Pluto’s Interactions With The Solar Wind Are Unique, Study Finds

Pluto has some characteristics less like that of a comet and more like much larger planets, according to an analysis of Pluto’s unique interaction with the solar wind, scientists say.

plutosintera

Using data from an instrument aboard the New Horizons spacecraft gathered on its Pluto flyby in July 2015, scientists have observed the material coming off of Pluto and seen how it interacts with the solar wind, and found it completely new – and unexpected.

“This is a type of interaction we’ve never seen before anywhere in our solar system,” said David J. McComas, lead author of the new study published today in the Journal of Geophysical Research – Space Physics, a publication of the American Geophysical Union.

McComas, professor in Princeton University’s Department of Astrophysical Sciences and vice president for the Princeton Plasma Physics Laboratory, leads the Solar Wind Around Pluto (SWAP) instrument aboard New Horizons; he also led development of SWAP when he was at the Southwest Research Institute (SwRI) in Texas. The research was funded as a part of the New Horizons project by NASA.

Space physicists say that they now have a treasure trove of information about how Pluto’s atmosphere interacts with the solar wind. Solar wind is the plasma, or charged particles, that spews off from the sun into the solar system at a supersonic 400 kilometers per second (1 million miles per hour), bathing planets, asteroids, comets and interplanetary space in a soup of mostly electrons and protons.

“The results are astonishing. We were fascinated and surprised” by the findings, McComas said.

Previously, most researchers thought that Pluto was characterized more like a comet, which has a large region of gentle slowing of the solar wind, as opposed to the abrupt diversion solar wind encounters at a planet like Mars or Venus. Instead, like a car that’s part gas- and part battery-powered, Pluto is a hybrid, the researchers say.

“This is an intermediate interaction, a completely new type. It’s not comet-like, and it’s not planet-like. It’s in-between,” McComas said. “We’ve now visited all nine of the classical planets and examined all their solar wind interactions, and we’ve never seen anything like this.”

1-plutosintera

“These results speak to the power of exploration. Once again we’ve gone to a new kind of place and found ourselves discovering entirely new kinds of expressions in nature,” said Alan Stern, New Horizons principal investigator at the Southwest Research Institute. “Many people were surprised by Pluto’s complex geology and atmosphere. This paper shows there’s even more that’s surprising there, including its atmosphere-solar wind interaction.”

Pluto continues to confound. Since it’s so far from the sun – an average of about 5.9 billion kilometers (3.7 billion miles) – and because it’s so small, scientists thought Pluto’s gravity would not be strong enough to hold heavy ions in its extended atmosphere. But, “Pluto’s gravity clearly is enough to keep material sufficiently confined,” McComas said.

Further, the scientists found that very little of Pluto’s atmosphere is comprised of neutral particles converted to electrically charged ions and swept out into space.

“This is backwards for many other planets, where the neutral particles stay relatively close to the planet,” said Michael Liemohn, a University of Michigan astrophysicist and Editor-in-Chief of JGR-Space Physics, who was not involved with the research but who helped edit the paper. “An ion particle becomes influenced by the electric and magnetic forces present in the solar system, which can be a very efficient acceleration processes. But at Pluto, McComas et al found that only a wisp of atmosphere leaves the planet as ions.”

The researchers were able to separate the heavy ions of methane, the main gas escaping from Pluto’s atmosphere, from the light ions of hydrogen that come from the sun using the SWAP instrument.

Among their Pluto findings:

– Like Earth, Pluto has a long ion tail, that extends downwind at least a distance of about 100 Pluto radii (119,000 kilometers (73,800 miles), almost three times the circumference of Earth), loaded with heavy ions from the atmosphere and with “considerable structure;”

– Pluto’s obstruction of the solar wind upwind of the planet is smaller than had been thought. The solar wind isn’t blocked until about the distance of a couple planetary radii (2,968 kilometers (1,844 miles), about the distance between Chicago and Los Angeles);

– Pluto has a very thin “Plutopause” – or boundary of Pluto’s tail of heavy ions and the sheath of the shocked solar wind that presents an obstacle to its flow.
The scientists write: “Pluto interaction with the solar wind appears to be a hybrid with the bow shock generated by mass-loading like at a comet, but the obstacle to the solar wind flow – the Plutopause – sustained by atmospheric thermal pressure as at Venus and Mars.”

