Centauri Dreams

We’ve seen circumstellar disks around numerous stars, significant because it is from such disks that planets are formed, and we would like to know a good deal more about how this process works. Now we have word of planet-forming disks around several low-mass objects that fit into the brown dwarf range, and one small star about a tenth the mass of the Sun. With the brown dwarfs, we’re working with objects small enough to be at the boundary between planet and star.

The work is led by Anne Boucher (Université de Montréal), whose team drew photometric data from the Two-Micron All-Sky Survey (2MASS) and the Wide-field Infrared Survey Explorer (WISE) mission, allowing the detection of the objects at infrared wavelengths. Boucher notes the strong attraction such objects hold for astronomers:

“Finding disks in low-mass systems is really interesting to us, because objects that exist at the lower limit of what defines a star and that still have disks that indicate planet formation can tell us a lot about both stellar and planetary evolution.”

Image: An artist’s conception of a planet-forming disk around a brown dwarf. Credit: Robin Dienel.

But let’s pause for a moment on the nature of the disks themselves. Dusty debris disks are considered to be signs of past planet formation, while gas-rich primordial disks mark the presence of active planet formation and accretion into the host star. Because disks can be detected by the presence of excess infrared, beyond what we would expect from the star by itself, we can use infrared observations to measure the differences in thermal emission from one infant system to another, showing us a range of disks at various stages of their evolution.

Thus we get massive primordial disks that are gas-rich and dense around younger systems, while dusty debris disks are colder and more depleted in gas, marking the remnants of planetary system formation even as they are replenished by the collisions of small objects in the system. These can last up to 100 million years; they’re colder and observable in the mid-infrared (primordial disks produce a strong signature in the near-infrared). We also see so-called ‘transition’ disks with an inner region depleted of dust, and an outer region still rich in it.

What Boucher and colleagues have been looking at are three objects ranging from 13 to 19 Jupiter masses, and a fourth of between 101 and 138 Jupiter masses. The paper makes the case that all four disk candidate stars are members of stellar associations — the TW Hydra, Columba, and Tucana-Horologium associations — allowing the researchers to draw conclusions about their age. A stellar association is a large grouping of stars similar in spectral type and origin, grouped more loosely than open or globular-type star clusters. These are young stars that have not had time to move any great distance from their place of formation.

The four disks are all thought to be in the planet-forming phase, none of them having attained the age of a dusty debris disk. Even so, two of these objects appear older — at between 42 and 45 million years — than we would normally associate with an active disk system. From the paper:

Disk temperatures and fractional luminosities were determined from this analysis. Their values indicate that the new disks are likely transitional or primordial. The spectral types of these new disk hosts are late (M4.5 to L0?), and correspond to low-mass BDs (13 ? 19 and 101 ? 138M_Jup) of young ages (? 7 ? 13 and ? 38 ? 49Myr). These four systems join a relatively small sample of low-mass objects known to harbor a circumstellar disk. There is still a lot to learn about primordial and (pre-)transitional disks around low-mass stars, and these four new candidates could play an important role to shed light on the formation and evolution processes of stars and planetary systems.

Also in question is the fraction of brown dwarfs that have developed planetary systems. Finding debris disks around young objects like these is one way into the problem, for we can watch the process of planet formation unfold around objects of different ages. Bear in mind as well that their lower luminosity means objects like these could be interesting targets for exoplanet searches using direct imaging methods. All four of the new disk candidates should likewise be prime candidates, the paper suggests, for the Atacama Large Millimeter Array.

The paper is Boucher et al., “BANYAN. VIII. New Low-Mass Stars and Brown Dwarfs with Candidate Circumstellar Disks,” accepted at The Astrophysical Journal (preprint).

A circumbinary planet is one that orbits two stars, and to date we haven’t found many of them. Word of a new detection comes from an event observed back in 2007 during a microlensing study called OGLE — Optical Gravitational Lensing Experiment. OGLE is a Polish undertaking designed to study dark matter using gravitational microlensing, but while dark matter remains as dark as ever, the project has been able to deliver useful findings on distant exoplanets.

A number of groups specializing in gravitational microlensing also contributed to this analysis. These are observation efforts not as well known to the public as Kepler or Gaia, but they’re doing exceptional work: MOA (Microlensing Observations in Astrophysics); MicroFUN (Microlensing Follow-Up Network); PLANET (Probing Lensing Anomalies NETwork); and Robonet. Subsequent Hubble Telescope data were then applied to the analysis, confirming the discovery.

