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Benefits of a ‘Snow Line’ Neptune

The formation of planets like Neptune under the core accretion model involves a protoplanetary core that reaches around 10 Earth masses before beginning to pull in surrounding gas, the latter being a runaway process that quickly builds the atmosphere around the object. Core accretion is most efficient at doing this just outside the snow line, but if we want to understand and test the theory, we need to know a lot more about how planets are distributed in this region.

And that’s a problem, because recent microlensing surveys have found that planets like Neptune are most abundant much more distant from their host stars. Outward migration can account for such worlds, but we know little about exoplanets that form at the snow line, which is where the condensation of ices can factor into the emergence of a new world.

Is this just an artifact of our still evolving microlensing detection techniques? Perhaps, and exceptions to the rule can therefore be helpful. Recent work that began with a discovery by a Japanese amateur astronomer has now blossomed into a full-scale study of a snow line Neptune around a star that, unlike most viewed by microlensing, is actually fairly close. The amateur, Tadashi Kojima in Gunma Prefecture, Japan, found the object in Taurus, the beginning of observations from numerous observatories that uncovered the microlensing behind the discovery.

The planet Kojima-1Lb orbits a star 1600 light years away, while the star it passed in front of is some 2600 light years out. Remember that the curvature of spacetime in the presence of massive objects accounts for this phenomenon, as warped space around the nearby star acts as a lens that focuses the light from the background star. Within this brightening, a transient but useful phenomenon, changes in intensity can reveal a planet orbiting the foreground star, as happened here. This discovery is unusual because most microlensed planets have been observed toward galactic center, which makes sense given the sheer abundance of stars there. This one is found close and toward the galactic anticenter.

Image: Diagram illustrating the microlensing event studied in this research. Red dots indicate previous exoplanet systems discovered by microlensing. Inset: Artist’s conception of the exoplanet and its host star. Credit: The University of Tokyo.

76 days of observation by a team led by Akihiko Fukui at the University of Tokyo took advantage of 13 telescopes around the world, including two at the National Astronomical Observatory of Japan’s Okayama Astrophysical Observatory. The work, as revealed in a paper just published in the Astronomical Journal, show a Neptune-class planet orbiting a star on the border line between K and M-class dwarf status. The planet is about 20 Earth masses and orbits at 1.08 AU, snow line distance for this system.

So we’ve got a helpful Neptune at the snow line. The paper draws an interesting but highly tentative conclusion from this detection:

The orbit of Kojima-1Lb is a few times closer to the host star than the other microlensing planets around the same type of star and is likely comparable to the snow-line distance at its youth. We have estimated that the detection efficiency of this planet in this event is ∼35%, which may imply that Neptunes are common around the snow line.

In other words, Fukui and colleagues calculate the a priori detection probability of this kind of planet at 35 percent, making this chance detection a possible indication of an abundance of such worlds around the snow line of other stars. The paper goes on to point out that the host star here is the brightest among all those studied in microlensed systems, offering the opportunity to do follow-up spectroscopic analysis to characterize the host star and to refine both mass and orbit of the planet through radial velocity studies.

The paper is Fukui et al. “Kojima-1Lb is a Mildly Cold Neptune around the Brightest Microlensing Host Star,” Astronomical Journal Vol. 158, No. 5 (November 1, 2019). Abstract / preprint.

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{ 8 comments… add one }
  • Alex Tolley November 5, 2019, 13:49

    I’m not clear based on this technique whether the planet needs to transit the star or not. If it does, then a BoE calc suggests that it could be determined by a transit method as each year is about 1.5x an Earth year, so 3 transits within 5 years?

    If this is correct, and their calculations of probability are correct, shouldn’t we be able to detect lots of these Neptunes at the snow line using transit methods, especially around M-dwarfs? If so, where are they?

    • Paul Gilster November 6, 2019, 8:34

      No transit needed for microlensing, but your question about snow line Neptunes around red dwarfs is certainly on point!

  • Geoffrey Hillend November 6, 2019, 1:56

    I don’t think the exoplanet has to transit the background star. Exoplanet microlensing involves two stars, the further distant background star and closer the lensing star and it’s orbiting lensing exoplanet. The distant background star has to be exactly aligned and behind the lensing star. When the light from the background or light source star passes by the lesning star, the light has to pass through the lensing stars gravitational field which brightens that light so the lensing star appears to be brighter due to the added light of the background star behind it. When the closer lensing star moves out of exact alignment, so, it’s orbiting exoplanet moves into alignment with the telescope on Earth. When light from the background star passes through the gravitational field of the lensing planet, there is another spike in brightness, but not as bright and the lensing background star combination of light, since the exoplanet’s gravitation field is much smaller and less of a lensing effect. A gravitational lens focuses and brightens the light that passes through it, so the observer in a telescope on Earth sees a increase in brightness of the lensing star.

  • Harry R Ray November 6, 2019, 10:35

    IT’S “OFFICIAL”! The exoplanet.eu website has entered Proxima c as a “CONFIRMED” exoplanet! Unfortunately they also DELETED HD 114762 so the total exoplanet count stays at 4,126. For details, go to https://solar-flux.forumotion.com

  • Geoffrey Hillend November 6, 2019, 16:46

    I made a mistake. The lesning planet would have to transit the background star or at least close to a transit for the light to pass through the exoplanet;s gravitational field to be microlensed.

  • Geoffrey Hillend November 6, 2019, 16:58

    I be specific, the lensing exoplanet could transit the background star, but does not have to transit it, but the planet has to be aligned closely enough so the light of the background star has to go through the lensing planet’s gravitational field.

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