimes around M-dwarfs since their orbital decay would likely be much slower. However, they are quick to caution against making any tentative conclusions before more research is conducted.

In the future, the team hopes to conduct measurements of the planet’s mass using the radial velocity method. This would likely involve a next-generation high-resolution spectrograph, like the Infrared Doppler (IFD) instrument or the CARMENES instrument – which are currently being built for the Subaru Telescope and the Calar Alto Observatory (respectively) to assist in the hunt for exoplanets around red dwarf stars.

One thing is clear though. This latest find is just another indication that red dwarf stars are where exoplanet-hunters will need to be focusing their efforts in the coming years and decades. These low mass, ultra-cool and low-luminosity stars are where some of the most interesting and extreme finds are being made. And what we stand to learn by studying them promises to be most profound!

Further Reading: arXiv

July 18, 2017July 18, 2017 by Matt Williams

Strange Radio Signals Detected from a Nearby Star

Artist's impression of rocky exoplanets orbiting Gliese 832, a red dwarf star just 16 light-years from Earth. Credit: ESO/M. Kornmesser/N. Risinger (skysurvey.org).

Astronomers have been listening to radio waves from space for decades. In addition to being a proven means of studying stars, galaxies, quasars and other celestial objects, radio astronomy is one of the main ways in which scientists have searched for signs of extra-terrestrial intelligence (ETI). And while nothing definitive has been found to date, there have been a number of incidents that have raised hopes of finding an “alien signal”.

In the most recent case, scientists from the Arecido Observatory recently announced the detection of a strange radio signal coming from Ross 128 – a red dwarf star system located just 11 light-years from Earth. As always, this has fueled speculation that the signal could be evidence of an extra-terrestrial civilization, while the scientific community has urged the public not to get their hopes up.

The discovery was part of a campaign being conducted by Abel Méndez – the director of the Planetary Habitability Laboratory (PHL) in Peurto Rico – and Jorge Zuluaga of the Faculty of Exact and Natural Sciences at the University of Antioquia, Colombia. Inspired by the recent discoveries around Proxima Centauri and TRAPPIST-1, the GJ 436 campaign relied on data from Arecibo Observatory to look for signs of exoplanets around nearby red dwarf stars.

*Arecibo Observatory, the world’s biggest single dish radio telescope, was and is still being used to image comet 45P/H-M-P. Courtesy of the NAIC – Arecibo Observatory, a facility of the NSF*

In the course of looking at data from stars systems like Gliese 436, Ross 128, Wolf 359, HD 95735, BD +202465, V* RY Sex, and K2-18 – which was gathered between April and May of 2017 – they noticed something rather interesting. Basically, the data indicated that an unexplained radio signal was coming from Ross 128. As Dr. Abel Mendez described in a blog post on the PHL website:

“Two weeks after these observations, we realized that there were some very peculiar signals in the 10-minute dynamic spectrum that we obtained from Ross 128 (GJ 447), observed May 12 at 8:53 PM AST (2017/05/13 00:53:55 UTC). The signals consisted of broadband quasi-periodic non-polarized pulses with very strong dispersion-like features. We believe that the signals are not local radio frequency interferences (RFI) since they are unique to Ross 128 and observations of other stars immediately before and after did not show anything similar.”

After first noticing this signal on Saturday, May 13th at 8:53 p.m., scientists from the Arecibo Observatory and astronomers from the Search for Extra-Terrestrial Intelligence (SETI) Institute teamed up to conduct a follow-up study of the star. This was performed on Sunday, July 16th, using SETI’s Allen Telescope Array and the National Radio Astronomy Observatory‘s (NRAO) Green Bank Telescope.

They also conducted observations of Barnard’s star on that same day to see if they could note similar behavior coming from this star system. This was done in collaboration with the Red Dots project, a European Southern Observatory (ESO) campaign that is also committed to finding exoplanets around red dwarf stars. This program is the successor to the ESO’s Pale Red Dot campaign, which was responsible for discovering Proxima b last summer.

*Images of the star systems examined by the GJ 436 Campaign. Credit: PHL/Abel Méndez*

As of Monday night (July 17th), Méndez updated his PHL blog post to announced that with the help of SETI Berkeley with the Green Bank Telescope, that they had successfully observed Ross 128 for the second time. The data from these observatories is currently being collected and processed, and the results are expected to be announced by the end of the week.