Image: This artist’s illustration shows a gas giant planet circling a pair of red dwarf stars. The Saturn-mass planet orbits roughly 480 million kilometers from the stellar duo. The two red dwarf stars are a mere 11 million kilometers apart. The illustration is based on Hubble Space Telescope observations that helped astronomers confirm the existence of a planet orbiting two stars in the system OGLE-2007-BLG-349, located 8,000 light-years away. Credit: NASA, ESA, and G. Bacon (STScI).

The distance of the stars in question — some 8000 light years away in the direction of galactic center — raises the issue of microlensing’s unique capabilities. Gravitational microlensing happens when a foreground star moves in front of a background star, causing the gravity of the former to ‘bend’ the light of the latter. The ‘bending’ is actually the light following the curvature of spacetime caused by the foreground star’s gravitational field, but whatever we call it, gravitational microlensing can help us detect planets that would otherwise be lost to us, for how the light is magnified offers many clues to the foreground star and any possible planets.

Both radial velocity and transit methods detect planets most readily when they are near to their star. A planet orbiting close in makes multiple transits in a shorter period of time than one further out, which is why the early Kepler detections were of planets like this, with the boundary of detection gradually moving outward with time. Radial velocity, which looks at the Doppler shift of light from the star as it is tugged by the orbiting planet, likewise shows its strongest signal for a close-in, large planet, and that’s why we had so many early detections of ‘hot Jupiters.’

But gravitational microlensing is another thing entirely, capable of finding planets and stars at a wide range of orbital distances around stars that can be many thousands of light years away. The OGLE-2007-BLG-349 event produced a light curve that needed additional data from the Hubble Space Telescope to confirm, allowing astronomers to separate the background source star and the lensing star in the crowded starfield.

What we wind up with is a planet orbiting about 480 million kilometers from its host stars, both of them red dwarfs no more than 11 million kilometers apart. David Bennett (NASA GSFC), lead author on the paper discussing the discovery, describes the process used in the work:

“The ground-based observations suggested two possible scenarios for the three-body system: a Saturn-mass planet orbiting a close binary star pair or a Saturn-mass and an Earth-mass planet orbiting a single star… [T]he model with two stars and one planet is the only one consistent with the Hubble data. We were helped in the analysis by the almost perfect alignment of the foreground binary stars with the background star, which greatly magnified the light and allowed us to see the signal of the two stars.”

Unlike Kepler’s ten discovered circumbinary planets, the two orbiting OGLE-2007-BLG-349 are much further away from their hosts. The paper discusses another two-star plus one planet event that has been interpreted as a planet orbiting one of the two stars (OGLE-2013-BLG-0341), and a second event dominated by a stellar binary signal, adding:

This suggests that there should be many more two-star plus one planet events in the data that we have already collected, but that we are not efficient at finding planetary signals in events that are dominated by stellar binary microlensing features. So, we recommend a systematic search for planetary signals in the light curves of strong stellar binary events.

As you might expect, the paper for today has a long list of co-authors. It’s Bennett et al., “The First Circumbinary Planet Found by Microlensing: OGLE-2007-BLG-349L(AB)c,” accepted at The Astronomical Journal and available as a preprint.

Proxima Centauri b, that highly interesting world around the nearest star, is about 0.05 AU out from its primary. The figure leaps out to anyone new to red dwarf stars, because it’s so very close to the star itself, well within the orbit of Mercury in our own system. But these are small, dim stars compared to our Sun, and hugging the star is essential to remain in the habitable zone. That also makes for very short years — Proxima b completes an orbit every 11.2 days.

Guillem Anglada-Escudé and colleagues reminded us in the discovery paper that among the many things we have to ask about this planet is whether or not it has a strong magnetic field. Because Proxima Centauri is known for flare activity, not to mention 400 times the X-ray flux the Earth receives. A magnetic field could help the planet hang on to its atmosphere, but just how strong would it need to be? Like any M-dwarf planet, then, Proxima b seems vulnerable.