Heather Elliott, astrophysicist at Southwest Research Institute and co-author on the paper, said that the study provides interesting comparisons. “Comparing the solar wind-Pluto interaction to the solar wind-interaction for other planets and bodies is interesting because the physical conditions are different for each, and the dominant physical processes depend on those conditions,” Elliott said.

What is significant, McComas said, is the range of diversity that bodies in the solar system have with the solar wind. Further, the findings offer clues to the magnetized plasmas that one might find around other stars. “The range of interaction with the solar wind is quite diverse, and this gives some comparison to help us better understand the connections in and beyond our solar system,” McComas said.

The scientists conclude: “The SWAP data will … be reanalyzed … for many years to come as the community collectively grapples with Pluto’s unique solar wind interaction – one that is unlike that at any other body in the solar system.”

New Horizons is the first mission in NASA’s New Frontiers program, managed by the agency’s Marshall Space Flight Center in Huntsville, Ala. The Johns Hopkins University Applied Physics Laboratory designed, built, and operates the New Horizons spacecraft and manages the mission under Principal Investigator Dr. Alan Stern’s direction for NASA’s Science Mission Directorate. SwRI leads the science mission, payload operations, and encounter science planning. The NASA Heliophysics program also supported the analysis of these observations.

Second Strongest Shock Wave Found In Merging Galaxy Clusters

The discovery by a physics doctoral student at The University of Alabama in Huntsville (UAH) of the second-strongest merger shock in clusters of galaxies ever observed has generated excitement that is opening doors to further scientific exploration.
cluster
Sarthak Dasadia, who is advised by assistant physics professor Dr. Ming Sun, discovered the very strong shock in the merging galaxy cluster Abell 655 using observations from the Chandra X-ray Observatory.

The shock to the north of this cluster is second in strength only to the Bullet Cluster shock.

The shock is traveling with an astonishing speed of 2,700 kilometers per second, about three times the local speed of sound in the cluster. By comparison, NASA’s Juno spacecraft in 2013 became the fastest human-made object when it was slingshot around Earth toward Jupiter at a relatively pedantic 40 kilometers a second.

“Studying mergers of galaxy clusters has proven to be crucial to our understanding of how such large scale objects form and evolve,” says Dasadia. Shocks provide unique opportunities to study high-energy phenomena in the intra-cluster medium — the hot plasma between galaxies.

“This could open a door, where people can do a number of different studies based on what I have found,” Dasadia says. Already, scientists are targeting shocks in galaxy clusters to study dark matter, the magnetic field in the intracluster space, particle acceleration and energy transfer in the intracluster medium.

In only 10 days, Dasadia’s research was accepted for publication by The Astrophysical Journal Letters. Dasadia recently received one year of research support from the Alabama EPSCoR Graduate Research Scholars Program (ALEPSCoR). He also gave an oral presentation on his research in August at the International Astronomical Union (IAU) General Assembly in Honolulu, Hawaii.

The universe is populated with galaxy clusters that are relaxed and unrelaxed, Dasadia says. The relaxed ones are mellow — they’ve been around a lot longer, have seen lots of past mergers and really aren’t dynamically active. It’s the unrelaxed clusters like Abell 665 that are good candidates to study merger features such as shocks and turbulence.

“These galaxy clusters are not boundary objects,” he says. “They do not have a very well-defined boundary around them.”

When the undefined boundaries of massive clusters of galaxies 3 million light-years across are drawn together in a slow-motion collision, their cold cores and surrounding hot gases are disrupted into shock waves and gas fronts of various temperatures.