In the meantime, scientists have come up with several possible explanations for what might be causing the signal. As Méndez indicated, there are three major possibilities that he and his colleagues are considering:

“[T]hey could be (1) emissions from Ross 128 similar to Type II solar flares, (2) emissions from another object in the field of view of Ross 128, or just (3) burst from a high orbit satellite since low orbit satellites are quick to move out of the field of view. The signals are probably too dim for other radio telescopes in the world and FAST is currently under calibration.”

Unfortunately, each of these possibilities have their own drawbacks. In the case of a Type II solar flare, these are known to occur at much lower frequencies, and the dispersion of this signal appears to be inconsistent with this kind of activity. In the case of it possibly coming from another object, no objects (planets or satellites) have been detected within Ross 128’s field of view to date, thus making this unlikely as well.

*The stars currently being examined as part of the GJ 436 campaign. Credit: PHL/Abel Méndez*

Hence, the team has something of a mystery on their hands, and hopes that further observations will allow them to place further constrains on what the cause of the signal could be. “[W]e might clarify soon the nature of its radio emissions, but there are no guarantees,” wrote Méndez. “Results from our observations will be presented later that week. I have a Piña Colada ready to celebrate if the signals result to be astronomical in nature.”

And just to be fair, Méndez also addressed the possibility that the signal could be artificial in nature – i.e. evidence of an alien civilization. “In case you are wondering,” he wrote, “the recurrent aliens hypothesis is at the bottom of many other better explanations.” Sorry, alien-hunters. Like the rest of us, you’ll just have to wait and see what can be made of this signal.

Further Reading: AFP, PHL

July 14, 2017January 31, 2024 by Matt Williams

Even Though Red Dwarfs Have Long Lasting Habitable Zones, They’d be Brutal to Life

Artist's concept of the TRAPPIST-1 star system, an ultra-cool dwarf that has seven Earth-size planets orbiting it. We're going to keep finding more and more solar systemsl like this, but we need observatories like WFIRST, with starshades, to understand the planets better. Credits: NASA/JPL-Caltech

Ever since scientists confirmed the existence of seven terrestrial planets orbiting TRAPPIST-1, this system has been a focal point of interest for astronomers. Given its proximity to Earth (just 39.5 light-years light-years away), and the fact that three of its planets orbit within the star’s “Goldilocks Zone“, this system has been an ideal location for learning more about the potential habitability of red dwarf stars systems.

This is especially important since the majority of stars in our galaxy are red dwarfs (aka. M-type dwarf stars). Unfortunately, not all of the research has been reassuring. For example, two recent studies performed by two separate teams from the Harvard-Smithsonian Center for Astrophysics (CfA) indicate that the odds of finding life in this system are less likely than generally thought.

October 27, 2016October 28, 2016 by Matt Williams

Planets Around Stars like Proxima Centauri are Probably Earth-Sized Water Worlds

Artist's impression of an "eyeball" planet, a water world where the sun-facing side is able to maintain a liquid-water ocean. Credit and Copyright: eburacum45/ DeviantArt

Proxima b is the subject of a lot interest these days. And why not? As the closest extrasolar planet to our Solar System, it is the best shot we have at studying exoplanets up close in the near future. However, a recent study from the University of Marseilles indicated that, contrary to what many hoped, the planet may be a “water world” – i.e. a planet where up to half of its mass consists of water.

And now, researchers from the University of Bern have taken this analysis a step further. Based on their study, which has been accepted for publication in the journal Astronomy and Astrophysics (A&A), they have determined that the majority planets that form within the habitable zones of a red dwarf star may be water worlds. These findings could have drastic implications for the search for habitable exoplanets around red dwarf stars.

The research was conducted by Dr. Yann Alibert from the National Centers for Competence in Research (NCCR) PlanetS center and Prof. Willy Benz from the Center of Space and Habitability (CSH). Both of these institutions, which are located at the University of Bern, are dedicated to understanding planetary formation and evolution, as well as fostering a dialogue with the public about exoplanet research.