This thinking has ramifications much closer to home. We are learning that CMEs have influenced the atmosphere of Mars and may have played a large role in how it evolved. Data from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft point in this direction, showing that a CME can compress the Martian magnetosphere, spinning off effects in the ionosphere and below. We are now in the realm of what is being called ‘space weather.’

Think about what happens here on Earth when the Sun throws off one of its enormous storms, a coronal mass ejection, or CME. Outflowing plasma can thoroughly disrupt communications and navigation equipment, with damage to satellites and even power blackouts, all this with our own planet’s magnetic field around us. And although stars like Proxima Centauri are much smaller than ours, it turns out that they are far more active, with CMEs often ten times more powerful.

Image: On August 31, 2012 a long filament of solar material that had been hovering in the sun’s atmosphere, the corona, erupted out into space at 4:36 p.m. EDT. The coronal mass ejection, or CME, traveled at over 1450 kilometers per second. The CME did not travel directly toward Earth, but did connect with Earth’s magnetic environment, or magnetosphere, with a glancing blow. causing aurora to appear on the night of Monday, September 3. Credit: NASA GSFC.

Add into the mix the fact that M-dwarfs can maintain high levels of magnetic activity for billions of years. Although Proxima Centauri is thought to be roughly the age of the Sun, about five billion years, it is clearly an active star. All of this is troubling for the prospects for astrobiology.

Now we have new work led by Christina Kay (NASA GSFC and Boston University) that makes an intriguing case. The probability of a habitable zone planet around an M-dwarf being hit by a CME may depend on the plane of the planet’s orbit. The work revolves around modeling of coronal mass ejections from the M-dwarf V374 Peg, allowing Kay and team to assess the effects of CMEs on a planet in the star’s habitable zone. The model is called Forecasting a CME’s Altered Trajectory (ForeCAT), and it predicts how a CME can wind up being deflected.

Now we get into interesting territory, for these models show that CME’s move in particular directions when deflected by the star’s magnetic field. A key factor here is what is known as the Astrospheric Current Sheet, which is the location of the minimum level of the background magnetic field. A strong magnetic field like that of V374 Peg can trap a CME at the Astrospheric Current Sheet, with the deflection depending primarily on the CME mass but also on its speed.

ForeCAT was used to analyze these interactions both in the case of a planet in the habitable zone of V374 Peg as well as a hot Jupiter orbiting a Sun-like star. The paper describes an intriguing result:

For both habitable zone mid-type M dwarf exoplanets and hot Jupiters [orbiting solar-type hosts] the probability of impact decreases if the exoplanet’s orbit is inclined with respect to the Astrospheric Current Sheet. The sensitivity to the inclination is much greater for the mid-type M dwarf exoplanets due to the extreme deflections to the Astrospheric Current Sheet. For low inclinations we find a probability of 10% whereas the probability decreases to 1% for high inclinations.

In other words, planetary orbits that line up with the astrospheric current sheet, which is generally aligned with the star’s equator, have a higher probability of being hit by a CME than planets in higher-inclination orbits. All of this has significant potential for affecting habitability:

From our estimation of 50 CMEs per day, we expect habitable mid-type M dwarf exoplanets to be impacted 0.5 to 5 times per day, 2 to 20 times the average at Earth during solar maximum. The frequency of CME impacts may have significant implications for exoplanet habitability if the impacts compress the planetary magnetosphere leading to atmospheric erosion.

At stake here is the minimum planetary magnetic field needed to retain an atmosphere (Kay and team believe a magnetosphere twice the size of the planetary radius is necessary). For mid-type M-dwarfs like V374 Peg, magnetic fields between tens to hundreds of Gauss are required to protect an exoplanet in the habitable zone. This is one to two orders of magnitude more than that of the Earth. The conclusion is stark: “We expect that rocky exoplanets cannot generate sufficient magnetic field to shield their atmosphere from mid-type M dwarf CMEs.”

The good news: The scientists argue that the minimum magnetic field strength will change depending on the M-dwarf’s spectral type, as well as on stellar activity and stellar magnetic field strength changes. Some types of M-dwarf may thus be more likely to retain an atmosphere than dwarfs like V374 Peg. Extending this work in their direction is a compelling next step.

The paper is Kay et al., “Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-Like Stars,” Astrophysical Journal Vol. 826, No. 2 (abstract / preprint). An AAS Nova essay on this work is also available.