Are Exo-planets Orbiting Red Dwarf Stars too Dry for Life?

In this artist's conception, gas and dust swirl around a young star. Eventually this material should form planets through gravitational accretion. Credit: NASA/JPL-Caltech View full size image

If water is the source of life, then finding the source of water certainly qualifies as a worthy astrobiological endeavor. Scientists have formulated certain scenarios for how our planet became wet and stayed wet, but other planets may not have been able to tap this same source.

One place where water availability could be a problem is around low-mass stars.

"Low-mass stars are appealing from an astrobiology point of view because there are so many of them," said Fred Ciesla of the University of Chicago. [5 Bold Claims of Alien Life]. The class of M dwarf stars— which weigh between 10 and 50 percent the mass of our sun — are the most common stars in our galaxy. A recent analysis of data from NASA's Kepler spacecraft showed that six percent of these red dwarf starslikely have habitable Earth-size planets.

Habitability in this case is defined by surface temperatures that are conducive to liquid water. But what if there is no water there to begin with? Indeed, previous research using computer simulations suggested that planets near M dwarfs might be devoid of water.

"These earlier papers definitely had an impact," Ciesla said. "They set the stage for current thinking for formation of planets."

An artist's depiction of a planet around an M dwarf star.
Credit: ESO/L. Calcada

View full size image

But Ciesla believes the problem should be looked at anew. Recent understanding about the formation of our solar system suggests that the source of planetary water is a complicated business that depends on the environment that the central star is born in. This could change the prognosis for water around M dwarfs.

Ciesla has funding from NASA’s Exobiology and Evolutionary Biology Program to re-explore the watery potential of small star planets.

No place like home

The way that water is generally thought to have arrived on Earth is by hitching a ride on space rocks and grains.

Early in our solar system's history, these rocks and grains were spread out in a large disk rotating around the sun. The material in the inner part of the disk was generally too warm to hold water, but rocks and grains beyond the so-called "frost line" at around 2.5 times the Earth-sun distance (which is 93 million miles, or 150 million kilometers) were cold enough to collect ice crystals.

The Earth and the other terrestrial planets presumably formed inside the frost line, so technically they should be dry. However, gravitational interactions in the outer part of the disk apparently stirred up the planetary ingredients, driving ice-carrying material into the inner solar system. The end result: an ocean-filled Earth brimming with life's possibility. [9 Exoplanets That Could Host Alien Life]

Could a similar process play out around other stars with less mass? In 2007, two separate research groups looked at this question. On computers, they basically re-did the planet-formation recipe but with half the ingredients. The results were disappointing for water enthusiasts. Planets in the habitable zone around M dwarf stars are dry, according to these models.

One of the reasons for this lack of water is that M dwarfs start out with smaller planetary disks. Researchers found that smaller disks have fewer gravitational interactions and therefore are less efficient at driving ice-carrying material into the inner disk region.  

Another knock against low-mass stars is that they are brighter when they are younger. This pushes the frost line farther out relative to the eventualhabitable zone.

"It becomes even harder to bring in water-bearing material when it starts so far out," Ciesla explained.

These are all sound arguments, based on a scaling down of our solar system, Ciesla said. But he wonders if something more than size differentiates our solar system from that of an M dwarf system. The difference, he thinks, may be aluminum.

Cooking with aluminum

Aluminum, in particular the isotope aluminum-26, may have played an important role in the delivery of water to the Earth.

Aluminum-26 is a radioactive isotope with a half-life of 700,000 years. Evidence from meteorites suggests that they — and the parent asteroids that they came from — once contained a significant amount of aluminum-26, which decayed long ago during the era of planet formation.