“When two cold cores collide, they may create a shock of heated gas,” Dasadia says. “Such mergers are actually among the most energetic events in the universe, other than the Big Bang itself.”

If talking about fronts and shock waves and temperature differentials sounds lot like the weather on Earth, Dasadia says that’s because there is not much difference as far as the physics involved.

“Technically, we observe the same features in space that we do on Earth,” he says. “This area has been studied extensively before at small scales, but few had done the work to discover what I found here at such big scales.”

He was able to measure the velocity of the collision and the dynamics of what is happening in it — or rather, what was happening in it. It took 3.2 billion years for the light in the observations to reach Earth, so the events all happened that far back in time. Dynamic observations included the energy in the collision, the gas movement, and measurements of the discrepancy between the visible and dark matter involved.

“It amazes me how long it takes for this information to even reach the Earth,” Dasadia says. “Then I am also amazed by our technology, by how much we have advanced in developing the telescopes and equipment it takes to be able to observe and study these interactions.”

Planet Nine: A World That Shouldn’t Exist

Earlier this year scientists presented evidence for Planet Nine, a Neptune-mass planet in an elliptical orbit 10 times farther from our Sun than Pluto. Since then theorists have puzzled over how this planet could end up in such a distant orbit.

planet9

New research by astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) examines a number of scenarios and finds that most of them have low probabilities. Therefore, the presence of Planet Nine remains a bit of a mystery.

“The evidence points to Planet Nine existing, but we can’t explain for certain how it was produced,” says CfA astronomer Gongjie Li, lead author on a paper accepted for publication in the Astrophysical Journal Letters.

Planet Nine circles our Sun at a distance of about 40 billion to 140 billion miles, or 400 — 1500 astronomical units. (An astronomical unit or A.U. is the average distance of the Earth from the Sun, or 93 million miles.) This places it far beyond all the other planets in our solar system. The question becomes: did it form there, or did it form elsewhere and land in its unusual orbit later?

Li and her co-author Fred Adams (University of Michigan) conducted millions of computer simulations in order to consider three possibilities. The first and most likely involves a passing star that tugs Planet Nine outward. Such an interaction would not only nudge the planet into a wider orbit but also make that orbit more elliptical. And since the Sun formed in a star cluster with several thousand neighbors, such stellar encounters were more common in the early history of our solar system.

However, an interloping star is more likely to pull Planet Nine away completely and eject it from the solar system. Li and Adams find only a 10 percent probability, at best, of Planet Nine landing in its current orbit. Moreover, the planet would have had to start at an improbably large distance to begin with.

CfA astronomer Scott Kenyon believes he may have the solution to that difficulty. In two papers submitted to the Astrophysical Journal, Kenyon and his co-author Benjamin Bromley (University of Utah) use computer simulations to construct plausible scenarios for the formation of Planet Nine in a wide orbit.

“The simplest solution is for the solar system to make an extra gas giant,” says Kenyon.

They propose that Planet Nine formed much closer to the Sun and then interacted with the other gas giants, particularly Jupiter and Saturn. A series of gravitational kicks then could have boosted the planet into a larger and more elliptical orbit over time.

“Think of it like pushing a kid on a swing. If you give them a shove at the right time, over and over, they’ll go higher and higher,” explains Kenyon. “Then the challenge becomes not shoving the planet so much that you eject it from the solar system.”

That could be avoided by interactions with the solar system’s gaseous disk, he suggests.

Kenyon and Bromley also examine the possibility that Planet Nine actually formed at a great distance to begin with. They find that the right combination of initial disk mass and disk lifetime could potentially create Planet Nine in time for it to be nudged by Li’s passing star.

“The nice thing about these scenarios is that they’re observationally testable,” Kenyon points out. “A scattered gas giant will look like a cold Neptune, while a planet that formed in place will resemble a giant Pluto with no gas.”

Li’s work also helps constrain the timing for Planet Nine’s formation or migration. The Sun was born in a cluster where encounters with other stars were more frequent. Planet Nine’s wide orbit would leave it vulnerable to ejection during such encounters. Therefore, Planet Nine is likely to be a latecomer that arrived in its current orbit after the Sun left its birth cluster.

