Tuning Up HPF: The Habitable Zone Planet Finder

If you had a hot new instrument like the Habitable Zone Planet Finder (HPF) now mounted at the Hobby-Eberly Telescope (McDonald Observatory, University of Texas), how would you run it through its paces for fine-tuning and verification of its performance specs? The team behind HPF has chosen to deploy the instrument during its commissioning phase on a nearby target, Barnard’s Star, which for these purposes we can consider something of an M-dwarf standard.

Working at near-infrared wavelengths, HPF uses radial velocity methods to identify low-mass planets around nearby M-dwarf stars. The choice of wavelength is determined by the mission: M-dwarfs (also known as ‘red dwarfs’) are prey to substantial magnetic activity that shows up as spots and flares that disrupt instruments working in visible light, not to mention the fact that they are small to begin with and thus faint on the sky. In the near-infrared, close to but not in the visible spectrum, this category of star appears brighter and its surface activity more muted.

I mentioned Barnard’s Star as a kind of standard because it precisely suits astronomers’ needs for calibrating such an instrument. Here let me quote from a Penn State blog on HPF (Penn State built the instrument), which lays out the ideal for commissioning:

While the ultimate goal of any Doppler spectrograph is to find lots of exoplanets, boring is better during the commissioning phase. The only way to test the stability and precision of your end-to-end measurement system–from the telescope, through the fiber optics, and ultimately the optics and detector of the spectrograph–is to make repeated measurements of a star with little or no variability. That way, any variability seen in the measurements must be caused by the instrument, rather than the star itself. In other words, the less variability we measure in observations of our stable “standard star,” the better the instrument is performing.

Barnard’s Star fits the bill beautifully. For one thing, it’s close by, at about 6 light years, making it the second-closest system to the Sun. At 14 percent of the Sun’s mass, it’s also typical of the kind of stars HPF will survey. But the real value lies in its age, for Barnard’s Star is thought to be extremely old, possibly as old as the Milky Way itself. The star rotates slowly and shows little stellar activity of the kind that would mask the radial velocity signal in other M-dwarfs.

Image: The new Penn State-led Habitable Zone Planet Finder (HPF) provides the highest precision measurements to date of infrared signals from nearby stars. Pictured: The HPF instrument during installation in its clean-room enclosure in the Hobby Eberly Telescope at McDonald Observatory. Credit: Guðmundur Stefánssonn, Penn State.

To increase precision at the HPF, Penn State has added a laser frequency comb (LFC) to the mix. Custom-built by the National Institute of Standards and Technology (NIST), the comb is a kind of ‘ruler’ that is used to calibrate the near-infrared signal from other stars. Work like this demands a calibration source because a spectrum from the observed star will ‘drift’ slightly, a movement that must be corrected when astronomers are looking for signals in the area of 1 meter per second to identify a small planet in the habitable zone of an M-dwarf. This is a kind of false Doppler effect likely due to physical issues in the instrument itself. Measuring the spectra of two sources at once — one of them being the stable frequency comb — allows the correction to be made, letting the true Doppler effect induced by planets around the star be observed.

Atomic emission lamps have been used for such calibration in the past, but laser frequency combs produce spectra with finely calibrated emission lines that are stable and of uniform brightness. Adding a laser comb to HPF ensures maximum performance, says Suvrath Mahadevan (Penn State), who is principal Investigator of the HPF project:

“The laser comb…separates individual wavelengths of light into separate lines, like the teeth of a comb, and is used like a ruler to calibrate the near-infrared energy from the stars. This combination of technologies has allowed us to demonstrate unprecedented near-infrared radial velocity precision with observations of Barnard’s Star, one of the closest stars to the Sun.”

Image: An example comparison of calibration spectra for astronomical spectrographs. Credit: HPF / Penn State.

Mahadevan adds that the technical challenges of reaching this level of precision are substantial. The instrument is highly sensitive to any infrared light emitted at room temperature, which means operations must take place at extremely cold temperatures. Thus far, the results speak for themselves, as discussed in a paper in Optica that describes the Barnard’s Star work (citation below).

