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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).

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Comments on this entry are closed.

  • ljk February 28, 2019, 15:04

    Penn State is also moving ahead with that other aspect of looking for alien planets, SETI research:

    https://www.sciencemag.org/news/2019/02/university-research-center-will-search-extraterrestrial-intelligence

    They even teach a course on SETI!

    https://sites.psu.edu/seticourse/

  • Thomas R Mazanec February 28, 2019, 18:29

    Barnard’s Star is a little variable…its variable star name is V2500 Ophiuchi.
    Does this project get its own listing on your list?

    • Paul Gilster February 28, 2019, 20:57

      Already there.

  • Bruce D. Mayfield March 1, 2019, 8:53

    It’s great to see the HET getting this upgrade. My wife and I got to see inside this mammoth 10 meter telescope on a guided tour at McDonald Observatory a few years ago. This scope does nothing but spectroscopy.

    “The Hobby–Eberly Telescope (HET) is a 10-meter (30-foot) aperture telescope located at the McDonald Observatory. It is one of the largest optical telescopes in the world and combines a number of features that differentiate it from most telescope designs, resulting in greatly lowered construction costs. For instance, the primary mirror is constructed from 91 hexagonal segments, which is less expensive than manufacturing a single large primary. Furthermore, the telescope’s main mirror is fixed at a 55° angle and can rotate around its base. A target is tracked by moving the instruments at the focus of the telescope; this provides access to about 70–81% of the sky at its location and allows a single target to be tracked for up to two hours.[1][2] The telescope is named for former Texas Lieutenant-Governor Bill Hobby and for Robert E. Eberly, a Penn State benefactor.”

  • Ashley Baldwin March 1, 2019, 14:58

    So just two hours tracking a star for the TPF per night . Compared to other telescopes with a fully tracking ALZ-ALT mount. That’s a lot less time per target to catch an expected transit and even less chance of catching an unexpected transit without a lot of separate observations . Although HARPs is mounted on a 3.6m scope that collects about an eighth as much light and is thus much less sensitive to a transit signal at a fixed point in time , it’s the time duration of an observation that matters more surely ? A greater chance of catching a ( complete) planetary transit of the star whilst that star is within the field of view . A big issue for any ground based scope viewing. Further limited by factors such as day ( night ) length and cloud cover – determined and/or complicated by both the observatory’s lattitude and longitude .

    • Bruce D. Mayfield March 1, 2019, 18:52

      The HET wasn’t built for long duration star tracking, it was designed to collect as much light as was economically possible in a short period, so that that starlight (or other light sources) could then be spread wide open into a fine spectrum. Most of what we can learn about stars is derived from spectroscopy.

      This scope won’t be finding many new exoplanets, but it will be very helpful in helping to characterize systems already known to possess planets.

      • Bruce D. Mayfield March 2, 2019, 1:55

        That last statement was wrong (I hope). I made the error because I was thinking of finding exoplanets by the transit method. HET equipped with the new HPF will be more effective in finding planets by the radial velocity method. My apologies to all.

        • Michael C. Fidler March 2, 2019, 11:17

          This scope was relatively inexpensive to build and I have seen dobsonian designs that have a third axis for better zenith coverage. It would be nice if some of the amatuer telescope mount designs were developed for such large inexpensive light buckets so these could be built in large numbers for the 10,000’s exoplanets that are going to be discovered in the next decade. Maybe inflatable mirrors, there is a lot of creative ideas out there – instead of building the Hale 200′ battleship designs!

          • Bruce D. Mayfield March 3, 2019, 0:55

            Indeed. More scopes are needed, both on high mountains and in space. Too many targets calling for time on far too few large telescopes.

            They’re using adaptive optics to cancel out bad seeing, why can’t such technology be designed to form thin sheet mirrors in the first place, thus avoiding the need for heavy mirror works?

            • Michael C. Fidler March 4, 2019, 18:48

              Mirror coatings have to be replaced after a few years and normal good coatings reflect 95% of light. On large telescopes there may be 10 mirrors so light loss is significant. They now have nearly zero 100% black, so what about quantum level reflectors with 100% reflection or maybe graphene, this development would increase light throughput by 100%, twice the light!

