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A Gravitational Wave Approach to Exoplanets

We should always be on the lookout for new ways of finding exoplanets. Right now we’re limited by our methods to stars within the neighborhood of the Sun (in galactic terms), for both radial velocity and transit detections are possible only around brighter, closer stars. The exception here is gravitational microlensing, capable of probing deep into the galaxy, but here the problem is one of numbers. We simply don’t make enough detections this way to build up the kind of statistical sample that the Kepler mission has provided in terms of transiting planets.

So how significant is this kind of selection bias, which thus far has been forced upon us? Without knowing the answer, we would do well to explore ideas like those put forward by Nicola Tamanini (AEI Potsdam) and colleague Camilla Danielski (CEA/Saclay, Paris). The two scientists are looking at the possibilities of gravitational wave astronomy, looking toward the launch, in the 2030s, of LISA, the Laser Interferometer Space Antenna.

Image: Artistic representation of gravitational waves produced by a compact binary white dwarf system with a jovian mass planetary companion. Credit: © Simonluca Definis.

This is rarified air indeed, and the kind of target in play is likewise a rarity, giant exoplanets orbiting detached double white dwarf binaries (DWDs). These are intriguing objects, eclipsing double white dwarfs, remnants of stars like our Sun that have passed beyond their red giant phase. Short-period DWDs with orbital periods less than one hour are rarer still. But they’re worth seeking out because these short-period binaries generate powerful gravitational waves.

What the authors propose in their new paper in Nature Astronomy is to use gravitational waves to find circumbinary planets, worlds that orbit both stars in the binary. We have no planets around white dwarf binaries in our catalog at present, but LISA should be able to remedy that by identifying DWDs both inside and outside the Milky Way. Perturbations in the gravitational wave signal would then flag the presence of a third gravitationally bound object, a giant planet. Thousands of DWDs are expected to be found, producing no shortage of targets.

Tamanini likens the method to Doppler modulations of the kind we use with radial velocity studies, this being their gravitational wave analog. But significantly, gravitational waves are not affected by the kind of stellar activity that can confound radial velocity signals. Nor are we hampered by distance to the degree we are when using electromagnetic means, for gravitational wave perturbations should be apparent from anywhere in the galaxy and nearby galaxies as well. The scientist believes LISA could detect exoplanets down to about 50 Earth masses throughout this range.

If Tamanini’s conclusions are valid, the method would therefore bring the kind of large statistical sample we derived from Kepler to the domain of post-main sequence stellar systems, which means we are pushing into regions in what he calls the ‘planetary Hertzsprung–Russell diagram’ that have not yet been explored. Valid over the entire galaxy, the data would be free of selection effect. Moreover, the paper points out that follow-up observations of close-in DWDs will be helpful in confirming the LISA identification and deepening our knowledge of its characteristics:

Imaging of CBPs [circumbinary planets] around DWDs can be used to test the presence of a second generation of exoplanets in the outer regions of a planetary system, and consequently to provide constraints on migration theories. Emission spectra of these objects will furthermore allow us to estimate their temperature and the main molecular component of their atmosphere, making direct connections to chemical element distributions in the atmosphere of white dwarfs. This would also allow to better understand the observed white dwarf pollution effect. On the other hand, if an existing CBP accretes mass after a common-envelope stage, it becomes brighter, further decreasing the already low planet-to-white dwarfs contrast, meaning that also first-generation, more mature exoplanets can be imaged.

So we are looking at a kind of exoplanet about which we know nothing, if it indeed exists, but bear in mind that about half ot the stellar population occurs in multiple star systems. LISA is expected to measure gravitational waves from thousands of DWDs. Exoplanets here would yield insights into the kind of planet that survives a star’s red giant phase, while probing the regime of any second generation planets — those that form after the red giant phase is complete. We further our knowledge even if LISA finds no exoplanets around DWDs, for then we’ve set statistical constraints on the last phase of planetary evolution.

The paper is Tamanini et al., “The gravitational-wave detection of exoplanets orbiting white dwarf binaries using LISA,” Nature Astronomy 8 July 2019 (abstract).

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{ 13 comments… add one }
  • Christian G July 11, 2019, 17:09

    Interesting…..but if I recall correctly, wouldn’t radiating gravitational waves cause the binary to lose energy, and the distance between the WDs to shrink until they merge? Does this paper or the LISA documentation talk about that, or am I mistaken?

