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Shaping the TESS Target List

Picking up on TESS (Transiting Exoplanet Survey Satellite), one of whose discoveries we examined yesterday, comes news of a document called the “TESS Habitable Zone Star Catalog.” The work of Cornell astronomers in collaboration with colleagues at Lehigh and Vanderbilt, the paper has just been published in Astrophysical Journal Letters (citation below), where we find 1,822 stars where TESS may find rocky terrestrial planets.

The listed 1,822 are nearby stars, bright, cool dwarfs, with temperatures roughly between 2,700 and 5,000 Kelvin, with a TESS magnitude brighter than 12 and reliable data from the Gaia Data Release 2 as to distance. Here TESS can detect 2 transits of planets that receive stellar irradiation similar to Earth’s, during the 2-year prime mission. 408 of these stars would allow TESS to detect transiting planets down to Earth size during one transit. The catalog is fine-tuned to the TESS instrumentation and mission parameters, the stars selected because they offer sufficient observing time to be able make these detections.

From the paper:

What distinguishes this catalog from previous work like HabCat (Turnbull & Tartar 2003), DASSC (Kaltenegger et al 2010) and CELESTA (Chandler, McDonald, & Kane 2016) is that the stars included here are specifically selected to have sufficient observation time by TESS for transit detection out to the Earth-equivalent orbital distance. We also use Gaia DR2 data, which allows us to exclude giant stars from the star sample and provides reliable distances for our full star sample. All the stars have been included in the TESS exoplanet Candidate Target List, ensuring that they will also have 2-minute cadence observation (provided they do not fall in TESS camera pixel gaps), providing a specific catalog for the TESS mission of stars where planets in the Habitable Zone can be detected by TESS. This data will be available to the community in the ongoing public TESS data releases.

In case you’re wondering, 137 stars in the catalog are within the continuous viewing zone of the James Webb Space Telescope, which will be able to observe them to characterize planetary atmospheres and search for biosignatures. Many more will be followed up after any TESS planet identifications by ground-based extremely large telescopes currently under construction.

Image: The TESS search space compared to that of the Kepler Mission. Credit: Zach Berta-Thompson.

The idea, then, is to help us shape our target lists for TESS by pointing to the most likely places of discovery. Meanwhile, Elisa Quintana (NASA GSFC) has been thinking about planets we can’t yet detect but which may indeed be present, using Kepler data as massaged by a mathematical model that has implications for TESS and future mission datasets. The difference is that in Quintana’s case, these are systems where we already know planets exist. The question: What other planets might yet be found in the same systems?

After all, we have to wonder what our methods may have missed. The Kepler mission has led us to believe that most stars in the galaxy have planetary companions, but around even relatively close stars, we may be seeing a subset of what’s actually there. Using the transit method, which Kepler employed to such brilliant effect, we only see the planets that move across the face of the star as seen from Earth. There could be others in the same system that do not.

Think about how rare a transit of Venus is. Even from our vantage so close to the planet, we see Venus cross the Sun only in pairs of transits eight years apart, separated by gaps of over a century. Indeed, the last transit of Venus of the 21st Century has already taken place (5,6 June, 2012); we have to wait until December of 2117 for the next. The orbit of Venus is responsible for the rarity of the phenomenon; it’s inclined by 3.4° relative to the Earth’s orbit. Exoplanetary systems are presumably not immune to such variation.

Quintana has been working as mentor with an 18 year-old high school student named Ana Humphrey, who developed the model to predict possible planets in such systems. Out of Humphrey’s work, which has garnered a a $250,000 prize in the Regeneron Science Talent Search, we learn that there may be as many as 560 ‘hidden’ planets in exoplanet systems identified by Kepler. Says Humphrey:

“I was completely fascinated by this idea of finding new planets using mass, based on data that we already had. I think it just shows that even if your data collection is complete, there’s always new questions that can be asked and can be answered.”

Image: Ana Humphrey won a $250,000 prize for calculating the potential for finding more planets outside our solar system. Credit: NASA GSFC.