Finally, Li and Adams looked at two wilder possibilities: that Planet Nine is an exoplanet that was captured from a passing star system, or a free-floating planet that was captured when it drifted close by our solar system. However, they conclude that the chances of either scenario are less than 2 percent.

UPDATE: New Sources of Charged Particles Discovered

Researchers from the University of Cambridge, used data from the European Space Agency’s (ESA) XMM-Newton space observatory to reveal for the first time strong winds gusting at very high speeds from two mysterious sources of x-ray radiation. The discovery, published in the journal Nature, confirms that these sources conceal a compact object pulling in matter at extraordinarily high rates.

two black holes eating star

Two black holes in nearby galaxies have been observed devouring their companion stars at a rate exceeding classically understood limits, and in the process, kicking out charged particles into surrounding space at astonishing speeds of around a quarter the speed of light.
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“We think these newly discovered sources we are calling ‘ultra-luminous x-rays’ come from special binary systems, sucking up gas at a much higher rate than an ordinary x-ray binary,” said Dr Ciro Pinto from Cambridge’s Institute of Astronomy, the paper’s lead author. “Some of these sources host highly magnetized neutron stars, while others might conceal the long-sought-after intermediate-mass black holes, which have masses around one thousand times the mass of the Sun. But in the majority of cases, the reason for their extreme behavior is still unclear.”

xmm_newton_esa

Pinto and his colleagues collected several days’ worth of observations of three ultra-luminous x-ray sources, all located in nearby galaxies located less than 22 million light-years from the Milky Way. The data was obtained over several years with the Reflection Grating Spectrometer on XMM-Newton, which allowed the researchers to identify subtle features in the spectrum of the x-rays from the sources.

In all three sources, the scientists were able to identify x-ray emission from gas in the outer portions of the disc surrounding the central compact object, slowly flowing towards it.

ngc_1313_galaxy

But two of the three sources – known as NGC 1313 X-1 and NGC 5408 X-1 – also show clear signs of x-rays being absorbed by gas that is streaming away from the central source at 70,000 kilometers per second – almost a quarter of the speed of light.

“This is the first time we’ve seen winds streaming away from ultra-luminous x-ray sources,” said Pinto. “And the very high speed of these outflows is telling us something about the nature of the compact objects in these sources, which are frantically devouring matter.”

NCG 5408

While the hot gas is pulled inwards by the central object’s gravity, it also shines brightly, and the pressure exerted by the radiation pushes it outwards. This is a balancing act: the greater the mass, the faster it draws the surrounding gas; but this also causes the gas to heat up faster, emitting more light and increasing the pressure that blows the gas away.

There is a theoretical limit to how much matter can be pulled in by an object of a given mass, known as the Eddington limit. The limit was first calculated for stars by astronomer Arthur Eddington, but it can also be applied to compact objects like black holes and neutron stars.

Eddington’s calculation refers to an ideal case in which both the matter being accreted onto the central object and the radiation being emitted by it do so equally in all directions.

But the sources studied by Pinto and his collaborators are potentially being fed through a disc which has been puffed up due to internal pressures arising from the incredible rates of material passing through it. These thick discs can naturally exceed the Eddington limit and can even trap the radiation in a cone, making these sources appear brighter when we look straight at them. As the thick disc moves material further from the black hole’s gravitational grasp it also gives rise to very high-speed winds like the ones observed by the Cambridge researchers.

“By observing x-ray sources that are radiating beyond the Eddington limit, it is possible to study their accretion process in great detail, investigating by how much the limit can be exceeded and what exactly triggers the outflow of such powerful winds,” said Norbert Schartel, ESA XMM-Newton Project Scientist.

The nature of the compact objects hosted at the core of the two sources observed in this study is, however, still uncertain.

Based on the x-ray brightness, the scientists suspect that these mighty winds are driven from accretion flows onto either neutron stars or black holes, the latter with masses of several to a few dozen times that of the Sun.

