Centauri Dreams
Imagining and Planning Interstellar Exploration
Learning More about Outer System Planets
What kind of planets are most common in the outer reaches of a planetary system? It’s a tricky question because most of the data we’ve gathered on exoplanets has to do with the inner regions. Both transit and radial velocity studies work best with large planets near their stars. But a new gravitational microlensing study looks hard at outer system planets, finding that planets of Neptune’s mass are those most likely to be found in these icy regions.
It should be no surprise that gravitational microlensing has produced few planets, about 50 so far, compared to the thousands detected through transit studies and radial velocity methods. After all, microlensing relies upon alignments that are far more unusual than even the transit method, in which a planet crosses the face of its star as seen from Earth. In microlensing, astronomers look for rare alignments between a distant star and one much nearer.
Given the right alignment, the ‘bending’ of spacetime caused by the nearer star’s mass allows researchers to study changes in the brightness of the background star, which can be clues to the existence of a planet. Microlensing can see not just planets close to their host stars but those far distant from the primary. Moreover, as the new work points out, we can use microlensing to figure out the mass ratio of the planet to the host star, and in about 40 percent of events, we can measure the mass of the host star and planet themselves.
A team led by Daisuke Suzuki (NASA GSFC) identified 1474 microlensing events between 2007 and 2012, drawing on data from the Microlensing Observations in Astrophysics (MOA) project, a collaboration between Japanese and New Zealand researchers that alerted astronomers to 3300 potential microlensing events in this time period. The analysis also incorporates data from the Optical Gravitational Lensing Experiment (OGLE).
Suzuki and colleagues homed in on the frequency of planets compared to the mass ratio of planet and star and the distances between them. A typical planet-hosting star is about 60 percent of the mass of the Sun. Its typical planet is between 10 and 40 times the mass of the Earth. By comparison, Neptune is about 17 Earth masses, while Jupiter is 318 times as massive as the Earth. Cold Neptune-mass worlds are thus identified as the most common kinds of planets beyond the ‘snow line,’ the point in a planetary system beyond which water remained frozen during planetary formation. In our Solar System, the snow line is at about 2.7 AU, roughly the middle of the main asteroid belt.
The paper surveys stars toward the galactic bulge, where the chances of a microlensing alignment are highest. Says Suzuki:
“We’ve found the apparent sweet spot in the sizes of cold planets. Contrary to some theoretical predictions, we infer from current detections that the most numerous have masses similar to Neptune, and there doesn’t seem to be the expected increase in number at lower masses. We conclude that Neptune-mass planets in these outer orbits are about 10 times more common than Jupiter-mass planets in Jupiter-like orbits.”
Image: This graph plots 4,769 exoplanets and planet candidates according to their masses and relative distances from the snow line, the point where water and other materials freeze solid (vertical cyan line). Gravitational microlensing is particularly sensitive to planets in this region. Planets are shaded according to the discovery technique listed at right. Masses for unconfirmed planetary candidates from NASA’s Kepler mission are calculated based on their sizes. For comparison, the graph also includes the planets of our solar system. Credit: NASA’s Goddard Space Flight Center
So based on the MOA data, planets forming in the outer reaches of a planetary system are likely to be Neptunes. But remember the limitations of the data here — we have relatively few detected exoplanets, and in fact, only 22 planets (with a possible 23rd) show up in the 1474 MOA events. What’s heartening is how we are going to go about expanding that dataset.
Tightening up the constraints on mass and distance to the lens systems will ultimately allow us to measure what the paper calls the ‘microlensing parallax effect,’ determining the distance of the system with the help of space telescopes far from the Earth. From the paper:
The ultimate word on the statistical properties of planetary systems will be achieved from the space based exoplanet survey (Bennett & Rhie 2002) of the WFIRST (Spergel et al. 2015) mission, and hopefully also the Euclid (Penny et al. 2013) mission. The high angular resolution of these space telescopes will allow mass and distance determinations of thousands of exoplanets because it will be possible to detect the lens star and measure the lens-source relative proper motion with the high resolution survey data itself. This will give us the same comprehensive picture of the properties of cold exoplanets that Kepler is providing for hot planets.