The current data series on Barnard’s Star shows a stability of about 1.5 meters per second, which tops anything achieved by an infrared instrument. This is actually close to the best earlier measurements of the star, which have come from the renowned HARPS spectrograph working at visible wavelengths (378 nm – 691 nm); these come in at 1.2 meters per second. The HPF goal is 1 meter per second, not yet attained, though the team continues to refine its numbers while searching for possible instrumental issues that may play a role. From the blog:

We would be remiss if we did not emphasize that working all of the kinks out of an ultra-precise Doppler spectrograph is a years-long process, and we are far from done making improvements to the instrument and our analysis techniques. With that said, our early observations of Barnard’s star are extremely promising!

Can HPF confirm the Pale Red Dot project’s super-Earth around Barnard’s Star? Not yet. Although the instrument has the precision to see Barnard’s Star b, a problem remains:

As it turns out, cosmic coincidence prevents us from having much information on Barnard b at this point. The orbit of the proposed planet is eccentric, which means the Doppler signal is more pronounced at some phases of its orbit than others. Through nothing but luck, our HPF-LFC observations completely missed the most dynamic section of the Barnard b phase curve. Thus, while our HPF measurements do not rule out the proposed planet, they cannot yet confirm it, either. This is just one of many examples of how exoplanet detection is a data-intensive process!

Image: The orbital model of Barnard b (blue), with HPF measurements (gold) folded to the orbital phase. Our measurements have not yet covered the maximum of the eccentric orbit. Credit: HPF team / Penn State.

The paper on applying laser frequency comb techniques to the HPF in studies of Barnard’s Star is Metcalf et al., “Stellar spectroscopy in the near-infrared with a laser frequency comb,” Optica Vol. 6, No. 2 (2019), pp. 233-239 (abstract).


Alternatives to DNA-Based Life

The question of whether or not we would recognize extraterrestrial life if we encountered it used to occupy mathematician and historian Jacob Bronowski (1908-1974), who commented on the matter in a memorable episode of his 1973 BBC documentary The Ascent of Man.

“Were the chemicals here on Earth at the time when life began unique to us? We used to think so. But the most recent evidence is different. Within the last few years there have been found in the interstellar spaces the spectral traces of molecules which we never thought could be formed out in those frigid regions: hydrogen cyanide, cyano acetylene, formaldehyde. These are molecules which we had not supposed to exist elsewhere than on Earth. It may turn out that life had more varied beginnings and has more varied forms. And it does not at all follow that the evolutionary path which life (if we discover it) took elsewhere must resemble ours. It does not even follow that we shall recognise it as life — or that it will recognise us.”

Bronowski wanted to show how human society had evolved as its conception of science changed — the title is a nod to Darwin’s The Descent of Man (1871), and the sheer elegance of the production reflected the fact that the series was the work of David Attenborough, whose efforts had likewise led to the production of Kenneth Clarke’s Civilisation (1969), among many other projects. If the interplay of art and science interests you, a look back at both these series will repay your time.

As to Bronowski, who died the year after The Ascent of Man was first aired, I can only imagine how fascinating he would have found new work out of the Foundation for Applied Molecular Evolution in Alachua, Florida. Led by Steven Benner, a team of scientists has addressed the question of alien life so unlike our own that we might not recognize it. Along the way, it has managed to craft a new informational system that, like DNA, can store and transmit genetic information. The difference is that Benner and team use eight, not four, key ingredients.

Image: This illustration shows the structure of a new synthetic DNA molecule, dubbed hachimoji DNA, which uses the four informational ingredients of regular DNA (green, red, blue, yellow) in addition to four new ones (cyan, pink, purple, orange). Credit: Indiana University School of Medicine.

DNA, a double-helix structure like the new “hachimoji DNA” (the Japanese term ‘hachi’ stands for ‘eight,’ while ‘moji’ means ‘letter’), is based upon four nucleotides that appear to be standard for life as we know it on Earth. ‘Hachimoji’ DNA likewise contains adenine, cytosine, guanine, and thymine, but puts four other nucleotides into play to store and transmit information.

