No Centauri Dreams posts this week — I’ll be back next Monday. I’ve been running hard and it’s time for a break. I’ll keep up with comment moderation as best I can, though I’m going to be trying to catch up with many long overdue commitments outside the interstellar field in coming days. As always, thanks to all for the continuing support. See you soon!
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.
I’ve always thought that the biggest driver for our next steps in space is the presence of asteroids. Asteroids affect us in two powerful ways, the first being that they are sources of potential wealth for companies like Deep Space Industries and Planetary Resources, as commercial operations use robotics and eventually humans to extract water and precious metals. Likewise significant is that near-Earth asteroids are a reminder that developing the tools for altering an asteroid trajectory is a good insurance policy for planetary protection.
Asteroids go past us all the time. I count eight that will move past the Earth between now and October 1, the closest — 2015 SO2 and 2015 DS53 — moving within 17 lunar distances. There’s nothing to worry about in this list, as all have zero chance of impacting the Earth. Looking ahead to the first 20 days of October, the closest pass will be by asteroid 462959, at 15 lunar distances. A lunar distance is 384,401 kilometers, and it’s how the Minor Planet Center tabulates these things. I’m also using the MPC’s assessment of impact risks.
Image: The Near-Earth asteroid Itokawa, from which the Japanese Space Agency (JAXA) returned samples in its Hayabusa mission. Credit: JAXA.
Asteroid impacts make for sensational speculation and the occasional overwrought movie, but I’d suggest the best approach to them is simple prudence. We have a mission — OSIRIS REx — on its way to an asteroid called Bennu to make numerous scientific investigations, teaching us about the early history of the Solar System. But the mission will also help us understand the factors that influence an asteroid’s trajectory, such as the Yarkovsky effect, which can slightly alter a small object’s movement because of the uneven heating it can experience. Learning more about such factors suggest better ways to nudge any errant asteroid in the future.
What I really favor is simply keeping asteroids in focus and educating the public about what we’re learning, so that we can get past the sensationalism and make better choices about future missions. It’s useful on that score to point to the Minor Planet Center’s Asteroid Data Explorer site, which allows viewers to quickly drill down into the actual data. If you want to rank asteroids by size, orbital period or closest approach, you can quickly summon up the needed information, with adjustable filters in place for fine-tuning. You can also rank nearby objects in terms of their potential for impact, invariably slight, but a number we always want to keep refining.
Operating at the Smithsonian Astrophysical Observatory, the Minor Planet Center tracks not just near-Earth objects but also comets and the outer satellites of the major planets. As to the term ‘minor planet’ itself, it refers to objects orbiting the Sun that are neither planets nor comets, which can be anything from a dwarf planet like Pluto to a Kuiper Belt Object or an asteroid, trojan or centaur. These turn up more quickly than I would have suspected, with the MPC listing 477 such objects discovered just this month. 45 Near-Earth Objects turned up in the same period, with 1244 NEOs discovered so far in 2016.
2015 DS53, a Near-Earth Object, passes within those 17 lunar distances I mentioned above in the early morning UTC on September 22. The list of close approaches maintained on the MPC site is a fascinating thing to track, reminding us how much our view of the Solar System has changed in the past half century. Few people in the 1950s would have imagined such a debris-laden system, with another belt of material outside the most distant known planet, and a vast halo of comets beyond that. And few would have thought there were so many NEOs.
As the Minor Planet Center reminds us, a known asteroid passes within a few million miles of our planet just about every day. And it has just introduced yet another way of tracking these through an online service called the Daily Minor Planet, which is being developed both by the MPC and volunteers from Oracle Corporation. Asteroid updates go straight to your inbox. Astronomer Matt Holman, director of the MPC, sees the project as an educational resource:
“Most people don’t realize how common asteroid flybys are. We want the Daily Minor Planet to educate readers in an entertaining way, so the next time they see a doom-and-gloom asteroid headline, they’ll know where to go to find the facts.”