WFIRST (Wide Field Infrared Survey Telescope) was formally designated as a NASA mission at the beginning of this year. To be launched in the mid-2020s, it will carry a 288 megapixel multi-band near-infrared camera and a coronagraph for the suppression of starlight. ESA’s Euclid mission, like WFIRST, has a gravitational microlensing component, with a launch date in late 2020. If we can use space-based resources like these to enrich our microlensing catalog, our understanding of the outer precincts of exoplanetary systems will surge.
The paper is Suzuki et al., “The Exoplanet Mass-Ratio Function from the MOA-II Survey: Discovery of a Break and Likely Peak at a Neptune Mass,” Astrophysical Journal Vol. 833, No. 2 (13 December 2016). Abstract / preprint.
A New Look at Ice on Ceres
Ceres, that interesting dwarf planet in the asteroid belt, is confirmed to be just as icy as we had assumed. In fact, a new study of the world, led by Thomas Prettyman (Planetary Science Institute), was the subject of a press conference yesterday at the American Geophysical Union fall meeting in San Francisco. Prettyman and team used data from the Dawn spacecraft’s Gamma Ray and Neutron Detector (GRaND) instrument to measure the concentrations of iron, hydrogen and potassium in the uppermost meter of Ceres’ surface.
Prettyman, who is principal investigator on GRaND, oversees an instrument that works by measuring the number and energy of gamma rays and neutrons coming from Ceres. The neutrons are the result of galactic cosmic rays interacting with the surface, some of them being absorbed while others escape. The number and kind of these interactions allows researchers to investigate surface composition. Hydrogen on Ceres is thought to be in the form of frozen water, allowing the researchers to study the global distribution of ice.
The result of the GRaND study: The elemental data show that the materials were processed by liquid water within the interior. The top layer of Ceres’ surface is hydrogen rich, with the higher concentrations found at mid- to high latitudes, a finding consistent with near surface water ice, with the ice table closest to the surface at the higher latitudes. Says Prettyman:
“On Ceres, ice is not just localized to a few craters. It’s everywhere, and nearer to the surface with higher latitudes. These results confirm predictions made nearly three decades ago that ice can survive for billions of years within a meter of the surface of Ceres. The evidence strengthens the case for the presence of near-surface water ice on other main belt asteroids.”
Image: This image shows dwarf planet Ceres overlaid with the concentration of hydrogen determined from data acquired by the gamma ray and neutron detector (GRaND) instrument aboard NASA’s Dawn spacecraft. The hydrogen is in the upper yard (or meter) of regolith, the loose surface material on Ceres. The color scale gives hydrogen content in water-equivalent units, which assumes all of the hydrogen is in the form of H2O. Blue indicates where hydrogen content is higher, near the poles, while red indicates lower content at lower latitudes. In reality, some of the hydrogen is in the form of water ice, while a portion of the hydrogen is in the form of hydrated minerals (such as OH, in serpentine group minerals). The color information is superimposed on shaded relief map for context. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI.
But we have no solid ice layer here. Instead, Ceres’ surface appears to be a porous mixture of rocky materials, with ice filling the pores, as this Institute for Astronomy (University of Hawaii) news release makes clear. The GRaND findings show about 10 percent ice by weight.
Also interesting is that the elemental composition of Ceres differs from CI and CM carbonaceous chondrite meteorites, which represent some of the most primitive, undifferentiated meteorites we know (Cl and CM are two of several different subgroupings within the carbonaceous chondrite family). These meteorites were also altered by water, but the GRaND data tell us that their parent body would have differed markedly from Ceres.
The researchers offer two explanations, the first being that large scale convection occurring within Ceres may have separated ice and rock components, leaving the surface with a different composition than the bulk of the object. The other possibility is that Ceres formed in a different location in the Solar System than the parent object of this class of meteorite.
A second paper on Ceres has also appeared, this one in Nature. It is the work of Thomas Platz (Max Planck Institute for Solar System Research, Göttingen) and colleagues, who focus on craters that are found in persistently shadowed regions. These ‘cold traps’ are cold enough (about 110 K) that little of their ice turns into vapor. Bright material found in some of these craters is thought to be ice, and Dawn’s infrared mapping spectrometer has indeed confirmed ice in at least one.
