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Ross 128b: Analyzing a Planet by the Light of its Star

Red dwarfs have a lot of things going for them when it comes to finding possibly habitable planets. A planet of Earth size in the HZ will produce a substantial transit signal because of the small size of the star (‘transit depth’ refers to the amount of the star’s light that is blocked by the planet), and the tight orbit the planet must follow increases the geometric probability of observing a transit. But planets that do not transit are also more readily detected because of the large size of the planet compared to the star, gravitational interactions producing a strong radial velocity signature, which is what we have in the case of Ross 128b.

About 11 light years from Earth, the planet was culled out of more than a decade of radial velocity data in 2017 using the European Southern Observatory’s HARPS spectrograph (High Accuracy Radial velocity Planet Searcher) at the La Silla Observatory in Chile. The location of the planet near the inner edge of its star’s habitable zone excited interest, as did the fact that Ross 128 is much less subject to flares of ultraviolet and X-ray radiation than our nearest neighbor, Proxima Centauri, which also hosts a planet in a potentially habitable orbit.

Image: Artist’s impression of the exoplanet Ross 128b. Credit: ESO.

What we know about Ross 128b is that it orbits 20 times closer to its star than the Earth orbits the Sun, but receives only 1.38 times more irradiation than the Earth, with an equilibrium temperature estimated anywhere between -60 degrees Celsius and 20°C, the host star being small and relatively cool. But bear in mind that what we get from radial velocity is a minimum mass, because we don’t know at what angle this system presents itself in our sky. Now a team led by Diogo Souto (Observatório Nacional, Brazil) is attempting to deduce more about the planet’s composition using an unusual method: Analyzing the composition of the host star.

If we learn the chemical abundances found in the star Ross 128, the thinking goes, we should be able to make reasonable estimates about the composition of any planets that orbit it. Souto and team are presenting new techniques for making these measurements, using data from the Sloan Digital Sky Survey’s APOGEE spectroscope. Measuring the star’s near-infrared light, where Ross 128 shines the brightest, the researchers have been able to derive abundances for carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium and iron.

“The ability of APOGEE to measure near-infrared light, where Ross 128 is brightest, was key for this study,” says co-author Johanna Teske (Carnegie Institution for Science). “It allowed us to address some fundamental questions about Ross 128 b’s `Earth-like-ness.’”

APOGEE is the Apache Point Galactic Evolution Experiment, an investigation using high-resolution spectroscopy to probe the dust that obscures the inner Milky Way. The project surveyed 100,000 red giant stars across the galactic bulge, but also observed M-dwarfs in the neighborhood of the Sun as a secondary study. Tightening up our knowledge of stellar parameters, the paper notes, offers an indirect route to studying exoplanet composition.

The assumption in this work is that the chemistry of a host star influences the contents of the disk from which planets form around it, which in turn affects the interior structure of any planet. Thus we can hope to tell from the amount of magnesium, iron and silicon available something about the exoplanet. This is the first detailed abundance analysis for Ross 128, and it shows that the star has iron levels similar to the Sun. The silicon level could not be measured, but the ratio of iron to magnesium points to a large core for the planet, larger than Earth’s.

Souto and team believe that knowledge of Ross 128b’s minimum mass (from the radial velocity data), coupled with their data on stellar abundances, can provide a broad estimate of the planet’s radius, a key factor because it would allow a calculation of its density. From the paper:

While both mass and radius are not available for Ross 128b, we can estimate its radius given its observed minimum mass and assuming the stellar composition of the host star is a proxy for that of the planet. We calculate the range of radii possible for Ross 128b using the ExoPlex software package (Unterborn et al. 2018) for all masses above the minimum mass of Ross 128b (1.35M; Bonfils et al. 2017). Models were run assuming a two-layer model with a liquid core and silicate mantle (no atmosphere). We increase the input mass until a likely radius of 1.5R was achieved, roughly the point where planets are not expected be gas-rich mini-Neptunes as opposed to rock and iron-dominated super-Earths…

Measurements of the temperature of Ross 128 coupled with the estimated radius of the exoplanet and its inferred composition allow the team to calculate Ross 128b’s albedo, the amount of light reflecting off its surface. These estimates allow the possibility of a temperate climate, taking into account the insolation flux (energy received from the host star) and equilibrium temperature. “Our results,” the authors write, “support the claim of Bonfils et al. (2017) that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone.”

But the paper urges caution in the interpretation:

However, this is not to say that Ross 128b is a “Exo-Earth.” Geologic factors unexplored in Bonfils et al. (2017) such as the planet’s likelihood to produce continental crust, the weathering rates of key nutrients into ocean basins or the presence of a long-term magnetic field could produce a planet decidedly not at all “Earth-like” or habitable due to differences in its composition and thermal history. Furthermore, other aspects of the M-dwarf’s stellar activity and its effect on the retention of any atmosphere and potential habitability should be studied, although we find no evidence of activity in the Ross 128 spectra.

