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WASP-189b: An Impressive Debut for CHEOPS

The European Space Agency’s CHaracterising ExOPlanet Satellite (CHEOPS) space telescope reached space in December of 2019, achieving a Sun-synchronous orbit some 700 kilometers up. The instrument has begun its observations of stars near the Sun that are already known to have planetary companions. The idea is to use the 30 cm optical telescope to constrain radius information for these worlds, previously identified in transit and radial velocity studies.

Transiting planets are particularly useful here, because tightening up their radius measurements means we get a better idea of their density, factoring in mass estimates provided by subsequent radial velocity follow-ups. It’s great to see the instrument already hard at work, with measurements of the giant planet WASP-189b, some 325 light years from the Sun, showing us a world that is one of the hottest known, with a likely temperature around 3400℃. By comparison, the surface temperature of the Sun is about 6000 ℃, while smaller M dwarfs can have temperatures below that of this incendiary planet. A planet hotter than some main sequence stars.

Image: CHEOPS observations of WASP-189b in front of and behind its star. Credit: ESA.

First detected in 2018, WASP-189b turns out to have an equatorial diameter of about 220,000 kilometres, making it 1.6 times larger than Jupiter, a figure 15% higher than previous estimates. Also noteworthy is its highly inclined orbit, which moves close to the poles of the star and is detected by studying both transit and occultation (as the planet moves behind the star). At 7.5 million kilometers, WASP-189b is 20 times closer to its primary than Earth, completing a revolution in a scant 2.7 days. Monika Lendl (University of Geneva), is lead author of the paper on this work:

“Because the exoplanet WASP-189b is so close to its star, its dayside is so bright that we can even measure the ‘missing’ light when the planet passes behind its star; this is called an occultation. We have observed several such occultations of WASP-189b with CHEOPS. It appears that the planet does not reflect a lot of starlight. Instead, most of the starlight gets absorbed by the planet, heating it up and making it shine.”

The A-class host star is itself notable, an object that rotates fast enough to deform itself, with an equatorial radius greater than the polar radius, so that it is cooler at the equator than at the poles. The poles thus appear brighter in the CHEOPS data because of this asymmetry, an effect that the authors are able to use to determine the spin-orbit angle of the planet. Needless to say, the planet’s highly inclined orbit raises questions abouts its formation, given that we would expect both star and planet to have developed from a common disk of gas and dust. Past gravitational interactions are likely the cause, forcing the planet to migrate inward.

From the paper:

WASP-189 b is one of the most highly irradiated planets known thus far, with a dayside equilibrium temperature of ∼ 3400 K (Anderson et al. 2018). It orbits an early-type star similarly to the extreme object KELT-9b (Gaudi et al. 2017), but with a longer orbital period of 2.7 days, placing it closer, in temperature, to ultra-short period planets orbiting F and G stars. As such, this object allows us to comparatively probe the impact of different stellar spectral energy distributions and, in particular, strong short wavelength irradiation on planetary atmospheres. As it is orbiting around an A-type star, the system is also relatively young (730 ± 130 Myr, see Section 2.2), providing us with a window into the atmospheric evolution of close-in gas giants.

So CHEOPS has given us a tighter look at WASP-189b, obviously useful information, but what this paper really demonstrates is the power of the space observatory at detecting extremely shallow signals, as is necessary to gauge brightness variations between pole and equator in stars like the primary here. From this we learn about the planet’s unusual polar orbit. We can look forward to measurements of much cooler planets at similar high levels of precision. The best-case scenarios, discussed in the paper, will be objects for which CHEOPS can make phase curve studies that reveal information on the distribution of clouds in planetary atmospheres The resulting transit and occultation catalog will be large and instructive.

Image: CHEOPS results of the observation of WASP-189b. Credit: ESA.

The paper is Lendl et al., “The hot dayside and asymmetric transit of WASP-189 b seen by CHEOPS,” in process at Astronomy & Astrophysics (abstract / preprint).

{ 10 comments… add one }
  • Henry Cordova September 30, 2020, 14:50

    My subjective impression from reading the last few entries here is that exoplanets and their stellar systems are extremely varied. No two are alike, and even in our own solar system the satellites seem all different, both in properties, type and origin.

    Galaxies, clusters, stars, all other astronomical objects seem to fall into families, types and classes. Sure, there are outliers and rogues, weirdos and exceptions, but they are mostly classifiable into finite and recognizable taxa.

    But the retinue of objects orbiting stars all seem to be composed of totally unique creatures. At first, I feared that extrasolar planets would be few and far between, that smooth, well-ordered systems like our own would be hard to find, but at least the ones we did locate would fall into some scheme that could be classified, organized, fitted into some predictable scheme of evolution. Instead, what we find is that planets are ubiquitous, but there doesn’t seem to be some process that generates them according to any predictable plan. Like the satellites in our own solar system, no two seem to be alike, and they resist falling into well-behaved families and groups. The plurality of worlds around other stars suggests they are common throughout the universe, but that earthlike worlds and stable planetary systems may well be the exception, not the rule.

    The limitations of our technology and the unforgiving influence of selection effects may be conspiring together to only show us the oddballs. Perhaps earthlike worlds around stable stars are common, we just haven’t seen any due to our crude investigative methods.

    I hope so. Being “special” isn’t a distinction I particularly care for. I’m not a religious man, but I do have faith in the Principle of Mediocrity. I just do not care for geocentric universes. I do not want to live in a cosmos where we are all alone.