As is the case with the Moon and Mercury, ice in such cold traps is thought to be the result of impacting bodies, although solar wind interactions are also a possibility. Each of these bodies has a small tilt compared to its axis of rotation, producing numerous permanently shadowed craters. “We are interested in how this ice got there and how it managed to last so long,” said co-author Norbert Schörghofer (University of Hawaii at Manoa). “It could have come from Ceres’ ice-rich crust, or it could have been delivered from space.”
But the comparison between what we find on Ceres and elsewhere in the Solar System reminds us how much we still have to learn about the process. From the paper::
The direct identification of water-ice deposits in PSRs [permanently shadowed regions] on Ceres builds on mounting evidence from Mercury and the Moon that PSRs are able to trap and preserve water ice. For the Moon, the abundance and distribution of cold-trapped ice is little understood. On Mercury, the cold traps are filled with ice, and the planet traps about the same fraction of exospheric water as Ceres, so either the PSRs on Ceres are not able to retain as much water ice as those on Mercury or the amount of available water is much lower.
The Prettyman paper is “Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy,” published online by Science 15 December 2016 (abstract). The Platz paper is “Surface water-ice deposits in the northern shadowed regions of Ceres,” published online by Nature Astronomy 15 December 2016 (abstract). Video of the press briefing at the AGU meeting can be accessed here.
Surviving the Journey: Spacecraft on a Chip
If Breakthrough Starshot can achieve its goal of delivering small silicon chip payloads to Proxima Centauri or other nearby stars, it will be because we’ve solved any number of daunting problems in the next 30 years. That’s the length of time the project’s leaders currently sketch out to get the mission designed, built and launched, assuming it survives its current phase of intense scrutiny. The $100 million that currently funds the project will go into several years of feasibility analysis and design to see what is possible.
That means scientists will work a wide range of issues, from the huge ground-based array that will propel the payload-bearing sails to the methods of communications each will use to return data to the Earth. Also looming is the matter of how to develop a chip that can act as all-purpose controller for the numerous observations we would like to make in the target system.
If the idea of a spacecraft on a chip is familiar, it’s doubtless because you’ve come across the work of Mason Peck (Cornell University), whose work on the craft he calls ‘sprites’ has appeared many times in these pages (see, for example, Sprites: A Chip-Sized Spacecraft Solution). Both Peck and Harvard’s Zac Manchester, who worked in Peck’s lab at Cornell, have been active players in Breakthrough Starshot’s choice of single-chip payloads and continue to advise the project.
Image: A small fleet of ‘sprites,’ satellites on a chip, as envisioned in low Earth orbit. Can single-chip spacecraft designs now be developed into payloads for an interstellar mission? Credit: Space Systems Design Studio.
Meanwhile, NASA itself has been working with the Korea Institute of Science and Technology (KAIST) on the design of single-chip spacecraft. A key issue, discussed at the International Electron Devices Meeting in San Francisco in early December, is how to keep such a chip healthy given the hazards of deep space. For Starshot, the matter involves not just the few minutes of massive acceleration (over 60,000 g’s) of launch from Earth orbit, but the 20 years of cruise time at 20 percent of the speed of light before reaching the target star.
The first part of the question seems manageable, as hardening electronics against huge accelerations is an area well studied by the military, so data are abundant. The cruise phase, though, opens up concerns about radiation. According to KAIST’s Yang-Kyu Choi, interstellar radiation can degrade performance through the accumulation of positively charged defects in the silicon dioxide depths of the chip. Such defects can produce anomalous current flow and changes to the operation of critical transistors. The matter of malfunctioning chips is discussed in this recent story in IEEE Spectrum.
At the San Francisco meeting, self-healing chips were the theme, drawing on work that comes out of the 1990s that showed heating could help radiation sensors recover their functionality. Mixing this with work on flash memory out of Taiwan’s Macronix International, an integrated device manufacturer in the Non-Volatile Memory (NVM) market, the new NASA study uses concepts developed at KAIST to make on-chip healing more efficient. From the IEEE story:
This study uses KAIST’s experimental “gate-all-around” nanowire transistor. Gate-all-around nanowire transistors use nanoscale wires as the transistor channel instead of today’s fin-shaped channels. The gate, the electrode that turns on or off the flow of charge through the channel, completely surrounds the nanowire. Adding an extra contact to the gate allows you to pass current through it. That current heats the gate and the channel it surrounds, fixing any radiation-induced defects.