We begin to see alternatives to the ways life can structure itself, pointing to environments where a different kind of structure could survive whereas DNA-based life might not. That could be useful as we’re beginning to put spacecraft into highly interesting environments like Europa and Enceladus, but to get the most out of our designs, we need to have a sense of what we’re looking for. What kinds of molecules could store information in the worlds we’ll be exploring?

Thus Mary Voytek, senior scientist for astrobiology at NASA headquarters:

“Incorporating a broader understanding of what is possible in our instrument design and mission concepts will result in a more inclusive and, therefore, more effective search for life beyond Earth.”

Creating something unusual right here on Earth is one way to approach the problem, but of course there are others, and I am reminded of Paul Davies work and his own notions of what he calls ‘weird life.’ The Arizona State scientist, a prolific author in his own right, has examined the concept of a ‘second genesis,’ a fundamentally different kind of life that might already be here, having evolved on our planet and remaining on it in what we might call a ‘shadow biosphere.’

Finding alternate life on our own planet would relieve us of the burden of creating new mechanisms to make life work in our labs, so perhaps the thorough investigation of deep sea hydrothermal vents, salt lakes and high radiation environments may cut straight to the chase, if such life is there. In any case, finding a second genesis would make it far more likely that we’re going to find life on other worlds, and such life, as Davies reminds us, might be right under our noses. Like ‘hachimoji DNA,’ such life would challenge and stimulate all our assumptions.

The paper is Hoskika et al., “Hachimoji DNA and RNA: A genetic system with eight building blocks,” Science Vol. 363, Issue 6429 (22 Feb 2019), pp. 884-887 (abstract). Thanks to Byron Rogers for an early tip on this work.


Ultima Thule at Highest Resolution

One of the most enjoyable interviews I’ve been involved with lately was with Ryan Ferris, who runs the podcast Cosmic Tortoise from Christchurch, New Zealand. Ryan’s questions were sharp and of a philosophic bent, plumbing issues like the purpose and direction of human exploration. From Thor Heyerdahl’s extraordinary experiments at shipbuilding and navigation to the impulses that took Polynesian sailors into unknown waters as they settled Pacific islands, is there an innate human impulse to explore? We kicked all this around, along with SETI, the ‘Oumuamua object, and the need for a re-orienting long-term approach to civilization.

Ultima Thule and the recent exploration of it by New Horizons fit comfortably within the narrative Ryan and I discussed, as an example of satisfying that drive to push into the unknown, and also as an early marker for the growth of infrastructure in the Solar System. The Kuiper Belt pushes us hard for now, but we learn with each mission. In the meantime, forays of growing complexity to the Moon and Mars, as well as nearby asteroids, will teach us many things about human and robotic operations in ways that can extend them more frequently to system’s edge.

I’m still jazzed about Ultima Thule. The New Horizons team is saying the recently released images have the highest resolution of any the spacecraft has taken during its mission. Note the surface detail including several bright areas, roughly circular, as well as dark pits near the terminator. We’ve got so much data yet to come as New Horizons continues to return its information, so there is much to figure out at this point. “Whether these features are craters produced by impactors, sublimation pits, collapse pits, or something entirely different, is being debated in our science team,” said John Spencer, deputy project scientist from SwRI.