For the sake of their study, titled “Formation and Composition of Planets Around Very Low Mass Stars“, Alibert and Benz carried out the first computer simulation designed to examine the formation of planets around stars that are ten times less massive than our Sun. This involved creating a model that included hundreds of thousands of identical low-mass stars, which were then given orbiting protoplanetary disks of dust and gas.

They then simulated what would happen if planets began to form from the accretion of these disks. For each, they assumed the existence of ten “planetary embryos” (equal to the mass of the Moon) which would grow and migrate over time, giving rise to a system of planets.

Ultimately, what they found was that the planets orbiting within the habitable zone of their parent star would likely to be comparable in size to Earth – ranging from 0.5 to 1.5 times the radius of Earth, with 1 Earth radii being the average. As Dr. Yann Alibert explained to Universe Today via email:

“In the simulations we have considered here, it appears that the majority of the mass (more than 99%) is in the solids. [W]e therefore start with a protoplanetary disk that is made of solids and gas and 10 planetary embryos. The solids in the disk are planetesimals (similar to present day asteriods, around 1 km in size), that can be dry (if they are located in the hot regions of the protoplanetary disk) or wet (around 50% per mass of water ice, if they are in the cold regions of the disk). The planetary embryos are small bodies, whose mass is similar to the moon mass. We then compute how much of the disk solids are capture by the planetary embryos.”

In addition, the simulations produced some interesting estimates on how much of the planets would consist of water. In 90% of cases, water would account for more than 10% of the planets’ mass. Compare that to Earth, where water covers over 70% of our surface, but makes up only about 0.02% of our planet’s total mass. This would mean that the exoplanets would have very deep oceans and a layer of ice at the bottom, owing to the extreme pressure.

Last, but not least, Alibert and Benze found that if the protoplanetary disks that these planets formed from lived longer than the models suggested, the situation would be even more extreme. All of this could be dire news for those hoping that we might find ET living next door, or that red dwarf stars are the best place to look for intelligent life.

“The fact that many planets are water rich could have potentially very strong (and negative) consequence on the habitability of such planets,” said Dr. Alibert. “In fact, we already showed in other articles (Alibert et al 2013, Kitzmann et al. 2015) that if there is too much water on a planet, this may lead to an unstable climate, and an atmosphere that could be very rich in CO2.”

However, Alibert indicates that these two studies were conducted based on planets that orbit stars similar to our Sun. Red dwarfs are different because they evolve much slower (i.e. the luminosity changes very slowly over time) and they are far more red than our Sun, meaning that the light coming from them has different wavelengths that will interact different with planetary atmospheres.

“So, to summarize, it could be that the presence of large amounts of water is not so bad as in the case of solar type stars, but it could also well be that it is even worse for reasons that we do not know,” said Alibert. “Whatever the effect, it is something that is important to study, and we have started working on this subject.”

But regardless of whether or not planets that orbit red dwarf stars are habitable, simulations like this one are still exciting. Aside from offering data on what neighboring planets might look like, they also help us to understand the wide range of possibilities that await us out there. And last, they give us more incentive to actually get out there and explore these worlds up close.

Only be sending missions to other stars can we confirm or deny if they are capable of supporting life. And if in the end, we should find that the most common star in the Universe is unlikely to produce life-giving planets, it only serves to remind us how rare and precious “Earth-like” planets truly are.

Further Reading: University of Bern, arXiv

October 24, 2016October 25, 2016 by Matt Williams

How Many Planets are There in the Galaxy?

Artist's impression of The Milky Way Galaxy. Based on current estimates and exoplanet data, it is believed that there could be tens of billions of habitable planets out there. Credit: NASA

On a clear night, and when light pollution isn’t a serious factor, looking up at the sky is a breathtaking experience. On occasions like these, it is easy to be blown away by the sheer number of stars out there. But of course, what we can see on any given night is merely a fraction of the number of stars that actually exist within our Galaxy.

What is even more astounding is the notion that the majority of these stars have their own system of planets. For some time, astronomers have believed this to be the case, and ongoing research appears to confirm it. And this naturally raises the question, just how many planets are out there? In our galaxy alone, surely, there must be billions!