It might seem natural to simply provide more shielding for the chip during the two decades of interstellar cruise, but shielding adds mass, a critical issue when trying to drive a payload to a significant fraction of the speed of light. Thus the self-healing alternative, which assumes potential damage but provides self-analysis of the problem and heat inside the chip to work the healing magic. We also gain from the standpoint of further miniaturization — at scales of tens of nanometers, nanowire transistors are significantly smaller than the kind of transistors on chips currently used in spacecraft, adding savings in chip size and weight.
According to the IEEE report, KAIST’s “gate-all-around” device is likely to see wide production in the early 2020s at it begins to replace the older FinFET (Fin Field Effect Transistor) technologies. From the standpoint of single-chip spacecraft, it’s heartening to learn that radiation repairs can be made over and over, with flash memory recovered up to 10,000 times. A scenario emerges in which a chip on an interstellar flight can be powered down, heated internally to restore full performance, and then restored to service.
Pondering interstellar performance for chips that weigh no more than a gram is cause for reflection. Within just a few years we’ve gone from the idea of massive fusion-driven designs like Project Daedalus to payloads smaller than smartphones. The idea invariably brings to mind Robert Freitas’ concept of a ‘needle’ probe that could be sent in swarms to nearby stars, loaded with nanotech assemblers that would construct scientific instruments and communications devices out of material they found in the destination system.
It wasn’t so long ago that former NASA administrator Dan Goldin was speaking of a probe as light as a Coke can, but the Freitas probe and Breakthrough Starshot go well beyond that. The trick here is not getting too far ahead of the curve of technological development. With a 30-year window, Starshot can anticipate breakthroughs that will solve some of its key challenges, but relying on the future to plug in a solution doesn’t always go as planned. Thus it’s heartening to see potential answers to the cruise problem already beginning to emerge.
OSIRIS-REx to Hunt for Earth ‘Trojans’
The so-called ‘trojan’ asteroids that cluster at 60° ahead and behind the planet Jupiter make up a surprisingly populous category. Consider that thus far we have found only one trojan at Earth’s Lagrangian points, while over 6000 have been discovered in Jupiter’s orbit. The total number of trojans larger than 1 km in diameter associated with Jupiter has been estimated to be about 1 million, which matches up well with objects of equivalent size in the main asteroid belt. These days ‘trojans’ can also refer to similar bodies associated with other planets. We know, for example, of about 20 trojans involved with Neptune.
That solitary Earth trojan, 2010 TK7, was discovered oscillating around Earth’s L4 Lagrangian point in 2010 by the NEOWISE team using NASA’s Wide-field Infrared Survey Explorer spacecraft. The object has a diameter of about 300 meters; its oscillations take it back and forth on a nearly 400 year cycle that at times puts it close to opposite the Sun with respect to the Earth. On the other side of its elongated loop of the L4 point, it can close to within about 20 million kilometers of Earth.
What catches the eye here is that 2010 TK7‘s orbit may have, within the past 2000 years, oscillated about the L5 Lagrangian point instead of L4. This seems to be an unstable libration, a fact that makes predicting its future course problematic. That’s why I’m interested in what scientists at the University of Arizona are doing with the OSIRIS-REx mission. Between February 9 and 20, the spacecraft’s onboard camera suite will be activated to search for other Earth trojans.
Image: In February 2017, the OSIRIS-REx spacecraft will undertake a search for Earth-Trojan asteroids while on its outbound journey to the asteroid Bennu. Earth trojans are asteroids that share an orbit with Earth while remaining near a stable point 60 degrees in front of or behind the planet. (Illustration: OSIRIS-REx/UA).
The problem with Earth’s trojans, assuming there is more than one, is that they are hard to find because from the Earth’s vantage point, they appear close to the Sun. OSIRIS-REx will deploy its MapCam imager to scan the regions where we might expect trojans to exist. The observations have a tactical purpose as well — consider them a warm-up for the encounter with the asteroid Bennu (1999 RQ36), the target of OSIRIS-REx. Checking for rocky material near Bennu will be critical as the spacecraft closes in for surface mapping in 2018. The mission will also attempt to return a sample of the asteroid to the Earth.