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UPDATE: New Sources of Charged Particles Discovered

The researchers, from the University of Cambridge, used data from the European Space Agency’s (ESA) XMM-Newton space observatory to reveal for the first time strong winds gusting at very high speeds from two mysterious sources of x-ray radiation. The discovery, published in the journal Nature, confirms that these sources conceal a compact object pulling in matter at extraordinarily high rates.

two black holes eating star

Two black holes in nearby galaxies have been observed devouring their companion stars at a rate exceeding classically understood limits, and in the process, kicking out charged particles into surrounding space at astonishing speeds of around a quarter the speed of light.
_____________

Mitch Battros and Science of Cycles Research Sponsorship Fundraiser – Be part of keeping ‘Science of Cycles’ alive and free. Your support is needed to keep this unique and valuable resource. Help sponsor us with your pledge as you see fit to the value you receive.    – CLICK HERE –

“We think these newly discovered sources we are calling ‘ultra-luminous x-rays’ come from special binary systems, sucking up gas at a much higher rate than an ordinary x-ray binary,” said Dr Ciro Pinto from Cambridge’s Institute of Astronomy, the paper’s lead author. “Some of these sources host highly magnetized neutron stars, while others might conceal the long-sought-after intermediate-mass black holes, which have masses around one thousand times the mass of the Sun. But in the majority of cases, the reason for their extreme behavior is still unclear.”

xmm_newton_esa

Pinto and his colleagues collected several days’ worth of observations of three ultra-luminous x-ray sources, all located in nearby galaxies located less than 22 million light-years from the Milky Way. The data was obtained over several years with the Reflection Grating Spectrometer on XMM-Newton, which allowed the researchers to identify subtle features in the spectrum of the x-rays from the sources.

In all three sources, the scientists were able to identify x-ray emission from gas in the outer portions of the disc surrounding the central compact object, slowly flowing towards it.

ngc_1313_galaxy

But two of the three sources – known as NGC 1313 X-1 and NGC 5408 X-1 – also show clear signs of x-rays being absorbed by gas that is streaming away from the central source at 70,000 kilometers per second – almost a quarter of the speed of light.

“This is the first time we’ve seen winds streaming away from ultra-luminous x-ray sources,” said Pinto. “And the very high speed of these outflows is telling us something about the nature of the compact objects in these sources, which are frantically devouring matter.”

NCG 5408

While the hot gas is pulled inwards by the central object’s gravity, it also shines brightly, and the pressure exerted by the radiation pushes it outwards. This is a balancing act: the greater the mass, the faster it draws the surrounding gas; but this also causes the gas to heat up faster, emitting more light and increasing the pressure that blows the gas away.

There is a theoretical limit to how much matter can be pulled in by an object of a given mass, known as the Eddington limit. The limit was first calculated for stars by astronomer Arthur Eddington, but it can also be applied to compact objects like black holes and neutron stars.

Eddington’s calculation refers to an ideal case in which both the matter being accreted onto the central object and the radiation being emitted by it do so equally in all directions.

But the sources studied by Pinto and his collaborators are potentially being fed through a disc which has been puffed up due to internal pressures arising from the incredible rates of material passing through it. These thick discs can naturally exceed the Eddington limit and can even trap the radiation in a cone, making these sources appear brighter when we look straight at them. As the thick disc moves material further from the black hole’s gravitational grasp it also gives rise to very high-speed winds like the ones observed by the Cambridge researchers.

“By observing x-ray sources that are radiating beyond the Eddington limit, it is possible to study their accretion process in great detail, investigating by how much the limit can be exceeded and what exactly triggers the outflow of such powerful winds,” said Norbert Schartel, ESA XMM-Newton Project Scientist.

The nature of the compact objects hosted at the core of the two sources observed in this study is, however, still uncertain.

Based on the x-ray brightness, the scientists suspect that these mighty winds are driven from accretion flows onto either neutron stars or black holes, the latter with masses of several to a few dozen times that of the Sun.