Indeed, as Quintana points out, systems like Kepler-186 show a large gap that exists between the four planets close in to the star and the outer planet. Another world the size of Earth could be there on an orbit inclined enough that we would not see it. Extend this over the range of multi-planet systems found thus far and there is ample room for additional discovery. Humphrey’s model manipulates the possible space between the hypothetical planet and its neighbors, to see what worlds of varying mass could be present without disrupting their orbits.

This could come in handy for TESS, which as we saw yesterday, is already producing planetary discoveries like TOI-197. Applying the new model to the exoplanet database being assembled by TESS would allow both it and future missions to predict systems in which hidden planets might be found. Such systems might then be studied both by transits and other methods.

In examining such questions, Quintana and Humphrey are simply extending a time-honored method of planetary discovery, one that led Johann Gottfried Galle, working with calculations from Urbain Le Verrier, to discover Neptune in 1846 (and yes, Neptune was observed before this but was not known to be a planet). The mathematical calculations that produced Neptune as planet captured the imagination of François Arago, who said that Le Verrier had discovered a planet “with the point of his pen.” Thus does one world grow out of another — it was data on Uranus and the irregularities of its orbit that led to our learning the true nature of Neptune.

Remarkably, Triton was discovered a mere 17 days after the discovery of Neptune, another case of data cascade. Applying the same concept to exoplanets has been a natural progression. We can actually see only a few such worlds through direct imaging. Fine-tuning our models to fit the methods and instruments at hand maximizes the opportunity to enlarge our catalog.

The paper is Kaltenegger et al., “TESS Habitable Zone Star Catalog,” Astrophysical Journal Letters Vol. 874, No. 1 (26 March 2019). Abstract / preprint.

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  • ljk April 3, 2019, 12:06

    https://arxiv.org/abs/1904.01062

    Advanced Aspects of the Galactic Habitability

    Vladimir Đošović, Branislav Vukotić, Milan M. Ćirković

    (Submitted on 1 Apr 2019)

    Context. Astrobiological evolution of the Milky Way (or the shape of its “astrobiological landscape”) has emerged as one of the key research topics in recent years.

    In order to build precise, quantitative models of the Galactic habitability, we need to account for two opposing tendencies of life and intelligence in the most general context: the tendency to spread to all available ecological niches (conventionally dubbed “colonization”) and the tendency to succumb to various types of existential catastrophes (“catastrophism”).

    These evolutionary tendencies have become objects of study in fields such as ecology, macroevolution, risk analysis, and futures studies, while a serious astrobiological treatment has been lacking so far.

    Aims. Our aim is to numerically investigate the dynamics of opposed processes of expansion (panspermia, colonization) and extinction (catastrophic mechanisms) of life in the Galaxy.

    Methods. We employ a new type of numerical simulation based on 1D probabilistic cellular automaton with very high temporal resolution, in order to study astrobiological dynamics.

    Results. While the largest part of the examined parameter space shows very low habitability values, as expected, the remaining part has some observationally appealing features that imply, among other things, a reduction in the amount of fine-tuning necessary for resolving the Fermi paradox.

    Conclusions. Advanced aspects of Galactic habitability are amenable to precision studies using massive parallel computer simulations. There are regions of parameter space corresponding to a quasi-stationary state satisfying observable constraints and possessing viable SETI targets.

    Subjects: Popular Physics (physics.pop-ph); Earth and Planetary Astrophysics (astro-ph.EP); Cellular Automata and Lattice Gases (nlin.CG)

    Cite as: arXiv:1904.01062 [physics.pop-ph]
    (or arXiv:1904.01062v1 [physics.pop-ph] for this version)

    Submission history

    From: Vladimir Đošović [view email]

    [v1] Mon, 1 Apr 2019 18:45:34 UTC (3,252 KB)

    https://arxiv.org/pdf/1904.01062.pdf

    • Michael C. Fidler April 4, 2019, 11:13

      Galactic Habitability or Galactic Ecology, interesting and something that struck me is the possibility that the dust and gases that formed our Sun also formed other stars in an open cluster. These stars would all have similar abundances of elements and may have developed planets and life like earth. The G dwarf type stars would be prime targets but even M to A dwarfs from that original cluster may have similar outcomes.