From a University of Arizona news release:
“The Earth-Trojan asteroid search provides a substantial advantage to the OSIRIS-REx mission,” said Dante Lauretta, OSIRIS-REx principal investigator and professor of planetary science at the Lunar and Planetary Laboratory. “Not only do we have the opportunity to discover new members of an asteroid class, but more importantly, we are practicing critical mission operations in advance of our arrival at Bennu, which ultimately reduces mission risk.”
So we may learn soon whether more Earth trojans exist, perhaps choosing one day to mine them for rare elements, although 2010 TK7 is not itself a good candidate for such operations (at least early on) because of its highly inclined orbit. Other near-Earth asteroids present better options.
Image: The inner Solar System, from the Sun to Jupiter. Also includes the asteroid belt (the white donut-shaped cloud), the Hildas (the orange “triangle” just inside the orbit of Jupiter), the Jupiter trojans (green), and the near-Earth asteroids. The group that leads Jupiter are called the “Greeks” and the trailing group are called the “Trojans” (Murray and Dermott, Solar System Dynamics, pg. 107). Credit: Wikimedia Commons.
And on the matter of nearby objects, you’ll recall 3753 Cruithne, a 5 kilometer wide asteroid whose orbit, an overlapping horseshoe, is not related to the kind of oscillations around the Lagrangian points associated with trojans. Another non-trojan near-Earth companion is (469219) 2016 HO3, considered by scientists to be a ‘quasi-satellite’ rather than a trojan. These asteroids are on curious and interesting orbits, and the suspicion is that we’ll find a good many more such objects in coming days.
Tidal Disruption by Black Hole?
The supernova considered to be the brightest ever recorded may have been evidence of something even more exotic. The explosion was caught by the All Sky Automated Survey for SuperNovae (ASAS-SN), the event itself dubbed ASASSN-15h. Yesterday we looked at what happens to a star roughly as massive as the Sun as it goes through a red giant phase and becomes a white dwarf, but stars significantly more massive than the Sun take no such route. A star a minimum of 8 times the mass of the Sun can explode as a Type II supernova.
But is that what ASASSN-15h really was? Detected in 2015 in a galaxy about 4 billion light years from Earth, the event has now been the subject of new work by an international team led by Giorgos Leloudas (Weizmann Institute of Science, Israel) and the Dark Cosmology Centre (Denmark). From this we get a new explanation: ASASSN-15h may have been the result of a rapidly spinning supermassive black hole tearing a relatively low mass star apart. The passing star, in other words, lacked the mass to become a supernova, but the tidal disruption caused by the black hole led to its extreme outburst.
Image: An artist’s depiction of a rapidly spinning supermassive black hole surrounded the rotating leftovers of a star that was ripped apart by the tidal forces of the black hole.
Photo Credit: ESO, ESA/Hubble, M. Kornmesser.
A spectacular tidal event in the center of a galaxy sounds like something out of a Greg Benford novel, but in addition to the intrinsic fascination of ASASSN-15h comes a way to use it to delve into the physics of a black hole. For the mass of the host galaxy implies a black hole of at least 100 million solar masses, one that would be too large to disrupt stars outside its event horizon. As explained in the paper on this work, stars can only be disrupted outside the horizon of a supermassive black hole if the black hole is below a certain size — larger supermassive black holes, in other words, do disrupt stars, but by swallowing them whole.
We learn, though, that a spinning black hole — a so-called Kerr black hole, after Roy Kerr, the New Zealand mathematician who worked out the mathematics of such objects — would allow the disruption to occur, producing what had been thought to be a supernova. We wind up with a new tool for the exploration of extreme phenomena, as the paper explains:
… the typical tidally-disrupted star comes from the lower end of the stellar mass function, and this hypothesis [of a supernova] is further challenged by the old age of the galaxy’s stellar population. Observations of active galactic nuclei suggest that rapid SMBH [supermassive black hole] spins are common. We demonstrated here that TDEs [tidal disruption events] present a method to probe the SMBH spins of quiescent galaxies. Given the inferred rapid spin of the SMBH, the fact that we did not detect a jet at radio wavelengths implies that black hole spin alone is not sufficient to launch powerful jets.