Indeed. The number of variables affecting ‘habitability’ is striking. So let’s say this: We have a planet for which mass-radius modeling based on the composition of its host star indicates a mixture of rock and iron, the relative amounts of each being set by the ratio between iron and magnesium. The derived values for insolation and equilibrium temperature are not inconsistent with previous studies indicating a temperate planet at the inner edge of its star’s habitable zone.

The work hinges on modeling of an exoplanet based on a deeper analysis of its host star than has previously been available for an M-dwarf. Tuning up such modeling will demand further data, in particular applying these methods to the host stars of transiting worlds (think TRAPPIST-1) to test their accuracy and reliability in characterizing planets we cannot see.

The paper is Souto et al., “Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra,” Astrophysical Journal Letters Vol. 860, No. 1 (13 June 2018). Abstract / preprint.

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{ 25 comments… add one }
  • Thomas Goodey July 13, 2018, 14:32

    “A planet of Earth size will produce a substantial transit signal because of the small size of the star and the tight orbit the planet must follow …”

    I don’t see why the tightness of the orbit would make any difference. I suppose the transit signal is the proportion of the star’s light intercepted by the planet’s disk. But the distance of the planet to the star will be immaterial to that proportion, at those huge distances.

    • Paul Gilster July 13, 2018, 15:44

      The tight orbit also makes it more likely that a transit will be observed — I’ve added that into the text to clarify the point.

    • Robin Datta July 13, 2018, 16:08

      Double the distance for a quarter of the signal; triple the distance for one-ninth of the signal, et cetera.

  • John walker July 13, 2018, 14:56

    Welcome news that the still poorly understood red dwarf population is here receiving a closer look. However, despite (the star)Ross 128’s current quiesense, Ross 128(planet) will still have experienced the brutal atmosphere stripping outbursts of a red dwarfs early phase. Even the discoverers of this world rejected the term “habitable” to describe it finding simply “temperate” more appropriate.

    This latest papers modelling also has its fair share of critics. Guillem Anglada Escude: “As far as I can tell, they measured elemental abundances on the star (that’s the new thing) and then do a lot of modeling and extrapolation. I would not call this a breakthrough, and inferring the planet’s properties this way takes a big leap of faith.”

    Let’s not jump the gun again on habitability. M Class systems are numerous but they are far from auspicious arenas for biosphere development.

    • Robin Datta July 13, 2018, 16:21

      Agreed.
      Can such systems have a sufficient abundance of comets with volatiles, particularly good ‘ol H₂O, to baptize a planet after the purgatory phase has abated?

      • Jens July 13, 2018, 17:46

        When does it abate?

    • Michael Fidler July 14, 2018, 12:45

      The larger the planet with an earth type composition the more likely it will have plate tectonics and volcanic outgassing. The probability that large amounts of water would also be trapped in the tectonic cycles would also replenish any atmosphere that was lost.

      • Alex Tolley July 15, 2018, 14:13

        With the discovery of so much water below the surface, I do wonder what the implications of atmosphere longevity are. Mars is a classic case where cold has resulted in very close to the surface glaciers despite no atmosphere. Is there a reservoir of liquid water in the rocks much deeper? Is it possible Venus has water trapped deep below its surface too?

        • Michael Fidler July 16, 2018, 3:45

          Venus is a good example of how dissimilar two planets can be. It would of been nice if a 2 earth mass planet would of been in the solar system, the only example is Neptune of larger planets and nothing in between. The impact theory as the starter of plate tectonics would seem to be the norm for larger then earth terrestrial planets and with a higher chance of large H2O oceans on such worlds, a larger reservoir of water in the deep subducted rocks. Venus seems to be more like Jupiter’s moon Io with it’s continues volcanic activity. This is another good reason we need more probes going to our nearest planetary neighbor, to have a better understanding of the geologic processes that are taking place on Venus surface and interior.

  • andy July 13, 2018, 17:40

    The inferred insolation is 1.79±0.26 times that of Earth, which is close (within the error bars) to Venus at 1.91 times Earth’s insolation. If the planet is able to build up a layer of reflective clouds on the dayside (as predicted by models of the atmospheres of slowly-rotating planets), it might be able to support liquid water, though the orbital period (and hence synchronous rotation period) is right at the transition where this effect begins to kick in. I’m fairly sceptical about the prospects for a habitable Ross 128 b but more data would definitely be welcome.

    Of course, if it does have a thick layer of clouds, that will make transit spectroscopy and detecting biosignatures harder… it seems that the universe conspires to make these kind of observations very difficult!

  • Andrew Palfreyman July 14, 2018, 1:46

    The fact that we almost have the tech to get a StarChip there within one human lifetime is indeed a heartening thought.