      The hunt for the Sun’s ancient siblings.

      “Thousands of stars were born in the same cluster as the Sun, but where are they now? To help solve this mystery, astronomers have analyzed the DNA of 340,000 stars.”

      By Amber Jorgenson | Published: Thursday, April 19, 2018

      http://www.astronomy.com/news/2018/04/the-hunt-for-the-suns-ancient-siblings

      The sun’s ‘long-lost BROTHER’ revealed: Star born from same gas cloud discovered – and it could host planets with alien life.

      The star, named HD 162826, is 15 per cent more massive than our sun.
      Scientists believe it was born from the same gas cloud as our own star.
      HD 162826 is located 110 light-years away in the constellation Hercules.
      As well as helping find alien life, star could shed light on how sun formed.

      https://www.dailymail.co.uk/sciencetech/article-2624257/The-suns-long-lost-BROTHER-revealed-Star-born-gas-cloud-discovered-host-planets-alien-life.html

  • Teresa Ospi April 3, 2019, 13:22

    This is related in some way to the fact that 85% of the exoplanets detected are within the tidal locking region. Surely we have missed many planets in those systems. It is as if they could only see Mercury and Venus in our Solar System.

    • Bruce D. Mayfield April 3, 2019, 17:41

      That is a very apt analogy. We haven’t even finished counting up the complete number of planets in this system yet, for that matter. As for the transiting planet method, the greater the distance from the star, the slimmer the odds become for detection.

      I’ve often wondered how many total planets some of those tight multi-planet Kepler systems might really have, if we could only “see” the whole system. If planets are packed clear out to 40 or more AU they could even have dozens. Our system might even be below average when it comes to total planet count!

  • Michael C. Fidler April 3, 2019, 19:55

    Here is the Pdf link to Ana Humphrey poster from the EXOPLANETS II conference on 2-6 July 2018 at Cambridge, UK:
    https://www.exoplanetscience2.org/sites/default/files/submission-attachments/humphreyanaexoplanetsii.pdf
    Doing a little digging I found a gold mine of posters from this gathering:
    https://www.exoplanetscience2.org/programme/posters.html

    Plenty of fascinating details about subjects we discuss here every day from the next generation of young astronomers/explorers!

    Wish I was 18 again! :-(

  • ljk April 4, 2019, 9:08

    TESS spots its first exocomet around one of the sky’s brightest stars

    Nestled in the disk of the young star Beta Pictoris, the newfound exocomet finally confirms a prediction made some 20 years ago.

    By Jake Parks | Published: Wednesday, April 3, 2019

    http://astronomy.com/news/2019/04/tess-spots-its-first-exocomet-around-one-of-the-skys-brightest-stars

  • Robert April 4, 2019, 13:01

    I thought it was typo…a quarter million dollar prize but it’s not. Wow! Funny thing is after proving such abilities, a student would likely get a free ride at any top school anyway.

  • Gary Wilson April 4, 2019, 14:50

    Ana clearly has a brilliant career ahead of her. A brilliant contribution to planet finding.

  • wdk April 5, 2019, 16:34

    Examining the pre-print, as many others might, I gathered that I was looking at the methodology for using the Tess catalog, but not the catalog itself. Well, using their filtering systems for the candidates, a list of
    stellar targets can be found at this location:
    https://filtergraph.com/tess_habitable_zone_catalog
    indicated in the pre-print.

  • Michael C. Fidler April 9, 2019, 9:38

    It looks like our nearby Red/M Dwarf exoplanets may not be all that bad after all! The UV levels may be significantly lower then all the doomsayers make it out to be. The possibility that some type of biofluorescence from the UV flaring could also create a photosynthesis process that another recent paper says was to low, because of only low power infrared rays from M Dwarfs. With these planets tidally locked the biofluorescence plant life may be near high noon where the UV radiations is strongest and animal life may be closer to the terminator were the atmosphere filters out the hasher radiations.

    Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems.