The researchers agree that their evidence does not allow absolute certainty that ASASSN-15lh was a tidal disruption event. But there are sound reasons for thinking it so, including the location of the event in a red, massive galaxy of the kind not known to host superluminous supernova explosions, which usually occur in younger, star-forming dwarf galaxies. Moreover, their 10 months of observations from telescopes including the Hubble instrument, ESO’s Very Large Telescope and the the Las Cumbres Observatory Global Telescope showed three phases consistent with a tidal disruption event, including a re-brightening in ultraviolet light, that reduce the chances that this was a supernova.
“Even with all the collected data we cannot say with 100% certainty that the ASASSN-15lh event was a tidal disruption event,” adds Leloudas in this ESO news release. “But it is by far the most likely explanation.”
The paper is Leloudas et al., “The Superluminous Transient ASASSN-15lh as a Tidal Disruption Event from a Kerr Black Hole,” accepted by Nature Astronomy (preprint).
Glimpsing Our Solar System’s Future
The star L2 Puppis (HD 56096), a red giant in the direction of the southern constellation Puppis (the Poop Deck), is the subject of interesting new investigations using data from the ALMA array in Chile. The star appears to belong on the asymptotic giant branch of the Hertzsprung-Russell diagram, a category dominated by highly evolved cool stars. The new study sees L2 Puppis as an analog for what our own Sun will become in billions of years. Thus Ward Homan (KU Leuven Institute of Astronomy, Belgium):
“We discovered that L2 Puppis is about 10 billion years old. Five billion years ago, the star was an almost perfect twin of our Sun as it is today, with the same mass. One third of this mass was lost during the evolution of the star. The same will happen with our Sun in the very distant future.”
Image: Composite view of L2 Puppis in visible light | © P. Kervella et al. (CNRS/U. de Chile/Observatoire de Paris/LESIA/ESO/ALMA).
But L2 Puppis is more than just an interesting glimpse at what our Sun could become. It also offers a view of the fate of our own planet in the form of what may be an exoplanet discovered orbiting the star about 300 million kilometers out. Because as the Sun eventually moves into the red giant phase and grows more than a hundred times larger than it is today, Mercury and Venus will be destroyed, but the Earth may just hang on as a rocky core eventually orbiting a white dwarf. How the red giant phase affects planets in a system is what the L2 Puppis study is all about. Will a future Earth survive, and will it, as seems likely, be lifeless?
Asymptotic giant branch stars are undergoing a transition from red giants into the white dwarf remnants they will leave behind, meaning there is extreme loss of mass through a strong stellar wind. The resulting white dwarf at L2 Puppis should be about the size of the Earth, though compressed to the point where a single teaspoon weighs 5 tons. Stars like this also go through extreme changes in brightness and temperature, but studying the effects of these changes on their planetary systems is tricky because the planets are becoming embedded in a late-phase circumstellar envelope that can obscure observation.
At a distance of just over 200 light years, the circumstellar dust disk surrounding L2 Puppis is seen almost edge-on. The new work has allowed astronomers to arrive at a mass estimate for the star that, when adjusted through evolutionary models, shows it to have had a mass very similar to the Sun when it was on the main sequence. At 10 billion years old, the star also shows signs of a companion that, based upon its estimated mass is either a planet or a low-mass brown dwarf now accreting material from the star’s stellar wind.
It’s too early to call this a planet because the researchers have no firm lower limit on its mass, but this is an interesting object with an orbital period of about 5 years. From the paper:
From its observed properties, L2 Pup and its companion emerge as a plausible analog of the solar system at an age of approximately 10 Gyr. It provides a view on the complex interactions occurring between a solar-type star entering the planetary nebula phase and its planetary system. The companion could also play an important role in the shaping of the bipolar envelope of L2 Pup and subsequently of the planetary nebula…
So we’re learning more about the complex interactions involved when a star at this phase of its life undergoes the changes that will eventually lead to its becoming a white dwarf. Future observations both at ALMA and with the European Extremely Large Telescope (now under construction in Chile’s Atacama Desert) should offer a further window into these processes.
The paper is Kervella et al., “ALMA Observations of the nearby AGB star L2 Puppis,” published online by Astronomy & Astrophysics 8 December 2016 (abstract / preprint).
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