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Confirming Proxima b

I’ve always liked the image of Proxima Centauri b that the ESO’s Martin Kornmesser has conjured directly below, and have used it in a couple of previous articles about the planet. Indeed, you’ll see it propagated widely when the topic comes up. But like all of these exoplanet artist impressions, it’s made up of educated guesses, as it has to be. We don’t even know, for example, whether the world we see here even has an atmosphere, as depicted.

Whether or not it does is important because it affects the possibilities for life around the star nearest to our own. Twenty times closer to its star than the Earth is to the Sun, Proxima b nonetheless receives roughly the same energy, meaning we could have surface temperatures there that would support liquid water on the surface. But the planet also receives 400 times more X-rays than the Earth, which leads the University of Geneva’s Christophe Lovis to ask:

“Is there an atmosphere that protects the planet from these deadly rays? And if this atmosphere exists, does it contain the chemical elements that promote the development of life (oxygen, for example)? How long have these favourable conditions existed? We’re going to tackle all these questions, especially with the help of future instruments like the RISTRETTO spectrometer, which we’re going to build specially to detect the light emitted by Proxima b, and HIRES, which will be installed on the future ELT 39 m giant telescope that the European Southern Observatory (ESO) is building in Chile.”

Image; This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. © ESO/M. Kornmesser.

RISTRETTO is exciting stuff, but I don’t want to get ahead of myself. Lovis (University of Geneva, or UNIGE) is responsible for data processing on ESPRESSO. The spectrograph that is currently the most accurate of any in our arsenal, ESPRESSO has indeed just confirmed Proxima Centauri b’s existence, the world having been first discovered by a team led by Guillem Anglada-Escudé some four years ago (and boy do I remember when that news came in).

Anglada-Escudé’s work was brilliant, as the ESPRESSO work confirms, for his team was working with the older HARPS spectrograph, a formidable instrument in its own right, but one with three times less precision than ESPRESSO. In fact, the ESPRESSO work that confirmed Proxima b, led by Francesco Pepe at UNIGE, currently works with an accuracy of 30 centimeters per second (cm/s), with the goal of eventually reaching 10 cm/s. By way of comparison, the ELODIE spectrograph that found 51 Peg b, the first main sequence exoplanet discovered, operated with an accuracy of 10 meters per second.

So the news is two-fold. First, we have a solid success from an instrument that is changing the game in terms of radial velocity detections, one that presages great things to come. Second, we have tightened up the numbers on Proxima b, which is now shown to have a minimum mass of 1.17 Earth masses, as opposed to the previous estimate of 1.3, in an orbit of 11.2 days. Both the 2016 discovery and the 2020 confirmation represent radial velocity work at the highest level.

Bubbling interestingly in the mix is the possibility of a second planet in the ESPRESSO results. We do have a signal in the data but the cause remains problematic. Radial velocity studies have to contend with photospheric and chromospheric phenomena on the surface of the star (associated with magnetic fields) that can look much like the signal of a planet. Here I turn to the paper on this work, which will appear in Astronomy & Astrophysics:

We find evidence for a second short period signal with a period of 5.15 days and a semi-amplitude of 0.4 m·s−1. If caused by a planetary companion, it would correspond to a minimum mass of 0.29 ± 0.08 M⊕ at an orbital distance of 0.02895 ± 0.00022 AU, with an equilibrium temperature of 330 ± 30 K. Further ESPRESSO observations will be needed to confirm the presence of the signal and establish its origin. We do not detect any additional companions up to 0.4 M⊕ at orbital distances shorter than the HZ of the star.

Take a look at that mass, less than one-third that of the Earth. If this is a planet, it’s the smallest planet ever measured with radial velocity methods. What an achievement if ESPRESSO can pull that one out of the noise!

The paper is Mascareño et al., “Revisiting Proxima with ESPRESSO,” in process at Astronomy & Astrophysics (preprint).

{ 31 comments… add one }
  • Astronist June 2, 2020, 8:53

    “If this is a planet, it’s the smallest planet ever measured with radial velocity methods” – I understand that this is only a minimum mass. Therefore it could be a larger planet, even a giant planet, in a highly inclined orbit, unless someone has pinned down the orbital inclination, and I’ve not heard anything about this.

    • Paul Gilster June 2, 2020, 10:39

      I pulled this from Francesco Pepe at UNIGE, the guy in charge of ESPRESSO. This is indeed a minimum mass, but as far as I know, it’s the smallest yet measured using RV. Says Pepe:

      “If the signal was planetary in origin, this potential other planet accompanying Proxima b would have a mass less than one third of the mass of the Earth. It would then be the smallest planet ever measured using the radial velocity method.”

  • Alex Tolley June 2, 2020, 10:11

    Does the X-ray intensity impact the atmospheric density and GHG composition over the long term? Because if it doesn’t then liquid water will remain at the surface and oceanic life will be shielded by the water column. This was certainly the case for UV on Earth before photosynthesis allowed the formation of ozone to block most of it.