This is an outreach project worth keeping an eye on. You can subscribe here. And I think Holman has it right when he refers to ‘asteroid headlines.’ The facts invariably knock the pins out from under the sensationalists who try to milk every natural phenomenon for controversy, but having a widely distributed fact-checking source like this one should tamp all that down.
What we need about asteroids is information and careful analysis. If some entrepreneurs are right, they’re going to play a large role in creating new wealth one day, and if I’m right, they’re going to act as a spur to help us develop the needed technologies to reach them quickly. What we learn will inevitably pay dividends in case we do discover a serious threat, which is why asteroids and their potential should play a major role in an informed space policy.
Comet 332P/Ikeya-Murakami has had a short but colorful history in our observations. First detected in 2010 by two amateur astronomers in Japan, the comet has been spinning off debris at least since 2015 and probably earlier. A large fragment, as big as Comet 332P itself, may have broken off in 2012. Still close to the comet, its discovery prompted a team led by David Jewitt (UCLA) to request time on the Hubble Space Telescope to study what was happening.
Among a long page of posted quotations on Jewitt’s UCLA website is this by Erwin Schrodinger: “The task is, not so much to see what no one has yet seen; but to think what nobody has yet thought, about that which everybody sees.” In this case, what everybody now sees is our most in-depth look at a comet’s disintegration ever. The trick is what to make of what we see.
The Hubble observations, taken in early 2016, show us 25 separate fragments, mixtures of dust and ice that are slowly separating from the comet at no more than the pace I make on my morning walk. The fragments make up about four percent of the parent comet, ranging in size from 20 meters to 60 meters. Comet 332P completes a rotation every two to four hours, and it’s thought that the fragments may be thrown from the surface by this spin, leaving them to fan out along a trail fully 4800 kilometers long.
Another quote Jewitt references on his web page comes from Heraclitus: “If you do not expect the unexpected, you will not find it; for it is hard to be sought out, and difficult.” A comet’s gradual death as it approaches the Sun would not be considered unexpected, but in this case what has to be sought out is the process. It turns out to be far from gentle. Says Jewitt:
“In the past, astronomers thought that comets die when they are warmed by sunlight, causing their ices to simply vaporize away. But it’s starting to look like fragmentation may be more important. In comet 332P we may be seeing a comet fragmenting itself into oblivion.”
Image: This NASA Hubble Space Telescope image reveals the ancient comet 332P/Ikeya-Murakami disintegrating as it approaches the sun. The observations represent one of the sharpest views of an icy comet breaking apart. The comet debris consists of a cluster of building-size chunks near the center of the image. They form a 4800-kilometer-long trail, larger than the width of the continental U.S. The fragments are drifting away from the comet, dubbed comet 332P, at a leisurely pace, roughly the walking speed of an adult.. Credit: NASA, ESA, David Jewitt/UCLA.
This is a short period comet that when discovered was 1.6 AU out, its detection taking place one month after perihelion in October of 2010. It has evidently survived a roughly 10 million year journey to the inner system, and Jewitt and colleagues estimate that it contains enough mass for about 25 more outbursts. With an orbital period of less than six years, we may be looking at a very short-lived spectacle, in astronomical terms. As Jewitt says:
“If the comet has an episode every six years, the equivalent of one orbit around the sun, then it will be gone in 150 years. It’s just the blink of an eye, astronomically speaking. The trip to the inner solar system has doomed it.”
The Hubble images captured 332P/Ikeya-Murakami breaking up almost 110 million kilometers from Earth, as comet and its debris orbit the Sun at approximately 80,000 kilometers per hour. The comet turns out to be smaller than originally thought, measuring about 500 meters across. Its fast rotation is probably the result of sunlight heating the surface, causing it to expel jets of dust and gas. The material we see in the image was probably shed between October and December of 2015.
The paper is Jewitt et al., “Fragmentation Kinematics in Comet 332P/Ikeya-Murakami,” published online by Astrophysical Journal Letters 15 September 2016 (abstract / preprint). David Jewitt’s entertaining selection of quotes can be found here. I can’t resist adding one last quote because it’s a personal favorite from Yogi Berra: “In theory there is no difference between theory and practice. In practice there is.”
