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Confirming Kepler-1658b: Tight Orbit around an Evolved Star

A planet designated Kepler-1658b is, after a good deal of investigation, demonstrated to be a ‘hot Jupiter,’ orbiting a star that is 50 percent more massive and three times larger than the Sun. The sizzling world is close enough to its star that were you to look into its sky from near the planet, the star would be 60 times larger than the Sun as seen from Earth. And while none of this makes Kepler-1658b unique in our catalog, what does stand out is how we learned all this.

For we are talking about the first planet candidate ever uncovered with the Kepler Space Telescope. Recall that for any transiting planet to be considered confirmed, we need a second kind of detection. The reason: Various astrophysical processes can mimic transit activity. Prudence dictates the backup, and in the case of Kepler-1658b, both the initial estimate of the star’s size and the size of the planet were underestimated. The result was that the putative world became thought of as a false positive whose numbers didn’t check.

Three of the stars in the Kepler field were already known to have planets from ground-based transit observations when the mission launched in 2009, and these three targets became the first designated Kepler Objects of Interest (KOI). What would be labeled KOI 4.01, Kepler’s first new planet candidate, was given a stellar radius of 1.1 times that of the Sun, with an initial transit depth implying that the star was orbited by a Neptune-class planet. A new paper on Kepler-1658b goes through the history of its description and subsequent catalog listings.

The detection remained a false positive, though, until a graduate student at the University of Hawai’i named Ashley Chontos reanalyzed the problematic find:

“Our new analysis,” says Chontos,” which uses stellar sound waves observed in the Kepler data to characterize the host star, demonstrated that the star is in fact three times larger than previously thought. This in turn means that the planet is three times larger, revealing that Kepler-1658 b is actually a hot Jupiter-like planet.”

Armed with that information, Chontos contacted Dave Latham (Smithsonian Astrophysical Observatory), whose team went to work culling the needed spectroscopic data to make a radial-velocity call on the planet. Newly confirmed, Kepler-1658b turns out to orbit at a distance only twice the diameter of the host star, making its orbit one of the closest we’ve found to an evolved star (one that has exhausted the hydrogen fuel in its core and, drawing on nuclear reactions outside the core, is passing through the subgiant phase and swelling into a red giant).

Image: Artist’s concept of a Kepler-1658-like system. Sound waves propagating through the stellar interior were used to characterize the star and the planet. Kepler-1658b, orbiting with a period of just 3.8 days, was the first exoplanet candidate discovered by Kepler nearly 10 years ago. Credit: Gabriel Perez Diaz/Instituto de Astrofísica de Canarias.

This is an interesting find for more than its significance as the first Kepler exoplanet. We know of few planets orbiting stars of this class, a deficit not yet understood. From the paper:

Systems like KOI 4.01 are interesting because giant planets at short orbital periods (P < 100 days) are rare around subgiant stars (e.g. Johnson et al. 2007, 2010; Reffert et al. 2015; Lillo-Box et al. 2016; Veras 2016), although the reason for this is still a topic of debate. On one hand, this may be related to the stellar mass. Subgiant host stars are thought to be more massive than main sequence stars targeted for planet detection. A higher mass could shorten the lifetime of the protoplanetary disk and lead to fewer short-period giant planets orbiting these type of stars (e.g. Burkert & Ida 2007; Kretke et al. 2009). Other authors have suggested that subgiants have fewer short-period planets because these objects may get destroyed by tidal evolution, which is likely stronger for more evolved stars (e.g. Villaver & Livio 2009; Schlaufman & Winn 2013). Distinguishing between those scenarios is further complicated by the fact that it is challenging to derive stellar masses of evolved stars (Lloyd 2011, 2013; Johnson et al. 2013; Ghezzi et al. 2018).

Thus Kepler-186b is a useful outlier that helps us put constraints on what happens as planets approach a death spiral into their star. And another exoplanet mission may provide a useful cross-check:

The Kepler field will be observed by the Transiting Exoplanet Survey Satellite (TESS; Ricker et al. 2015) in mid-2019. Extending the baseline of transit observations to over a decade for Kepler-1658 will allow for a stronger constraint on orbital period decay in more evolved systems. Extrapolating our period decay analysis to the time Kepler-1658 would be observed by TESS would rule out another order of magnitude for the tidal quality factor in subgiant stars.