Image: The most detailed images of Ultima Thule — obtained just minutes before the spacecraft’s closest approach at 12:33 a.m. EST on Jan. 1 — have a resolution of about 33 meters (110 feet) per pixel. Their combination of higher spatial resolution and a favorable viewing geometry offer an unprecedented opportunity to investigate the surface of Ultima Thule, believed to be the most primitive object ever encountered by a spacecraft. This processed, composite picture combines nine individual images taken with the Long Range Reconnaissance Imager (LORRI), each with an exposure time of 0.025 seconds, just 6 ½ minutes before the spacecraft’s closest approach to Ultima Thule (officially named 2014 MU69). The image was taken at 5:26 UT (12:26 a.m. EST) on Jan. 1, 2019, when the spacecraft was 6,628 kilometers (4,109 miles) from Ultima Thule and 6.6 billion kilometers (4.1 billion miles) from Earth. The angle between the spacecraft, Ultima Thule and the Sun – known as the “phase angle” – was 33 degrees. Credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute, National Optical Astronomy Observatory

The success at Ultima Thule and the possibility of another KBO encounter in an extended mission will keep New Horizons in our thoughts for a long time to come — even after its KBO adventures have ended, we’ll still be tracking a live, outbound spacecraft just as we follow the Voyagers. The degree of precision exhibited in the Ultima Thule work is made possible by the stellar occultations we’ve discussed here in past months as well as data from the European Space Agency’s Gaia mission, so critical for star locations during the occultation campaigns.

Image: New Horizons scientists created this movie from 14 different images taken by the New Horizons Long Range Reconnaissance Imager (LORRI) shortly before the spacecraft flew past the Kuiper Belt object nicknamed Ultima Thule (officially named 2014 MU69) on Jan. 1, 2019. The central frame of this sequence was taken on Jan. 1 at 5:26:54 UT (12:26 a.m. EST), when New Horizons was 6,640 kilometers (4,117 miles) from Ultima Thule, some 6.6 billion kilometers (4.1 billion miles) from Earth. Ultima Thule nearly completely fills the LORRI image and is perfectly captured in the frames, an astounding technical feat given the uncertain location of Ultima Thule and the New Horizons spacecraft flying past it at over 14.3 kilometers per second. Credit: NASA/Johns Hopkins Applied Physics Laboratory/Southwest Research Institute.

Closing to within 3,500 kilometers of its target, the spacecraft moved three times closer to Ultima than when it flew past Pluto/Charon in July of 2015. No wonder Alan Stern is exultant:

“Getting these images required us to know precisely where both tiny Ultima and New Horizons were — moment by moment – as they passed one another at over 32,000 miles per hour in the dim light of the Kuiper Belt, a billion miles beyond Pluto. This was a much tougher observation than anything we had attempted in our 2015 Pluto flyby.

“These ‘stretch goal’ observations were risky, because there was a real chance we’d only get part or even none of Ultima in the camera’s narrow field of view,” he continued. “But the science, operations and navigation teams nailed it, and the result is a field day for our science team! Some of the details we now see on Ultima Thule’s surface are unlike any object ever explored before.”

The golden age of human discovery continues; indeed, it is just beginning. If you want to explore the raw imagery from the LORRI instrument, have a look at the New Horizons LORRI website. And give some thought to context. One thing we should recall as we ponder future exploration is a vast, island-dotted Pacific, and ancestors who navigated it by wind, currents and stars.


Hayabusa2: Asteroid Touchdown

For those of you who’ve been asking, I think the best way to keep up with the Hayabusa2 mission to asteroid Ryugu is via Twitter, @haya2e_jaxa. The news continues to percolate via websites and various publications, with a sustained ripple when the spacecraft successfully tested its sample mechanism and touched down on the asteroid. I’ll remind you too that the mission team now offers updated systems information in English on its Haya2NOW page for obsessives like me who want a really fine-grained look at what’s going on.

Hayabusa 2 is once again at what JAXA calls its ‘home position’ about 20 kilometers above the asteroid as the multi-part sample selection process continues. JAXA’s news release on the touchdown was to the point:

National Research and Development Agency Japan Aerospace Exploration Agency (JAXA) executed the asteroid explorer Hayabusa2 operation to touch down the surface of the target asteroid Ryugu for sample retrieval.

Data analysis from Hayabusa2 confirms that the sequence of operation proceeded, including shooting a projectile into the asteroid to collect its sample material. The Hayabusa2 spacecraft is in nominal state. This marks the Hayabusa2 successful touchdown on Ryugu.