Number of Planets per Star:

To truly answer that question, we need to crunch some numbers and account for some assumptions. First, despite the discovery of thousands of extra-solar planets, the Solar System is still the only one that we have studied deeply. So it could be that ours possesses more star systems than others, or that our Sun has a fraction of the planets that other stars do.

So let’s assume that the eight planets that exist within our Solar System (not taking into account Dwarf Planets, Centaurs, KBOs and other larger bodies) represent an average. The next step will be to multiply that number by the amount of stars that exist within the Milky Way.

Number of Stars:

To be clear, the actual number of stars in the Milky Way is subject to some dispute. Essentially, astronomers are forced to make estimates due to the fact that we cannot view the Milky Way from the outside. And given that the Milky Way is in the shape of a barred, spiral disc, it is difficult for us to see from one side to the other – thanks to light interference from its many stars.

As a result, estimates of how many stars there are come down to calculations of our galaxy’s mass, and estimates of how much of that mass is made up of stars. Based on these calculations, scientists estimate that the Milky Way contains between 100 and 400 billion stars (though some think there could be as many as a trillion).

Doing the math, we can then say that the Milky Way galaxy has – on average – between 800 billion and 3.2 trillion planets, with some estimates placing that number as high a 8 trillion! However, in order to determine just how many of them are habitable, we need to consider the number of exoplanets discovered so far for the sake of a sample analysis.

Habitable Exoplanets:

As of October 13th, 2016, astronomers have confirmed the presence of 3,397 exoplanets from a list of 4,696 potential candidates (which were discovered between 2009 and 2015). Some of these planets have been observed directly, in a process known as direct imaging. However, the vast majority have been detected indirectly using the radial velocity or transit method.

In the case of the former, the existence of planets is inferred based on the gravitational influence they have on their parent star. Essentially, astronomers measure how much the star moves back and forth to determine if it has a system of planets and how massive they are. In the case of the transit method, planets are detected when they pass directly in front of their star, causing it to dim. Here, size and mass are estimated based on the level of dimming.

In the course of its mission, the Kepler mission has observed about 150,000 stars, which during its initial four year mission consisted primarily of M-class stars. Also known as red dwarfs, these low-mass, lower-luminosity stars are harder to observe than our own Sun.

*Histogram showing the number of exoplanets discovered by year. Credit: NASA Ames/W. Stenzel, Princeton/T. Morton*

Since that time, Kepler has entered a new phase, also known as the K2 mission. During this phase, which began in November of 2013, Kepler has been shifting its focus to observe more in the way of K- and G-class stars – which are nearly as bright and hot as our Sun.

According to a recent study from NASA Ames Research Center, Kepler found that about 24% of M-class stars may harbor potentially habitable, Earth-size planets (i.e. those that are smaller than 1.6 times the radius of Earth’s). Based upon the number of M-class stars in the galaxy, that alone represents about 10 billion potentially habitable, Earth-like worlds.

Meanwhile, analyses of the K2 phase suggests that about one-quarter of the larger stars surveyed may also have Earth-size planet orbiting within their habitable zones. Taken together, the stars observed by Kepler make up about 70% of those found within the Milky Way. So one can estimate that there are literally tens of billions of potentially habitable planets in our galaxy alone.

In the coming years, new missions will be launching, like the James Webb Space Telescope (JWST) and the Transitting Exoplanet Survey Satellite (TESS). These missions will be able to detect smaller planets orbiting fainter stars, and maybe even determine if there’s life on any of them.

Once these new missions get going, we’ll have better estimates of the size and number of planets that orbit a typical star, and we’ll be able to come up with better estimates of just many planets there are in the galaxy. But until then, the numbers are still encouraging, as they indicate that the chances for extra-terrestrial intelligence are high!

We have written many articles about galaxies for Universe Today. Here’s How Many Stars are there in the Milky Way?, How Many Planets are there in the Solar System?, What are Extra-Solar Planets?, Planets Plentiful Around Abundant Red Dwarf Stars, Study Says, Life After Kepler: Upcoming Exoplanet Missions.

If you’d like more info on galaxies, check out Hubblesite’s News Releases on Galaxies, and here’s NASA’s Science Page on Galaxies.

We have also recorded an episode of Astronomy Cast about galaxies – Episode 97: Galaxies.

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