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TOI-1259A: Implications of a White Dwarf Companion

TESS, our Transiting Exoplanet Survey Satellite, continues to roll up interesting planet candidates, with over 2450 TESS Objects of Interest (TOI) thus far identified. The one that catches my eye this morning showed up in the lightcurve of TOI-1259A, a K-dwarf some 385 light years away. The planet designated TOI-1259Ab is Jupiter-sized but some 56 percent less massive, with a 3.48 day orbit at 0.04 AU, and an equilibrium temperature of 963 K.

This system gets interesting, though, not so much for the planet but the other star, a white dwarf (TOI-1259B) in a wide orbit at 1648 AU from the K-dwarf. A team of astronomers led by David Martin (Ohio State University) finds an effective temperature of 6300 K, a radius of 0.013 solar radii and a mass of 0.56 solar masses, a set of characteristics that allow the team to estimate that the system is just over 4 billion years old.

Image: SDSS image of the planet host TOI-1259A and its bound white dwarf companion TOI-1259B. Credit: Martin et al., 2021.

Let’s home in on the issue of white dwarfs, for they force a significant question: How does the evolution of a star affect the evolution of the planetary system around it? The system containing TOI-1259Ab may prove useful in helping us understand the processes at work.

But first, what about white dwarfs themselves? A star like the Sun will expand into a red giant perhaps five billion years from now, ultimately leaving a white dwarf remnant, with whatever planetary survivors still intact left transformed by the process.

So it’s not surprising that about 50 percent of the white dwarfs studied show atmospheres polluted by heavy elements. That would be an indication of material from the surrounding system accreting onto the white dwarf. Still noteworthy, though, is the fact that you would expect such heavy elements to settle out in the presence of the white dwarf’s high gravity. The process of accretion must, then, be relatively common, allowing the stellar atmosphere to be constantly replenished.

White dwarfs produce their own set of challenges when it comes to exoplanet discovery. Finding planets around them is relatively rare. From the paper on the TOI-1259Ab work, I learned that these stars show a lack of the kind of sharp spectral features that would allow precise characterization using radial velocity methods. The push and pull of orbiting worlds is less evident than it would be around other classes of star.

That seems to throw us back on transit methods, something both Kepler and TESS turned into an art, but here we’re dealing with the problem that white dwarfs have a small radius. They’re roughly the size of the Earth, which means that transit probabilities are reduced and so are transit durations. Moreover, according to Martin et al., white dwarfs are faint enough that their light curves are noisy, a problem for astrometric methods — think Gaia — as well.

So while we’ve found atmospheric pollutants at these stars, and have tagged transits of planetary debris, the first confirmed planet orbiting a white dwarf wasn’t found until recently (for more on WD 1856+534, see On White Dwarf Planets as Biosignature Targets).

But back to TOI-1259Ab, which is not a white dwarf planet, but a tightly orbiting Jupiter-sized planet around a K-dwarf. Here the white dwarf is a distant but definitely bound second star. It turns out that even systems with planets and white dwarf companions are rare, as the paper notes:

Only a few bona fide planets have been discovered with degenerate outer companions (Table 2), the first being Gliese-86b (Queloz et al. 2000; Els et al. 2001; Lagrange et al. 2006). Mugrauer (2019) found 204 binary companions in a sample of roughly 1300 exoplanet hosts, of which eight of the companions were white dwarfs. Mugrauer & Michel (2020) found five white dwarf companions to TESS Objects of Interest, including TOI-1259, but without radial velocity data to confirm the TOIs as planets. Some of these planets were also in the El-Badry & Rix (2018) catalogue.

Stellar systems that include white dwarfs have much to teach us, and in the case of TOI-1259Ab, we have a world that has now been confirmed through radial velocity follow-up, and a white dwarf that influenced it. Driving this research forward will be the question of how systems with a degenerate outer companion object evolve, for there are implications here for planetary dynamics. This system should be an interesting target for the James Webb Space Telescope. Consider: The transit depth is 2.7 percent on the K-dwarf host star, which is 0.71 percent of the Sun’s radius. Moreover, its location places the system near the TESS and JWST continuous viewing zones.

The authors believe that the white dwarf in this system is far enough from the K-dwarf that it would not affect the formation of planets, but go on to point out that while it was on the main sequence, it progenitor star would have been both more massive and also closer, which would have made it a factor in orbital dynamics for TOI-1259Ab. The planet’s tight orbit may thus be at least partially the result of migration forced by the now degenerate white dwarf companion.

On the matter of stellar age, it’s worth noting that white dwarfs cool steadily as they age, which helps astronomers constrain the age of the star and the system around it using its temperature and luminosity. Let me quote the paper on this, because star age is so tricky to determine for other stellar types:

If the WD’s mass is known, the initial mass of its progenitor star can be inferred through the initial-final mass relation (IFMR), and this initial mass constrains the pre-WD age of the WD progenitor. Therefore if we have a well-constrained distance to the WD then its total age, i.e. the sum of its main sequence lifetime and its cooling age, can be robustly measured from its spectral energy distribution (SED). Under the reasonable ansatz that the WD and K dwarf formed at the same time, we can then measure the total system age from the WD.

Another useful insight offered by white dwarfs, the study of which may help us explain unusual system architectures like this one, as well as informing us on outcomes as stars and their companions are transformed over time.

The paper is Martin et al., “TOI-1259Ab – a gas giant planet with 2.7% deep transits and a bound white dwarf companion,” submitted to Monthly Notices of the Royal Astronomical Society (preprint).