But really, Twitter carries the excitement of the mission via tweets like these:

and photos like the one below, which was sent out to thank worldwide supporters for their thoughts and encouragement. Look at the size of the Hayabusa2 team! Congratulations to all of you.

Upon touchdown within the 6-meter circle selected on the asteroid, the spacecraft fired a tantalum ‘bullet’ into the surface to drive particles outward that the sampling instrument could collect. The craft then rose again, as vividly attested in the photo below, where its shadow is obvious. Two more samples are to be taken before Hayabusa 2 departs the asteroid, the final sampling involving a larger crater deliberately blasted into the asteroid to probe sub-surface materials.

Image: Image captured roughly 1 minute after touchdown at an estimated altitude of about 25m (error is a few meter). The color of the region beneath the spacecraft’s shadow differs from the surroundings and has been discolored by the touchdown. At the moment, the reason for the discoloration is unknown but it may be due to the grit that was blown upwards by the spacecraft thrusters or bullet (projectile). The photograph was taken with the Optical Navigation Camera – Wide angle (ONC-W1) on February 22, 2019 at an onboard time of around 07:30 JST. (Credit: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST.)

You may recall that the original first sample collection was scheduled for last October, but had to be delayed because the surface of the asteroid turned out to be rougher than expected. JAXA has been operating two small robotic rovers — MASCOT and MINERVA-II — on the surface, which produced the information that centimeter-sized gravel and larger were to be found there. As the agency reported online, a key question was whether the material was fine enough be released from the asteroid during the sampling events planned, which is why an artificial gravel experiment was performed in Tokyo at the end of December.

Image: Target simulating the surface of Ryugu (Credit: JAXA, University of Tokyo).

As JAXA went on to report:

In the ground test performed during the initial development, even large rocks with similar strengths to carbonaceous chondrite meteorites were crushed when a projectile made of metal (tantalum) with a mass of 5g was injected at about 300 m/s. It was confirmed that material formed from the resulting small pieces could be gathered by the sampler. So in this test, it was predictable that the bullet would crush material that it struck, but what would be the behavior of the gravel surrounding the focus of the shot?

From the results of the experiment, the fragments of gravel that were crushed were released into the surrounding gravel where they collided like billiards to break up the material. The resulting sample amount exceeded the initial assumption that would be released from the surface (Figure 4).

While the diameter of the collision site (crater) made by the impact of the projectile is smaller than when compared to that in a fine regolith layer, it was a sufficient size in comparison with the inner diameter of the open tip of the sampler horn.

The plan is for Hayabusa2 to depart Ryugu in December of this year, with return to Earth toward the end of 2020. Assuming a successful sample return, Hayabusa2 will mark the first time samples from a C-type (carbonaceous) asteroid — the most common, constituting 75% of those known — have been returned to Earth. Naturally we’ll also keep an eye on OSIRIS-REx and its operations at 101955 Bennu, another carbonaceous asteroid, for both sample returns should give us a window into early building blocks of our planet. The OSIRIS-REx sample return is scheduled for 2023.


A White Dwarf with Puzzling Rings

Backyard Worlds: Planet 9 is a project worth investigating. Using a database drawn from NASA’s Wide-field Infrared Survey Explorer (WISE), Backyard Worlds: Planet 9 is probing the cosmos at infrared wavelengths. Volunteers search the WISE data in a ‘citizen science’ effort that has already discovered more than 1,000 likely brown dwarfs. Now we have news of an intriguing white dwarf showing apparently multiple rings of gas and dust.

A ringed white dwarf isn’t unique. In fact, dust and rings have been observed around white dwarfs that were considerably younger than the one in question, J0207. As described in a paper in Astrophysical Journal Letters, the object is about 145 light years away in the constellation Triangulum and is thought to be about 3 billion years old. With a temperature of 5,800 degrees Celsius (10,500 degrees Fahrenheit), J0207 produced a strong infrared signal.