    • AlexTru October 1, 2020, 2:06

      But the retinue of objects orbiting stars all seem to be composed of totally unique creatures

      I am sure that we do not have enough data about planet systems to begin any classification, even this data that we have is very limited and preliminary, so cannot agree with tour statement.
      I am sure that when we will collect more data about extraterrestrial planets, a classification and planets formation laws will became more obvious.

      • Henry Cordova October 1, 2020, 9:15

        My remarks were inspired not just by our exoplanets catalogue (which is actually quite extensive) but by our own solar system’s satellites. Prior to our missions to the outer planets, the expectation was that these would all be either lumps of slag or ice, pretty much the same–dull and uninteresting.

        What we see is geysers, volcanoes, oceans, tectonic ice plates, atmospheres, hydrocarbon lakes, precipitation and a distinctive and unique geomorphology on almost every world, some quite dramatic and spectacular. Refer to the earlier article on Saturn’s moons; each is a treasure, and Saturn is just one family.

        The solar system building process rejects conformity, uniformity and standardization, it seems to favor diversity–if I may be permitted the use of such a politically biased word…

        • AlexTru October 2, 2020, 1:28

          Limit of our knowledge, does not mean – no classification.
          Same situation (lack of classification), was in different branches of science.
          As example, Quantum mechanic, in the beginning scientists was surprised by wide spectrum of new discovered elementary (colored, strange, charm etc.) particles, but finally it was somehow classified by Standard Model and diversity vanished …
          In situation with planets we even do not have rich data about discovered planets, even the planets of Solar System.
          And information about more remote planets now mostly virtual, than real…

    • Robin Datta October 1, 2020, 19:18

      An apparent homogeneity as the first impression of massive quantities may conceal a diversity that seems to defy categorization, but which with further study, does fall into distinct groups.

      The possibility of xenobiology so alien as to be nigh unrecognizable is part of the speculation. Even a sentience that moves on a slower time scale, with their seconds akin to our days or weeks, might evade recognition.

    • Michael Fidler October 1, 2020, 20:26

      A prime example is our attitude toward Venus, how much money has been spent to explore our nearest neighbor compared to a much more difficult too reach Mars? We put our values on objects instead of learning from them and have a unicorn syndrome toward extraterrestrial life.

  • Geoffrey Hillend September 30, 2020, 17:22

    We’re not in a geocentric universe. We are not even in a geocentric solar system since that idea was discredited by Copernicus and the scientific revolution that followed him. We are in a heliocentric system.

    A hypothesis for the formation of WASP-189b might be that it formed from it’s own, separate collapsing disk at the same time as the parent star collapsed, so it begin already with an inclined orbit so it didn’t have to form in the protoplanetary disk around the equator of the star like most exoplanetary systems in a flat, disk. Gas giants might form earlier than rocky planets.

    Also even if the light of the exoplanet is not that bright, we can still always subtract the light of star plus planet from only the star light since when the planet goes behind the star, only the star light is visible.

    When a exoplanet passes in front of the star it is called in transit and at that time there is an absorption spectra, but when the planet is not in front of the star it is called out of transit so it’s orbit allows us to see the phases or illumination of the exoplanet by the light of the star. At this time an emission spectra can be taken. the emission spectra is when one is not looking directly at the planet in front of the star. Now if the planet is large and close to the star it will be very hot like WASP-189b, a black body infra red radiator. My point is that we always have both an absorption spectra and emission spectra which can only be seen when the planet is at a point in it’s orbit when it is not in front or behind the star. The emission spectra can be fainter if the exoplanet is small. From what I recall reading is the absorption spectra occurs when the planet has to pass in front of the star or be in transit.

  • Michael Fidler October 1, 2020, 20:40

    Is WASP-189b even a planet or what is left of a M or Brown Dwarf? The intense irradiation from the A-class host would ablate much of its mass.
    The other problem is the polar orbit around a oblate star would eventually cause the planet to reach an orbit in the equatorial plane of the flattened star. As with any new strange object could this be a Dyson Sphere that is a power generating device for an advance civilization?

  • Geoffrey Hillend October 2, 2020, 15:32

    Quote by Michael Fidler: “The other problem is the polar orbit around a oblate star would eventually cause the planet to reach an orbit in the equatorial plane of the flattened star.” This is not necessarily the case if the gas giant formed like a proto star in a binary system which both form at the same time. The gas giant formed at the same time of the star and immediately went into an odd orbit with an inclination. This is just a theory though where the gas giant did not have to form in a disk around the star’s equator.

    I don’t buy the migration theory of gas giants which have to form inside the cold, snow line and then migrate closer to the star. There are just too many discovered stars with gas giants close to them and the astrophysical probabilities don’t always add up since there is not always another nearby gas giant, star or body to cause such an orbital perturbance.

    • Michael Fidler October 3, 2020, 7:08

      Yes, but this system is 730 million years old and this object is orbiting the primary in 2.7 days. Look at it this way, the primary has a mass like a tube at its equator that pulls on the much less massive planet. Over a relatively short period that mass would cause the planet to pull into the plane of the stars equator. Think, lunar mascons effecting the orbit, speeding up and slowing down as it passes the equator of the star causing the orbital inclination to change. Effects from mass loss of the planet or strong stellar winds might change the orbit but this would be continuing process and should be visible in a short time from timing changes in the transits. The main point is that the oblate spheroid shape of this fast rotating massive A class star should bring the orbit of the planet into the equatorial plane of the star quickly.

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