The decay of aluminum-26 likely had a major impact on the large space rocks, called planetesimals, that eventually glommed together to make planets. These mountain-sized planetesimals trapped the heat from aluminum-26 decays, and that heat would have melted ice carried by the planetesimal. Some of the ice-melted water would have reacted with the rock (producing the hydrated mineralsfound on many meteorites). But some of the water would have been lost to space. check the video here:http://www.space.com/20400-the-search-for-another-earth.html 

If this story is right, then aluminum "cooking" robbed Earth and the other terrestrial planets of some water and other volatiles that were destined for delivery. But this is not a general rule of planetary formation.

"Not every planetary system is going to have aluminum-26," Ciesla said.

Aluminum-26 is made in massive stars. It's not entirely clear how our solar system got so much of it, but it likely came from a nearby supernova or stellar wind.

Another planetary system may have had no such source for aluminum-26, so its planetesimals may have kept more of their water throughout the planet formation process.

Using sophisticated computer simulations, Ciesla is currently testing whether this extra water might make a difference around low-mass stars. He admits that there may be other effects that cancel out the influence of the wetter ingredients.

"It could be two steps forward, two steps back," Ciesla said.

Refilling theories

Sean Raymond, who worked on one of the earlier studies of planets around M dwarfs, agrees that the previous conclusions are outdated.

In the outer regions of the solar system — beyond an invisible boundary called the "frost line" — it was cold enough for lightweight volatiles such as water and ammonia to condense onto the nascent giant planets. Thus, the outer planets formed from not only rocks and metals, but collected volatiles to become gassy giants. In the relatively warm inner region, the smaller planets Mercury, Venus, Earth, and Mars formed mainly from rocks and metals.

He says other researchers have looked at the effects of aluminum-26, but he thinks Ciesla may be the first to take into account the increased mass of water-rich planetesimals and how that might increase gravitational interactions.

"I definitely think that if planetesimals around low-mass stars were wetter, then the planets that form would also be wetter, presumably wet enough for life," Raymond said. 

However, he and others have developed a new model for our solar system that seems to solve water delivery and other problems in planet formation research – without a role for aluminum. This so-called Grand Tack model  assumes that early on Jupiter crept inward toward the sun but was yanked back outward when Saturn formed. This migration of the gas giants presumably helped drive water-rich material toward the inner solar system where Earth and the other terrestrial planets were forming.

"If the formation of planets around low-mass stars includes migration, then water is also naturally delivered to Earth-like planets," Raymond said. "So, the idea that water delivery is a big problem for these planets is probably overstated."

In fact, there may be an abundance of water for these planets. If Ciesla is right about the effects of aluminum-26 and Raymond is right about migration, then we might expect many exoplanets to be "water worlds," Raymond said.

If so, then future missions, such as NASA's James Webb Space Telescope, might be able to spot water molecules in the atmospheres of some of these distant planets. That would give us a clearer understanding of the source for planetary water.

NASA | Downloads the Future

check the video here: https://www.youtube.com/watch?v=ptfLfrWI648&feature=player_detailpage

LLCD will be NASA's first-step in creating a high performance space-based laser communications system. The LLCD mission consists of space-based and ground-based components. The Lunar Laser Space Terminal (LLST) is an optical communications test payload to fly aboard the LADEE Spacecraft and it will demonstrate laser communications from lunar orbit.The ground segment consists of three ground terminals that will perform high-rate communication with the LLST aboard LADEE. The primary ground terminal, the Lunar Laser Ground Terminal (LLGT) is located in White Sands, NM and was developed by MIT/Lincoln Laboratory and NASA. The ground segment also includes two secondary terminals located at NASA/JPL's Table Mountain Facility in California and the European Space Agency's El Teide Observatory in Tenerife, Spain. The main goal of LLCD is proving fundamental concepts of laser communications and transferring data at a rate of 622 megabits per second (Mbps), which is about five times the current state-of-the-art from lunar distances. Engineers expect future space missions to benefit greatly from the use of laser communications technology.