    • Paul Gilster July 11, 2019, 21:31

      I don’t remember seeing the issue discussed in the paper, Christian, but yes, I think white dwarfs in this configuration would gradually coalesce. I’ll go back through the paper to see if this is mentioned somewhere and report back if I find anything.

  • Bruce D. Mayfield July 11, 2019, 20:35

    I had heard and read about this new prospective exoplanet detection method yesterday, and I’d wondered just how common or rare such close, short-period DWD systems might be. Thanks for answering my question Paul.

  • Thomas R Mazanec July 11, 2019, 21:30

    Interesting that such DWDs are Supernova candidates…

  • Geoffrey Hillend July 11, 2019, 22:17

    Impressive. It can detect the gravity waves of the DWD’s as they orbit. LIGO could only detect gravity waves from the merger or collision of two white dwarfs, two neutrons stars and two black holes etc but not the waves while these stars are still orbiting each other. Consequently, LISA “is expected to detect and resolve around 25,000 galactic compact binaries.” Wikipedia.

  • Geoffrey Hillend July 11, 2019, 22:26

    This is also an interesting article and idea to detect exoplanets down to 50 Earth masses with LISA and DWD systems.

  • Charley July 11, 2019, 22:46

    I’m a little bit leery of this idea behind gravitational waves; I know that this is supposed to be the manifestation that space-time is in possession of this an enormous elasticity which is given as the be-all and end-all of current physical understanding, but I’d be a little bit cautious here.
    There has been alternate explanations for such phenomena as the precession of the perihelion of mercury, which do not lean on general relativity to explain it. In addition, current understanding of the rotation of galaxies is not explicable due to any currently understood gravitational theories, so it might be a little bit of a good idea to remain cautious in expecting this to be a useful candidate to tease out unseen perturbation elements. I believe that within the next 50 years at the maximum we’re going to start looking elsewhere for our ideas behind gravitation. At least, I find it conceivable.

  • Geoffrey Hillend July 12, 2019, 17:00

    I’ve never looked at any or for any alternate explanations for the precession of the perhilion of Mercury because I think there are not any that are as good as Einstein’s general relativity to predict observations. According to Einstein’s field equations matter and energy warp space and time that warping is gravity. The Sun emits not only gravitational wave particles which are the quanta of an energy field that is subject to the inverse square law. The precession of Mercury is due to the fact that spacetime warped or curved around large gravitational masses like stars. That curved space is an energy field which perturbs the orbit of Mercury so it precesses. All the rest of the orbits of the planets in the solar system precess. I am not an expert in general relativity, but I see the precession of Mercury’s orbit as a deflection or addition or subtraction of the Mercury’s orbital energy by the energy of the gravitational field emitted by the Sun or it is a property of the fact that Mercury’s orbit is really motion through warped, curved space-time.

    I also think that general relativity supports galactic rotation, but I don’t want to explain my ideas here, but I will hint I didn’t come up with them, so they are nothing new.

    My scientific intuition tells me LISA might be more sensitive than LIGO because it uses warped space to measure gravity waves instead of the expansion and contraction of the effects of gravity or the warping of space time on solid,material objects like the distance between two mirrors, tunnels, etc. which might need a very strong gravity wave from a collision of compact stellar bodies like white dwarfs, neutron stars and black holes in order to be detected by LIGO?

    • Charley July 12, 2019, 23:03

      The relation between the particles of gravitation (gravitons?) And the space-time warping is a subject which I don’t pretend to understand; however, it can be said that space-time does not represent I’ve been led to believe any kind of material or energy type of entity. So how does gravitons fit into this picture?
      Turning attention to the procession of the perihelion of mercury you have alternate explanations which basically states that the sun is a flattened sphere (although there is some contesting of that idea) and the fact that it possesses this flattening can be enough to produce the observed procession:
      https://www.mathpages.com/home/kmath280/kmath280.htm
      Finally, there is some contention that GR does not fully explain galactic rotations, I’ve seen a few different YouTube videos, which suggest that it is not quite the answer to totally account for the rotational anomaly.
      https://www.youtube.com/watch?v=uqMl5HQLCLM
      https://www.youtube.com/watch?v=Y5SvUaMw_eA

    • Alex Tolley July 13, 2019, 19:14

      The Sun emits not only gravitational wave particles which are the quanta of an energy field that is subject to the inverse square law.