The paper is Chontos et al., “The Curious Case of KOI 4: Confirming Kepler’s First Exoplanet Detection,” accepted at the Astronomical Journal (preprint).

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{ 14 comments… add one }
  • Bruce D. Mayfield March 8, 2019, 23:43

    This low hanging fruit of the Kepler mission is getting cooked good, and it’s afixin’ ta get recycled, mostly, at least astronomically speaking.

    Hot jupiters probably aren’t on most space fans list of favorite exoplanet, but here’s one thing they’ve got going for ’em in the loooong term: Their gas (along with their star’s unburnt H and He) is going to get recycled back into the interstellar medium when the star expels its outer layers on its way to becoming a white dwarf.

    The universe runs on gas, and it has built in recycling mechanisms!

  • Gary Wilson March 9, 2019, 14:52

    Very interesting. Can anyone explain in layman’s terms how you analyze stellar sound waves from the Kepler data?

    • Bruce D. Mayfield March 9, 2019, 18:19

      I think they show themselves as swellings or pulsations on the star’s surface. This is detectable because it effects radial velocity measurements. Some of the star’s apparent motion is just it’s surface bobbing up and down from internal acoustic waves. This needs to be filtered out to nail down real motions induced by orbiting planets.

  • Geoffrey Hillend March 10, 2019, 17:22

    Very interesting article. Sound waves are created by the stars convection zone and cause the whole star to vibrate and expand and contract which dims and brightens the star which can be detected through telescopes.

    I was interested in how astronomers and Kepler detect sound waves and found this NASA article; https://exoplanets.nasa.gov/news/1516/symphony-of-stars-the-science-of-stellar-sound-waves/

    Sound waves can also be detected by doppler shift, the red and blue shift of the star. Astroseismology also can be used to find the diameter which determines the size.

  • Gary Wilson March 11, 2019, 13:14

    Ok, that’s very helpful. thank you Bruce and Geoffrey!

  • ljk March 11, 2019, 15:18

    There may be 50 billion free-floating planets in our galaxy

    By Paul Scott Anderson in Space | March 10, 2019

    There are at least 200 billion stars in our galaxy, and perhaps even a greater number of planets. Now a new study suggests there could be an additional 50 billion rogue planets, not orbiting any stars.

    https://earthsky.org/space/50-billion-free-floating-planets-in-milky-way

    • Andrei March 11, 2019, 20:50

      Even though these planets are far from enough having enough mass to explain the dark matter problem, such planets might make up a part of the solution to this long standing problem in astronomy.

    • Harold Daughety March 12, 2019, 1:09

      I have wondered how much of the postulated “dark matter” may be rogue planets and smaller chunks of rocks and metals.

  • Harry R Ray March 12, 2019, 9:24

    FYI! The Exoplanet.eu planet count just hit 4,000 today.

  • Gary Wilson March 12, 2019, 12:02

    I think dark matter is now thought to create a “halo” of matter around our galaxy helping to explain the velocity of stars around the galactic centre. This effect would not be explained by rogue planets.

  • Gary Wilson March 12, 2019, 12:17

    Vera Rubin did a lot of fine work on galactic rotation rates which provided early clues to the existence of dark matter. Fifty billion planets is a lot but the mass involved is nowhere near enough to explain the phenomenon.

    • Bruce D. Mayfield March 12, 2019, 19:00

      Right. Consider that in our solar system the Sun contains 99.86% of its mass. Even if there prove to be many more rogue planets than this 50 billion estimate it wouldn’t even come close to solving the dark matter mystery.

  • Robert Feyerharm March 15, 2019, 18:05

    Based on the results of this article it’s conceivable that the estimates of star sizes (and therefore any orbiting planets detected by Kepler) could be significantly off. Maybe all of the “super-Earths” Kepler detected are in fact mini gas giants or actually closer to Earth size?

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