'New Physics' Observations Challenge Standard Model of Universe

"Physics beyond the Standard Model" embraces the theoretical developments needed to explain the deficiencies of the Standard Model, such as the origin of mass, neutrino oscillations, matter–antimatter asymmetry, and the nature of dark matter and dark energy, as well as the fact that the Standard Model itself is inconsistent with general relativity, to the point that one or both theories break down within known space-time singularities like the Big Bang and black hole event horizons.

Now, new observations presented at the Europhysics Conference on High-Energy Physics in Grenoble, France, of the top quark -- the heaviest of all known fundamental particles -- could overturn the standard model.

Data from collisions at the Tevatron particle accelerator at Fermilab in Batavia, Illinois, suggest that some of the top quark's interactions are governed by an as-yet unknown force, communicated by a hypothetical particle not possible under the standard model called the top gluon. According to one interpretation, a top quark bound by to its anti-matter partner, the antitop, would act as a version of the elusive Higgs boson, conferring mass on other particles.

Regina Demina, a physicist at the University of Rochester in New York, and her colleagues analyzed eight years' worth of particle-collision data recorded by one of the Tevatron's two detectors, known as DZero. Top quarks produced during collisions can fly off in the direction of the accelerator's proton beam or its antiproton beam; Demina and her team discovered that more travel towards the proton beam than is predicted in the standard model of physics. A physics beyond the standard model appears to be needed to explain the discrepancy.

According to Nature.com, a possible new model was suggested by Christopher Hill, a theorist at Fermilab who 20 years ago but updated in 2003 proposed how a top quark and its antiparticle could impart mass to the W and Z bosons, particles that carry the weak nuclear force responsible for radioactive decay. The work rests on an analogy with some types of low-temperature superconductors, materials that have no electrical resistance at temperatures just a few degrees above absolute zero. In some superconductors, electrons pair up, bound by particle-like vibrations in the material. The bound electrons limit the range over which the electromagnetic force can act within the material, an effect that in turn imparts an effective mass to nearby photons -- particles of light, which carry the long-range electromagnetic force and are normally weightless.

In a similar manner, Hill suggested that top quarks and anti-top quarks might pair up throughout the cosmos, bound by a force carried by an as-yet undiscovered particle dubbed the top gluon.

"It's as if the entire universe was a special kind of superconductor," says physicist Matthew Schwartz of Harvard University in Cambridge, Massachusetts who shows in a study posted online on 16 June, Schwartz that Hill's model could also account for the top-quark asymmetry observed at the Tevatron. The details have to do with the way the up quark, a component of the proton, couples with the top quark in the new theory.

The theory, reports Nature, explains the origin of mass throughout the universe as a team effort, First, the top gluon would act to make both the top quark and the antitop heavy, just like the force binding electrons in a superconductor makes nearby photons heavy. Then, the top-anti-top pair would itself explain the origin of mass throughout the rest of the universe, conferring mass, for instance, on the W and Z bosons, the carriers of the weak nuclear force. The relatively heavy mass acquired by the W and Z particles limits the range of the weak force, breaking the symmetry between this force and the long-range electromagnetic force that theorists believe exists at very high energies.

The asymmetry observed at DZero is not certain enough to constitute proof of the existence of the top gluon, but it does independently match findings reported earlier this year by researchers at the Tevatron's other detector, the Collider Detector at Fermilab (CDF).

Schwartz's theory is easily testable. The top gluon has a predicted energy within the current range of the world's most powerful particle collider -- the Large Hadron Collider (LHC) near Geneva, Switzerland -- so it could be found within a year, says Schwartz.Dmitri Denisov, a spokesman for the DZero experiment, agrees that the results are similar to the directional preference of the top quark seen with CDF. He cautions, however, that the standard model of particle physics is so complicated that it is difficult to accurately describe with equations. The observed top-quark asymmetry is being compared to an imperfect surrogate for the true standard model, so the supposed discrepancy might fall within the uncertainty of the model.