    While X-rays are damaging to cells, I have to think that life may have adapted to this in a variety of ways, from more efficient cell repair to ecosystems that allow for carbon fixing life at the surface to have a transient lifecycle at the surface before being replaced by cells from below. Even trees on the surface may have thick barks to protect the living interior, with rapid replacement of exposed leaves if needed.

    Should we detect unambiguous biosignatures, it will be the planets that are exposed to exotic conditions that might be most interesting for future probes to visit and observe how life has adapted to these conditions.

  • Michael Fidler June 2, 2020, 10:12

    With the goal of eventually reaching 10 cm/s, could we see some clarification as to what process needs to be done to get down to the 10 cm/s? A recent paper on Proxima b dealing with how ocean and continet size relates to habilbility;

    A comment I made in a earlier post about PROXIMA CENTAURI d;

    “What is fascinating is a possible new planet in a 5.15 day orbit with a mass about 2.7 times Mars and a high noon temperature of 134 degrees Fahrenheit. The dusk and morning might be very comfortable and being tidally locked it may have a large ocean or large glaciers on the night side. 134 degrees is a hot day on a earth’s deserts so a large Mars type planet with ice and water underground in large reservoirs. Maybe Canals!”

    It would be nice to find a good infographic of the habitable zone around Proxima Centauri with the orbits of b,c and d on it. Anyone know a good computer-generated imagery (CGI) artist?

    • andy June 4, 2020, 2:57

      I wouldn’t read too much into the temperature value: this is an equilibrium temperature, which is a measure of the energy balance between received and outgoing radiation. Such values generally do not correspond to physical temperatures, especially when atmospheres are involved.

      • Michael Fidler June 4, 2020, 9:44

        So what would the equilibrium temperature of Venus be? The question is this planet d in the habitable zone around proxima centauri or not?

        • Paul Gilster June 4, 2020, 11:24

          The signal in the noise is extremely tiny even for ESPRESSO, but further work may clarify the situation. I don’t think we should be calling this ‘planet d’ yet.

          • Andrei June 5, 2020, 4:10

            Yes this and several other claims are based on so tentative data that it usually would not be accepted as a claim for a discovery in other fields of science. So yes a conformation by other teams and methods are necessary.

            • Paul Gilster June 5, 2020, 6:06

              Let me add, too, that the ESPRESSO team does not call this a planet, but simply considers it an interesting data point that merits follow-up.

  • ljk June 2, 2020, 17:06

    And Proxima c!


    New Tricks from Old Data: Texas Astronomer Uses 25-year-old Hubble Data to Confirm Planet Proxima Centauri c

    2 June 2020

  • Wojciech J June 2, 2020, 17:34

    Hopefully future telescopes will allow us to learn more about this fascinating planet, possibly even take first simple photos of its surface using technologies like this one:

    I don’t have high hopes for biosphere in Centauri System(though it would be amazing beyond belief), however even simple presence of multi-planetary system with asteroid rings(as it seems to be the case here) provides humanity with immense opportunity in far future interstellar exploration.A stepping stone or possible new settlement with numerous and vast resources at arms length in terms of galactic distances. As a species we are in very lucky position.

  • Geoffrey Hillend June 2, 2020, 17:57

    Since Venus still has an atmosphere and it is not in the life belt, most likely Proxima B will have one. The question is what is it’s chemical composition. Nitrogen, Ozone and Oxygen block the x rays and cosmic rays in the upper atmosphere, thermosphere in the ionosphere. Ozone blocks the ultra violet. I don’t expect to see an atmosphere without any water due it’s stronger gravity than Mars. There must be some water loss due to photo dissociation and solar wind stripping. Water vapor, carbon dioxide, sulfur dioxide, and hydrogen sulfide are volcanic gases. An atmosphere can also be replenished by tidal heating from tidal locking.

    • Andrei June 5, 2020, 3:07

      Sorry, things are not quite that simple.
      Our Sun is one unusually well behaved star, while Proxima is the opposite with frequent and violent flares. And Proxima B is extremely close to this star so several models have suggested both atmosphere and other volatiles have been stripped off long ago.
      The little hope that remain is that this planet originally was a water world or mini-Neptune that had just enough gas and liquids removed to make it somewhat habitable today.
      Though a long shot, it is still possible and that’s what some put their bets on here. :)

      • Michael Fidler June 5, 2020, 14:45

        Yes, these planets can be stripped until they are bare bones but the next billion years a star can ripe thru their comet belt and regenerate a stable atmosphere. We tend to put blinders on when dealing with different worlds but look at earth bombarted over and over again. No I believe we are a little short sighted because nature has trillions of ways to make fools of us.

  • anon86 June 2, 2020, 19:38

    What’s the most likely mass? Do we have models that imagine what this planet might be like?

    • anon86 June 3, 2020, 1:21

      I am wondering about the transition to gas giant. We see two competing forces. the radiation stripping the atmosphere & the planet being larger than Earth so maybe holding more gases. Can there be an equilibrium between these tendencies or is it likely that only one wins?