    • Robin Datta July 14, 2018, 8:42

      When the technology becomes avalable it should not be limited by human lifetimes. If an unmanned probe can be designed and manufactured to be viable and functional for millennia, it should be launched as an investment for far future generations, even with the possibility that it may be obsolete by then.

      • Andrew Palfreyman July 15, 2018, 1:12

        Beg to differ. Either we continue to progress, in which case we overtake this antique curiosity multiple times, perhaps even scooping it up to plonk it into a museum. Or our civilisation crashes, in which case such a probe will be of no use to anyone, ever.

        • Alex Tolley July 15, 2018, 14:08

          Suppose the Romans had launched such a probe. Here we are, a civilization later, and we would be able to benefit from its data 1500-2000 years later. Local civilizations may collapse, but that doesn’t mean that there will be no future civilization to follow us.

          We have no guarantee of continued technological progress in a certain direction (e.g. spaceflight propulsion) so it makes sense to do what the technology allows now, rather than waiting. Indeed, doing something may well be the driver to development, whereas doing nothing results in academic papers but no engineering progress.

          • Joe H. July 18, 2018, 13:00

            Will we be able to interpret data 1500-2000 years from now? I suspect so, but it will be a challenge. Remember when WordStar was state of the art word processing? Try finding software to read those files now. It can probably be done, but it’s not easy.

  • andy July 15, 2018, 8:42

    Looks like there are additional planets in the LHS 1140 system: an inner Earth-size planet and an outer Neptune-like world. In the preprint from Feng et al. they note that the inner Earth-size planet is not secure from the RV detection, but apparently transits have been detected.

  • ljk July 16, 2018, 9:06

    Almost one year ago exactly we were talking about unusual radio signals coming from the Ross 128 star system:

    https://www.centauri-dreams.org/2017/07/19/keeping-an-eye-on-ross-128/

    Any follow-up news on this since?

    • andy July 18, 2018, 14:48
      • ljk July 18, 2018, 16:36

        The article you link to (from July of 2017) has them taking a survey (!) asking what the Ross 128 signals could be! Science by popular vote? Whatever happened to actually searching for evidence?

        A paper was released in late October of 2017 with the SETI officials concluding it may have been terrestrial geosynchronous satellites:

        http://astrobiology.com/2017/10/breakthrough-listen-follow-up-of-the-reported-transient-signal-observed-at-the-arecibo-telescope-in.html

        But before those of who you are comfortable with a humans-only galaxy all make a collective Wheh!, note that the conclusions in the paper are once again a guess (what, no survey?) and no actual satellites that might be the culprit are even listed, never mind further investigated to see if they could produce the actual signals. And is it not a rule that satellites should not be signaling in the vicinity of Arecibo?

        I could find nothing else on the Ross 128 signals, as no doubt the authorities and the media feel satisfied with their guess-based conclusions from almost a year ago.

        Was it a signal from ETI? Probably not, but it is more than a little disappointing that the astronomers in charge did not try to investigate further, or longer. This is how and why we are going to miss any real signals from ETI, who likely do not operate like their fictional counterparts.

  • Anton M. July 16, 2018, 11:49

    Well, in our Solar system and only among rocky planets, we have Earth with average density of 5 and Mars with half of that, so good luck guessing the density.

  • ljk July 17, 2018, 9:27

    Finding a planet with a 10 years orbit in a few months

    Current techniques tend to only detect exoplanets with short orbital periods, however a new method developed by UNIGE researchers allows to find within months planets with periods lasting several years.

    https://www.unige.ch/communication/communiques/en/2018/cdp180717/

    To discover and confirm the presence of a planet around stars other than the Sun, astronomers wait until it has completed three orbits. However, this very effective technique has its drawbacks since it cannot confirm the presence of planets at relatively long periods (it is ideally suited for periods of a few days to a few months). To overcome this obstacle, a team of astronomers under the direction of the University of Geneva (UNIGE) have developed a method that makes it possible to ensure the presence of a planet in a few months, even if it takes 10 years to circle its star: this new method is described for the first time in the journal Astronomy & Astrophysics.

    • Mark Zambelli July 21, 2018, 5:34

      Thanks for that UNIGE link Larry… very interesting.

      As for any news re. following up that radio outburst, I too can find nothing. IIRC, the idea that it may’ve been a comet was ruled-out early on and the geostationary satellite idea never really seemed convincing.

      • ljk July 23, 2018, 6:37

        Satellites and comets are apparently the SETI equivalent of swamp gas and the planet Venus as the go-to explanations for unknown signals. Which is fine so long as they are scientifically proven to be the reason, not via a poll or a guess.

  • Spaceman July 17, 2018, 13:40

    Question: would earth still be habitable today if life had never arose here in the first place? Can a planet be habitable to humans without being inhabited?

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