A research team working with the LHC's Compact Muon Solenoid detector reported on 21 July that they see no evidence of the top-quark asymmetry. But Schwartz notes that the asymmetry is much harder to see at the LHC than at the Tevatron, because the LHC starts with an intrinsically symmetrical setup -- smashing a proton beam into another proton beam -- so it's more difficult to discern if the top quark has a directional preference at the LHC than at the Tevatron. "I suspect that you can't rule out anything with this data," he says, "and it doesn't negate any models."

The image at the top of the page suggests a surplus over Standard Model predictions of a type of particle decay called “B to D-star-tau-nu.” In this conceptual art, an electron and positron collide, resulting in a B meson (not shown) and an antimatter B-bar meson, which then decays into a D meson and a tau lepton as well as a smaller antineutrino. 

Magnetic loop on magnetar SGR 0418

MYSTERIOUS MAGNETAR BOASTS ONE OF STRONGEST MAGNETIC FIELDS IN UNIVERSE

Scientists using ESA’s XMM-Newton space telescope have discovered that a curious dead star has been hiding one of the strongest magnetic fields in the Universe all along, despite earlier suggestions of an unusually low magnetic field.The object, known as SGR 0418+5729 (or SGR 0418 for short), is a magnetar, a particular kind of neutron star.A neutron star is the dead core of a once massive star that collapsed in on itself after burning up all its fuel and exploding in a dramatic supernova event. They are extraordinarily dense objects, packing more than the mass of our Sun into a sphere only some 20 km across – about the size of a city.

A small proportion of neutron stars form and live briefly as magnetars, named for their extremely intense magnetic fields, billions to trillions of times greater than those generated in hospital MRI machines, for example. These fields cause magnetars to erupt sporadically with bursts of high-energy radiation.

SGR 0418 lies in our galaxy, about 6500 light years from Earth. It was first detected in June 2009 by space telescopes including NASA’s Fermi and Roscosmos’ Koronas-Photon when it suddenly lit up in X-rays and soft gamma rays. It has been studied subsequently by a fleet of observatories, including ESA’s XMM-Newton.

“Until very recently, all indications were that this magnetar had one of the weakest surface magnetic fields known; at 6 x 10¹² Gauss, it was roughly a 100 times lower than for typical magnetars,” said Andrea Tiengo of the Istituto Universitario di Studi Superiori, Pavia, Italy, and lead author of the paper published inNature.“Understanding these results was a challenge. However, we suspected that SGR 0418 was in fact hiding a much stronger magnetic field, out of reach of our usual analytical techniques.”

Magnetars spin more slowly than neutron stars, but still complete a rotation within a few seconds. The normal way of determining the magnetic field of a magnetar is to measure the rate at which the spin is declining. Three years of observations of SGR 0418 had led astronomers to infer a weak magnetic field.

The new technique developed by Dr Tiengo and his collaborators involves searching for variations in the X-ray spectrum of the magnetar over extremely short time intervals as it rotates. This method allows astronomers to analyse the magnetic field in much more detail and has revealed SGR 0418 as a true magnetic monster. “To explain our observations, this magnetar must have a super-strong, twisted magnetic field reaching 10¹⁵ Gauss across small regions on the surface, spanning only a few hundred metres across,” said Dr Tiengo.“On average, the field can appear fairly weak, as earlier results have suggested. But we are now able to probe sub-structure on the surface and see that the field is very strong locally.”A simple analogy can be made with localised magnetic fields anchored in sunspots on the Sun, where a change in configuration can suddenly lead to their collapse and the production of a flare or, in the case of SGR 0418, a burst of X-rays.

“The spectral data provided by XMM-Newton, combined with a new way of analysing the data, allowed us to finally make the first detailed measurements of the magnetic field of a magnetar, confirming it as one of the largest values ever measured in the Universe,” adds Norbert Schartel, ESA’s XMM-Newton Project Scientist.“We now have a new tool to probe the magnetic fields of other magnetars, which will help constrain models of these exotic objects.”