  • James Jason Wentworth June 2, 2020, 21:57

    While this has been obsoleted (at least with regard to Proxima Centauri c, which has now been confirmed), it would seem that “outlier spikes” in radial velocity measurements–while they *could* be due to photosphere “hiccups” (like the random jumping of water that is boiling in a saucepan)–could be identified by watching for and measuring them over time. An exoplanet will yield a regular, repeating pattern of “spikes” (the pattern takes a long time to occur and recur, for more distantly-orbiting exoplanets), while photosphere movements that produce “false planet positive” indications will, when recorded over time, be random. Also:

    The discovery of two (so far) planets orbiting Proxima Centauri also bodes well for our other red dwarf neighbors (Barnard’s Star, Wolf 359, Lalande 21185, Luyten 726-8, Ross 154, Ross 248, etc.) possibly having planets. With the increasingly-fine radial velocity measurement capabilities, perhaps we will, before too much longer, be able to “disentangle and detect” any Earth-sized planets orbiting Alpha Centauri A and/or Alpha Centauri B. These two stars are the stellar equivalent of the far side of the Moon. With it being the closest other world, it was so frustrating and infuriating to NOT know–until 1959–anything about the 41% of its averted side that we could not see from the Earth’s surface. Likewise, our two closest stellar neighbors (besides Proxima), the only Sun-like ones in that system where anyone ^really^ hopes to possibly find Earth-like, life-bearing planets, are in full view, yet we can’t (yet) detect any such planets–if there are any–orbiting either or both of them.

  • Zanstel June 3, 2020, 4:04

    I don’t think that X-Rays could be a problem for ocean life. We are too fixed in Earth-like life, but even for Earth, in most of the planet’s life, the living creatures lived only on the seas.

    Some meters of water must be enough to block lethal radiation. And we know that at least simple forms of archeas can survive that, so they could replace the algae in that environment and do the capture energy while biomass later could move to the deep, where complex life forms are safe from the worst radiation.

    That seems a biocompatible environment. Just not for humans.

    • Wojciech J June 3, 2020, 10:26

      ”That seems a biocompatible environment. Just not for humans.”
      I highly doubt any exo-biosphere will be compatible for humans(at least any not heavily modified ones). Settling other planets is a very romantic dream but ultimately I think we will probably mostly stick to space habitats. That doesn’t mean there won’t be exploration and settlement of some sorts, but I think limited, think O’Neill habitats over alien worlds researching them.

    • Ivar Brouwer June 3, 2020, 17:52

      These notions might apply to settling within our solar system, but once humankind kan travel to the stars, we might not really resemble biologicals anymore. Sending a micro probe might be in the cards in the foreseeable future, but beyond that settling Proxima is far far away.

    • anon86 June 3, 2020, 22:34

      One thing I vaguely remember hearing, which maybe someone more knowledgeable can comment on, is that radiation could burn away the atmosphere and then, due to water vapor evaporating, strip away the oceans.

      Also this is solid review material:

    • Mike Serfas June 7, 2020, 19:10

      Whether within Callisto or upon a super-Earth, an environment with copious water, friendly gravity, warmth within and merely chilly temperature outside seems reasonably suitable for human colonization along the rough outline of https://www.poetryfoundation.org/poems/43991/kubla-khan : read twelve lines “And from this chasm….” — fountains of piped geothermal heat, a descending river of condensate, transparent domes of ice reinforced with walls and towers to greenhouse the gardens of imagination in a dimmed but whitened sunlight.

  • Michael Fidler June 4, 2020, 13:40

    Since this is our closet neighbor it would make for a designater now before more planets are discovered around Proxima. You have had to clarify which ones in the case of c. Alpha Centauri Bb has been around for a long time but with little evidence , so the d designater should be used until proven not to exist to keep the confusion from newer discovery. Can we take a vote?

  • Wojciech J June 5, 2020, 13:56

    Now that this planet has been confirmed, doesn’t it make the first confirmed closest exoplanet to our Solar System? This seems quite significant and I believe we should start to think what specific unique name it should have instead of general Proxima B-perhaps Sagan?

    • Mike Serfas June 10, 2020, 15:20

      To name a planet after a person seems presumptuous. In deference to the no doubt foolhardily optimistic view that the universe has spun a thread for humanity to follow, I would suggest Clotho, one of the three Fates with a say in matters of new life. (Though there is a 97 Klotho in another system, it would seem unreasonable to disambiguate a rock in one system from a planet in another; if each exoplanet in the cosmos ends up with a name to its own that seems uncommonly well organized already)

      • Ron S. June 11, 2020, 12:22

        Extrapolated from findings to date there are many times more planets in our galaxy than human beings that have lived throughout our existence, and will be born into the foreseeable future. Eventually everyone can have a planet (or ten) named after them, and their pets. Of course if there exist ET civilizations they may have stamped their names on the planets first.

  • Ronald June 8, 2020, 16:45

    And also very exciting, news of probably the closest earth-sun analogue found so far, that is, an earthlike planet in an earthlike orbit around a sunlike star, found in the Kepler data:

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