______________

_science of cycles33

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Although Boiling, Water Does Shape Martian Terrain

At present, liquid water on Mars only exists in small quantities as a boiling liquid, and only during the warmest time of day in summer. Its role has therefore been considered insignificant until now. However, an international team including scientists from the CNRS, Université de Nantes and Université Paris-Sud and headed by Marion Massé, from the Laboratoire de Planétologie et Géodynamique de Nantes (CNRS/Université de Nantes)has now shown that even though water that emerges onto the surface of Mars immediately begins to boil, it creates an unstable, turbulent flow that can eject sediment and cause dry avalanches. The flow of small amounts of a boiling liquid therefore significantly alters the surface. The discovery of this exotic process, unknown on our planet, radically changes our interpretation of the Martian surface, making it difficult to undertake a direct comparison of flows on the Earth and on Mars. These findings are published on 2 May 2016 in the journal Nature Geoscience.

mars

It is well known that water boils at 100 °C. But this is only true at sea level, since boiling point depends on atmospheric pressure: the higher the altitude, the thinner the atmosphere, and the lower the boiling point. For instance, at the top of Mount Everest, water boils at 60 °C. But on Mars, where the atmosphere is much thinner than on Earth, it can boil at temperatures as low as 0 °C. During the Martian summer, when the subsurface water ice begins to melt and emerge at the surface, where the mean temperature reaches 20 °C, it immediately starts to boil. This is also the case for the flows of saline water discovered last year. So could an evaporating liquid alter the Martian landscape?

To find out, a team of researchers from the Open University (UK) used a former diving decompression chamber to reproduce the low pressure of the Martian atmosphere. At the same time, another team from the GEOPS laboratory (CNRS/Université Paris-Sud) carried out the same experiment, but this time in a cold chamber at Earth’s atmospheric pressure. In both chambers, a block of pure water ice, followed by one of saline water ice, were melted at a temperature of 20 °C (as on Mars in summer) on a sand-covered slope.

The experiments showed that, in the flows produced under terrestrial conditions, the water gradually seeped into the sand, leaving no trace on the surface after drying. However, what was observed in the Martian chamber was very different. The water produced by the melting ice started to boil as soon as it reached the surface, and the gas released caused the ejection of sand grains . These gradually formed small ridges at the front of the flow, which, as they grew larger, became unstable and actually produced avalanches of dry sand. The process was even more violent at lower pressures. Contrary to what is observed on Earth, the surface, once dry, therefore exhibited a series of ridges.

This process is not as efficient in the case of saline water since it is more stable than pure water under Martian conditions. However, since saline water is more viscous, it can carry along sand grains and form small channels, a process that can sometimes become explosive under low pressure.

These findings, analyzed along with other laboratories worldwide, including the Institut d’Astrophysique Spatiale (CNRS/Université Paris Sud), provide new insight into the effect of the flow of water — whether saline or not — on the surface of Mars. Far from making its action insignificant, the water’s instability considerably increases its impact on surface morphology. This broadens the potential range of processes that could explain the activity on the Martian surface, such as that observed in spring on the planet’s slopes during the melt of winter frost made up of CO2 and water ice, as well as the dark flows (Recurring Slope Lineae) seen in summer.

mars

The possible presence of liquid water on the surface of Mars is a key question in the search for environments potentially favorable to life. Until now, detecting liquid water depended on identifying morphologies similar to those produced on Earth by the flow of liquid water, such as channels, gullies, or simply the seasonal appearance of dark traces caused by dampening of the surface. However the flows produced in the laboratory show that morphologies produced under either Martian or terrestrial conditions are very different. Direct comparison between landforms produced on Earth and on Mars does not therefore appear to be appropriate for detecting the appearance of a liquid on Mars, thus altering our interpretation of the Martian surface.