    “The closest potentially habitable worlds outside our Solar system orbit a different kind of star than our Sun: smaller red dwarf stars. Such stars can flare frequently, bombarding their planets with biologically damaging high-energy UV radiation, placing planetary atmospheres
    at risk of erosion and bringing the habitability of these worlds into question. However, the surface UV flux on these worlds is unknown. Here we show the first models of the surface UV environments of the four closest potentially habitable exoplanets: Proxima-b, TRAPPIST-1e,
    Ross-128b, and LHS-1140b assuming different atmospheric compositions, spanning Earth-analogue to eroded and anoxic atmospheres and compare them to levels for Earth throughout
    its geological evolution. Even for planet models with eroded and anoxic atmospheres, surface UV radiation remains below early Earth levels, even during flares. Given that the early Earth was inhabited, we show that UV radiation should not be a limiting factor for the habitability
    of planets orbiting M stars. Our closest neighbouring worlds remain intriguing targets for the search for life beyond our Solar system.

    https://arxiv.org/abs/1904.03956

    • Michael C. Fidler April 9, 2019, 9:44

      Or the other possibility, that a biofluorescence jungle could exist on the red dwarf facing side with plenty of animal life living beneath it in the shade… ;-})

    • Michael C. Fidler April 9, 2019, 19:24

      Life could be evolving right now on nearest exoplanets.

      “Excitement about exoplanets skyrocketed when rocky Earth-like planets were discovered orbiting in the habitable zone of some of our closest stars – until hopes for life were dashed by the high levels of radiation bombarding those worlds.
      Proxima-b, only 4.24 light years away, receives 250 times more X-ray radiation than Earth and could experience deadly levels of ultraviolet radiation on its surface. How could life survive such a bombardment? Cornell University astronomers say that life already has survived this kind of fierce radiation, and they have proof: you.”

      https://phys.org/news/2019-04-life-evolving-nearest-exoplanets.html

      Protective biofluorescence would be most useful during the increase in UV flux during flares, and could cause a temporary change in the planet’s surface brightness in the visible. Constant high UV radiation environment present in the anoxic planet model could favour continuous fluorescence (O’Malley-James & Kaltenegger 2018). Lab experiments with green fluorescent proteins have successfully produced high fluorescent efficiencies of up to 100 per cent (see O’Malley-James & Kaltenegger 2018 and references therein). Because biofluorescence is independent of the visible flux of the host star and only dependent on the UV flux of the star, emitted biofluorescence can increase the visible flux of a planet orbiting an active M-star by several orders of magnitude (O’Malley-James & Kaltenegger 2018) during a flare.

      https://academic.oup.com/mnras/article/485/4/5598/5426502

  • Michael C. Fidler April 9, 2019, 19:49

    Made In Space unveils small satellite interferometry tool.

    Traditional space-based interferometry missions bring along large deployable structures to separate their telescopes or other instruments. Hinges and mechanical systems on the deployable structures allow them to be folded in launch fairings and extended in orbit.
    Made In Space proposes instead equipping satellites weighing roughly 150- to 300-kilograms with technology to manufacture in orbit a 20-meter optical boom interferometer with a modular internal optics bench the firm developed with Lowell Observatory.

    “Depending on the ambition of the project, this is something we could design and have ready for flight on a two- or three-year timescale,”

    https://spacenews.com/made-in-space-interferometry/

    Tools For Advancing Space Exploration: Understanding Interferometry /April 9, 2019
    https://madeinspace.us/blog-detailed/2019/4/9/3vaud85kz69rjxy845kq2c7eoomwf5

    Made In Space, Inc. Announces Manufacturing System For Smallsat Interferometry.
    https://madeinspace.us/press-release-optimastsci

    Optimast-SCI:
    https://static1.squarespace.com/static/56d9b0528259b560ad38cde1/t/5cab518fe5e5f0b640cd3ef5/1554731413859/Optimast-sci+table.jpg?format=2500w

    https://madeinspace.us/optimast-sci

    Optimast-SCI’s in-space configuration can resolve faint objects at less than 5 milliarcsecond resolving power and enable new high resolution, space-based missions in astrophysics, planetary science, Earth remote sensing, and space situational awareness, all with no atmospheric distortion.