  • Ronald March 5, 2019, 9:54

    Slightly off-topic, but related and very relevant:
    https://arxiv.org/ftp/arxiv/papers/1902/1902.04720.pdf

    According to a new study that recently appeared online, titled “A Limited Habitable Zone for Complex Life“, habitable zones could be much narrower than originally thought. These finds could have a drastic impact on the number of planets scientists consider to be “potentially habitable”.

    This would be because, as was already known, at greater AU distance from the parent star a higher atmospheric CO2 content is necessary to keep the planet habitable.
    And exactly that high CO2 concentration makes the planet unsuitable for complex life, defined as multi-cellular Eukaryotic.

    The authors estimate that the CLHZ (you guessed it: complex life HZ) is only 20-28% of the total (conservative) HZ.
    In our solar system situation that would reduce the HZ outer limit from about 1.7 AU to about 1.15 (1.1 – 1.17) AU.

    Truly, this does not really surprise me, as I was always somewhat puzzled by the extremely high CO2 concentrations required to keep a planet habitable in the outskirts of the HZ, in the traditional HZ models (Kasting, Kopparapu).

    The first Eukaryotes (that require a high O2 concentration and not too much CO2) appeared on Earth about 1.8 gy ago, the first multicellular Eukaryotes about 0.8 (- 1 ?) gy ago. The Sun was dimmer then, by resp. about 18% and 8%.
    Hmm, that’s interesting, 18% dimmer corresponds to being at about 1.1 AU now.

    BTW, at the same time it would make red dwarfs (M stars) even more unsuitable, because they require very high CO2 in most of their HZ.

  • Ashley Baldwin) March 10, 2019, 7:32

    I think there is an interesting but pivotal corollary to the publication that launched the TPF. That is the fact that as “proof of concept” the instrument was used for an 86 day baseline RV observation run looking at Barnard’s star. A reasonable choice given it is the sort of late M dwarf this instrument was built to observe and the fact that it’s close proximity and relative quiescence make it a “softer” target for an inaugural observation ( the text describes how each individual observation lasted about an hour – a significant fraction of the maximum two hours that the Hobby Ebberly Telescope can track an individual target given the limitations of its novel mount)

    The entirety of this observation occurred coincidentally in parallel with part of the longer observation period of the same target that Ribas et al used to identify Barnard’s b. Tellingly, the Penn State “TPF” team observations did not show a transit signature consistent with a planetary candidate. As their results paper was “in press” at the time of the Barnard’s b announcement they have been obliged to comment on the apparently conflicting results . This is done in a supplemental appendix to their own article which “attempts” to address the disparity .

    Though “only” proof of concept ( does TPF work as hoped as opposed to actually hoping to find something ) the observation run was conducted under the usual rigorous RV process. The authors comment how the observation at 86 days was shorter than Ribas’ team and occurred during the most stable period of the eccentric orbit of the erstwhile Barnard’s b planet thus making it harder to spot for the TPF. They don’t comment on how long Barnard’s b would take to transit. Bear in mind that planetary transits, especially for planets with a larger semi major axis ,around lower mass stars – with a resultant lower orbital velocity, can take hours . Kepler’s 3rd law. This for an instrument that as pointed out can only track its target star for an hour maximum . However the two teams actual individual observations are laid out in the supplement side by side by way of comparison . For the benefit of the informed reader to judge for themselves. No obfuscation ,which I think is telling.

    This means someone is likely wrong here remember ! It is made clear in the text that the TPF performance was at least as sensitive and stable as those of the Ribas instrument – if inferior it would undermine the entire case as TPF being the great step forward in RV spectroscopic terrestrial planet discovery ( though proof of concept data is only really aimed at matching the status quo so “as good as” is good enough ) . So the TPF results are atleast as good as the admittedly longer Ribas. The Penn State team also had the benefit of having seen the Ribas results in advance and will undoubtedly have cross checked both them and their own findings beforehand .

    Which leaves us with an interesting dynamic regarding the validity of the Barnard’s b discovery.

    Watch that space.

  • Ronald March 12, 2019, 16:06

    And another interesting study on K stars as the Goldilocks spectral class for habitable planets ánd their detection:

    https://iopscience.iop.org/article/10.3847/2041-8213/ab0651/meta