Three Potentially Habitable Worlds Found Around Nearby Ultracool Dwarf Star

Astronomers using the TRAPPIST telescope at ESO’s La Silla Observatory have discovered three planets orbiting an ultracool dwarf star just 40 light-years from Earth. These worlds have sizes and temperatures similar to those of Venus and Earth and are the best targets found so far for the search for life outside the Solar System. They are the first planets ever discovered around such a tiny and dim star. The new results will be published in the journal Nature on 2 May 2016.

habitable planets

A team of astronomers led by Michaël Gillon, of the Institut d’Astrophysique et Géophysique at the University of Liège in Belgium, have used the Belgian TRAPPIST telescope to observe the star 2MASS J23062928-0502285 now also known as TRAPPIST-1. They found that this dim and cool star faded slightly at regular intervals, indicating that several objects were passing between the star and the Earth. Detailed analysis showed that three planets with similar sizes to the Earth were present.

TRAPPIST-1 is an ultracool dwarf star — it is much cooler and redder than the Sun and barely larger than Jupiter. Such stars are both very common in the Milky Way and very long-lived, but this is the first time that planets have been found around one of them. Despite being so close to the Earth, this star is too dim and too red to be seen with the naked eye or even visually with a large amateur telescope. It lies in the constellation of Aquarius (The Water Carrier).

Emmanuël Jehin, a co-author of the new study, is excited: “This really is a paradigm shift with regards to the planet population and the path towards finding life in the Universe. So far, the existence of such ‘red worlds’ orbiting ultra-cool dwarf stars was purely theoretical, butnow we have not just one lonely planet around such a faint red star but a complete system of three planets!”

Michaël Gillon, lead author of the paper presenting the discovery, explains the significance of the new findings: “Why are we trying to detect Earth-like planets around the smallest and coolest stars in the solar neighbourhood? The reason is simple: systems around these tiny stars are the only places where we can detect life on an Earth-sized exoplanet with our current technology. So if we want to find life elsewhere in the Universe, this is where we should start to look.”

Astronomers will search for signs of life by studying the effect that the atmosphere of a transiting planet has on the light reaching Earth. For Earth-sized planets orbiting most stars this tiny effect is swamped by the brilliance of the starlight. Only for the case of faint red ultra-cool dwarf stars — like TRAPPIST-1 — is this effect big enough to be detected.

Follow-up observations with larger telescopes, including the HAWK-I instrument on ESO’s 8-metre Very Large Telescope in Chile, have shown that the planets orbiting TRAPPIST-1 have sizes very similar to that of Earth. Two of the planets have orbital periods of about 1.5 days and 2.4 days respectively, and the third planet has a less well determined period in the range 4.5 to 73 days.

“With such short orbital periods, the planets are between 20 and 100 times closer to their star than the Earth to the Sun. The structure of this planetary system is much more similar in scale to the system of Jupiter’s moons than to that of the Solar System,” explains Michaël Gillon.

Although they orbit very close to their host dwarf star, the inner two planets only receive four times and twice, respectively, the amount of radiation received by the Earth, because their star is much fainter than the Sun. That puts them closer to the star than the habitable zone for this system, although it is still possible that they possess habitable regions on their surfaces. The third, outer, planet’s orbit is not yet well known, but it probably receives less radiation than the Earth does, but maybe still enough to lie within the habitable zone.

“Thanks to several giant telescopes currently under construction, including ESO’s E-ELT and the NASA/ESA/CSA James Webb Space Telescope due to launch for 2018, we will soon be able to study the atmospheric composition of these planets and to explore them first for water, then for traces of biological activity. That’s a giant step in the search for life in the Universe,” concludes Julien de Wit, a co-author from the Massachusetts Institute of Technology (MIT) in the USA.

This work opens up a new direction for exoplanet hunting, as around 15% of the stars near to the Sun are ultra-cool dwarf stars, and it also serves to highlight that the search for exoplanets has now entered the realm of potentially habitable cousins of the Earth. The TRAPPIST survey is a prototype for a more ambitious project called SPECULOOS that will be installed at ESO’s Paranal Observatory.