      I believe that gravitons are purely theoretical so far. Despite various theories to combine quantum theory and GR for gravity, or starting afresh with String Theory, AFAIK we have no evidence that such gravitons exist. Warped space-time appears to exist at the macro level, but we do not know what happens at the quantum level, if anything even does.

      If gravitons exist, and if you could create a [tractor] beam, I don’t even know what that would look like in warped space.

  • Geoffrey Hillend July 13, 2019, 17:17

    Newton’s laws including the inverse square law do not explain the precession of Mercury’s perhilion. Newton considered gravity to be instantaneous, but it works at the speed of light in general relativity and reality. The inverse square law does not explain the effect of matter and energy on space-time, so it does not explain the precession of the perhilion of Mercury. General relativity is a curved space-time, the result of an energy field and is not explained by Newton’s law of universal gravitation.

    The article Newtonian Percession of Mercury’s Perhilion does not explain Mercury’s perhilion precesson. The effects of the other planets in the solar system do not account for Mercury’s orbital precession. GR explains it.

    I don’t wish to comment on GR and galactic rotation as I am saving my ideas for my own paper on cosmological gravitation I am writing.

    • Charley July 15, 2019, 20:43

      @Geoffrey Hillend , I’d like to push back a bit on your declaration that Newtonian principles do not express the explanation behind the procession of the perihelion of mercury. Let me boil it down into two points: first off, no one knows the extent of the flattening and/or the composition of our star the sun. If the solution to the Newtonian equations are not EXACTLY for a perfectly spherical body and/or a uniform mass density object procession of mercury will occur. I would argue that neither condition of uniformity and are composition can be guaranteed in the case of the sun.
      Secondly, based on anecdotal evidence competent professional physicist have literally been thrown out of offices of other physicist when they dared to present their own conclusions, which differed from the orthodoxy of Einstein’s field equations. That in and of itself does not prove that Einstein’s field equations are incorrect, that just shows that reasonable people can disagree in a reasonable manner.
      Out of my own burning interest you stated that you would not wish to comment on GR or the galactic rotation since you are in the process of writing a paper on cosmological gravitation. Am I to gather from that comment that you are a professional physicist and are dealing in areas of gravitational research? Your comment would seem to suggest that in fact you have a somewhat fundamental disagreement with GR yourself-and that sort of whetted my appetite, and the reason for my question.

  • Geoffrey Hillend July 14, 2019, 17:11

    There is no reason to think that gravitons do not exist based on our knowledge of waves and the Heisenberg uncertainty principle which should in theory apply to all fields and waves. In quantum field theory, high frequency particles or photons from the electromagnetic spectrum like gamma rays and x rays have very short wavelength and high energy. A short wavelength is very small. For example x rays and even shorter wavelength gamma waves go right through matter easily since their wavelength is so small it fits between the spaces between atoms and molecules so with x rays, we can see into our bodies. These very small wavelength photons are like point particles.

    The Heisenberg uncertainty principle applies to gravity waves. We can mix many wavelengths, so they destructively interfere to isolate the position of a gravity wave, so instead of a whole train of waves of crests and troughs is reduced to just one crest which would be a high frequency gravity wave. It would have a very short wavelength and behave like a particle. Of course when we isolate the position of the wave then the momentum becomes uncertain. If we look at the velocity and momentum, the wavelength and energy become certain, but the position becomes uncertain and we have a many crests and troughs, or rises and falls.

    There is no quantum field theory of gravity since the quantum general relativity is non-renormalizable and is background independent a problem with the Hamiltonian which is based on “idealized, vibrating, massless, coupled balls and springs.” Dr. Eric Davis, youtube video. Since quantized general relativity can’t have the infinite values removed from it, so it can’t calculate the probability a particle will absorb and emit a graviton. Wikipedia, Graviton.

    This does not mean there can’t be a quantum of gravity or particle which is simply a discrete energy level. It does mean there can’t be a Hamiltonian applied to gravity. In the future we might find a better mathematical foundation than the ball and spring Hamiltonian for predictions in quantum field theory, so quantum electrodynamics might be refined and improved and the ball and spring method could become obsolete without a need for renormalization, and infinites, but this is only a speculation that a quantized general relativity might improve or change QED.

    A tractor beam is absolutely possible, but it would have a range limit based on energy and inverse square law. I know how it would look and work as far as the warping of space is concerned, but I don’t want to give away my ideas. We could also have artificial gravity which would be good for long journeys through space and we can even has a propulsion system without a reaction mass with gravity control.

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