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{ 11 comments… add one }
  • Drew LePage January 19, 2021, 15:38

    The first observational evidence of the existence of exoplanets (but not recognized as such until nine decades later) was a spectrum of the nearby white dwarf, van Maanen’s Star, taken on October 24, 1917 which showed the presence of heavy metal “pollution” in its atmosphere:

    https://www.drewexmachina.com/2014/10/21/the-first-observational-evidence-of-extrasolar-planets/

    • Edwin January 19, 2021, 19:06

      Thanks Andrew

      Very interesting to follow up and read

      Cheers Edwin

  • Geoffrey Hillend January 19, 2021, 17:06

    I agree with the idea that the planets survived the red giant phase. Maybe they migrated inwards from the gravity of the red giant.

    • Michael January 20, 2021, 10:37

      It would be due to tidal interactions and solar wind drag not gravity.

  • Alex Tolley January 19, 2021, 18:01

    For such binaries, there presumable must be Lagrange points in the system. Could any planets migrate and be trapped in the L4, L5 points, and even transiently at the L1, L2, and L3 points? If so, would it make any sense to look for planets at the stable points?

    • Douglas Muir January 20, 2021, 7:14

      Any planet in a Trojan would be ~1600 AU away from both stars! That seems unlikely.

      Also, the white dwarf would have lost a lot of mass during its red giant / planetary nebula phase, causing it to migrate outwards, probably by several hundred AU. (Okay, strictly speaking the K dwarf would have migrated, since it started as the smaller of the two.) It’s super unlikely a planet would have stayed in a stable orbit through that.

      Doug M.

      • Alex Tolley January 20, 2021, 13:37

        Any planet in a Trojan would be ~1600 AU away from both stars! That seems unlikely.

        Can you explain this to me? What is the issue here? That the distance is so great that there is very little stable attraction at the Trojan point to hold a planet? Some other factor?

        • Douglas Muir January 21, 2021, 5:09

          Current planetary formation theories don’t allow for the formation of large planets that far out. The material in the protostellar disk is just too thin. That’s not an absolute certainty — the outer Solar System has surprised us before. But right now it looks like the way to bet.

          If you’re thinking of a planet accreting in the Trojan point, recall again that there’s just not that much material to accrete. Trojan points are passive accumulators; they’ll very slowly sweep stuff up over astronomical time, but they don’t actually pull stuff in from a distance. At 900 AU, there wouldn’t be that much to sweep up.

          In theory a planet could form somewhere else and somehow migrate to the distant Trojan. It’s not actually impossible. But it would be very unlikely.

          That’s putting aside the facts that (1) the Trojan itself has moved as the F star lost mass and became a white dwarf, and (2) as George King notes, with two companions that close in mass, the Trojan wouldn’t be long-term stable anyway.

          Finally, note that a hypothetical planet at 1600 AU from two not-very-bright dwarf stars would for all intents and purposes be in interstellar space.

          Doug M.

          • Alex Tolley January 21, 2021, 10:49

            I appreciate that very helpful perspective.

    • George King January 20, 2021, 16:03

      The prospect of Trojan planets at L4/L5 Lagrange points for I would guess either one of the binary companions is intriguing.

      I would suppose that both companions would appear on such a Trojan planet more as stars rather than “suns” in what likely would be a perpetual night sky. One where (as the planet rotated) the companions would be fixed in the sky in relation to each other but would both have high proper motion in relation to all the other stars. An interesting sight to see for any civilization that survived to that point, by perhaps fusion power and either synthesized nutrients or evolving to a post-biological existence. And an interesting puzzle to unravel for any sentient life that developed instead along the way in some form of extremophile evolution well outside of the habitable zone for life as we know it. One can only imagine the mythologies that would develop before they figured out the celestial mechanics of what actually was going on, as we ultimately did in our much simpler system.

      But, alas, per Wikipedia, it appears that “[t]he L4 and L5 points are stable provided that the mass of the primary body (e.g. the Earth) is at least 25 . . . times the mass of the secondary body (e.g. the Moon).” https://en.wikipedia.org/wiki/Lagrange_point#Stability (footnote cite omitted)

      If I’m reading the paper correctly, the masses for these companions are much closer now these days to equivalent masses, with the K-dwarf having approximately .68 solar masses and the white dwarf having approximately .561. The resulting instability at the Trojan L4/L5 Lagrange points (per Wikipedia anyway) perhaps may have something to do with the barycenter being closer to the midline between the two companions, canted a bit toward the slightly more massive K-dwarf. As opposed to the barycenter being within a much more massive body such as the Sun in relation to the Earth.

      Always perilous when I venture from liberal arts to math and especially orbital dynamics. But the prospect of a planet surviving the whole process and still being bound–somehow–to the system is intriguing, in terms of, among other things, how the universe would appear from such a body. Too much science fiction in that old creaking wood-floored library in Danvers, Mass., I guess.

  • Douglas Muir January 20, 2021, 7:26

    According to the paper, the white dwarf probably started life as an F type star of about 1.6 solar masses, orbiting about 900 AU out from the K dwarf, give or take. That’s right at the bleeding edge of being able to affect planetary orbits, so it’s just plausible that the F star might have been responsible for the gas giant’s current orbit.

    Mass loss during the F star’s red giant / planetary nebula stage would have caused the two stars to drift further apart, to their current 1600 AU distance. The white dwarf now has about half its starting mass and is nearly twice as far away, so it’s no longer a significant influence on planetary orbits around the K dwarf.

    Doug M.

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