Bear in mind that a white dwarf is a remnant, a star left behind when its Sun-like predecessor, running out of nuclear fuel, has gone through red giant phase and ejected half of its mass, leaving the hot dwarf behind. As has been depicted in numerous science fiction tales, a swollen red giant can engulf its inner planets while pushing more distant planets and asteroids outward. Something of the future of our own Solar System is thus suggested.

Image: Backyard Worlds: Planet 9 volunteers scour infrared images from NASA, searching animated blinks for moving objects. Like other white dwarf stars, J0207 shows a bluish tinge in visible light (top), but also sports an orange hue in the infrared (bottom), indicating the unexpected presence of circumstellar dust rings. Credit: Digitized Sky Survey/WISE/NEOWISE, Aaron Meisner (NOAO).

What made J0207 stand out for discoverer (and paper co-author) Melina Thévenot, a citizen scientist based in Germany, was infrared emission indicating the star was surrounded by a dusty disk. We could be looking at a process of tidal disruption here, in which asteroids and comets in the stellar neighborhood are brought closer to the star by gravitational interactions with surviving planets. In any case, approaching the white dwarf, they would be torn apart by the star’s gravity. The ring that emerges will eventually dissipate, spiralling down onto the surface of the star.

Having discovered something odd about J0207, Thévenot assumed there were problems with the data. Consulting the European Space Agency Gaia archives for brown dwarfs, she identified J0207, and compared it to the source in the WISE data. Its brightness and distance confirmed it was not a brown dwarf, at which point she passed her results on to Backyard Worlds: Planet 9, where Adam Schneider (ASU), John Debes (Space Telescope Science Institute), and Marc Kuchner (NASA GSFC), who leads the Backyard Worlds: Planet 9 project, could examine them.

It would be Debes and Kuchner who, working with UC-San Diego astronomer Adam Burgasser, arranged follow-up observations of J0207 with the Keck II instrument in Hawaii. What makes J0207 stand out in relation to other white dwarfs with dust disks is its age. Given a process in which asteroids are ground apart by gravitational interactions with the star, we should get to the end of the line — with all such materials incorporated into the star — in a fairly short time. This is especially true if the white dwarf is not part of a binary, which J0207 is not.

Says Debes:

“This white dwarf is so old that whatever process is feeding material into its rings must operate on billion-year time scales. Most of the models scientists have created to explain rings around white dwarfs only work well up to around 100 million years, so this star is really challenging our assumptions of how planetary systems evolve.”

Image: The star, designated LSPM J0207+3331, is the oldest, coolest white dwarf known to be surrounded by a ring of dusty debris. This illustration depicts the ring with two distinct components, which scientists think best explains the system’s infrared signal, and an asteroid broken up by the white dwarf’s gravity. Credit: NASA Goddard Space Flight Center/Scott Wiessinger.

We may also be looking at a second point of interest, for the J0207 disk shows evidence of being composed of more than a single ring-like component. That would be a first in white dwarf observations, and for it I turn to the paper:

…the infrared excess seen for this disk requires a second, colder ring of dusty material that could potentially signal the presence of a gap in the system, or a component of dust that extends beyond the outer edge of the inner disk. If the second ring is confirmed, it would be the first example of a two-component ring system around a dusty white dwarf. If the dust disk has a gap near 0.94 R, this implies the possibility of a body that continuously clears dust from the system, since the PR drag timescale is so short.

So we have what the paper calls “an interesting test of dust disk accretion“ in J0207, given the age of the star. The researchers suggest further investigation in the form of optical spectra of this white dwarf to look for metal lines from accreted dust to examine the actual accretion rate.

The paper is Debes et al., “A 3 Gyr White Dwarf with Warm Dust Discovered via the Backyard Worlds: Planet 9 Citizen Science Project,” Astrophysical Journal Letters Vol. 872, No. 2 (19 February 2019). Abstract / Preprint.

I can’t say enough how much I support Backyard Worlds: Planet 9 and other citizen science projects that are not only producing high-quality results but also involving the wider public in our ongoing exploration of the cosmos.