Crumbling planets might trigger repeating fast radio bursts

Jan 27, 2020
It’s one more hypothesis among many for the source of these flares


Interactions between a planet and a magnetic neutron star (illustrated) might be the source of repeating, millisecond-long bursts of cosmic radio waves. MARK GARLICK/SCIENCE PHOTO LIBRARY/GETTY

By Liz Kruesi
APRIL 18, 2022 AT 7:00 AM

Fragmenting planets sweeping extremely close to their stars might be the cause of mysterious cosmic blasts of radio waves.

Milliseconds-long fast radio bursts, or FRBs, erupt from distant cosmic locales. Some of these bursts blast only once and others repeat. A new computer calculation suggests the repetitive kind could be due to a planet interacting with its magnetic host star, researchers report in the March 20 Astrophysical Journal.

FRBs are relative newcomers to astronomical research. Ever since the first was discovered in 2007, researchers have added hundreds to the tally. Scientists have theorized dozens of ways the two different types of FRBs can occur, and nearly all theories include compact, magnetic stellar remnants known as neutron stars. Some ideas include powerful radio flares from magnetars, the most magnetic neutron stars imaginable (SN: 6/4/20). Others suggest a fast-spinning neutron star, or even asteroids interacting with magnetars (SN: 2/23/22).

“How fast radio bursts are produced is still up for debate,” says astronomer Yong-Feng Huang of Nanjing University in China.

Huang and his colleagues considered a new way to make the repeating flares: interactions between a neutron star and an orbiting planet (SN: 3/5/94). Such planets can get exceedingly close to these stars, so the team calculated what might happen to a planet in a highly elliptical orbit around a neutron star. When the planet swings very close to its star, the star’s gravity pulls more on the planet than when the planet is at its farthest orbital point, elongating and distorting it. This “tidal pull,” Huang says, will rip some small clumps off the planet. Each clump in the team’s calculation is just a few kilometers wide and maybe one-millionth the mass of the planet, he adds.

Then the fireworks start. Neutron stars spew a wind of radiation and particles, much like our own sun but more extreme. When one of these clumps passes through that stellar wind, the interaction “can produce really strong radio emissions,” Huang says. If that happens when the clump appears to pass in front of the star from Earth’s perspective, we might see it as a fast radio burst. Each burst in a repeating FRB signal could be caused by one of these clumps interacting with the neutron star’s wind during each close planet pass, he says. After that interaction, what remains of the clump drifts in orbit around the star, but away from Earth’s perspective, so we never see it again.

Comparing the calculated bursts to two known repeaters — the first ever discovered, which repeats roughly every 160 days, and a more recent discovery that repeats every 16 days, the team found the fragmenting planet scenario could explain how often the bursts happened and how bright they were (SN: 3/2/16).

The star’s strong gravitational “tidal” pull on the planet during each close pass might change the planet’s orbit over time, says astrophysicist Wenbin Lu of Princeton University, who was not involved in this study but who investigates possible FRB scenarios. “Every orbit, there is some energy loss from the system,” he says. “Due to tidal interactions between the planet and the star, the orbit very quickly shrinks.” So it’s possible that the orbit could shrink so fast that FRB signals wouldn’t last long enough for a chance detection, he says.

But the orbit change could also give astronomers a way to check this scenario as an FRB source. Observing repeating FRBs over several years to track any changes in the time between bursts could narrow down whether this hypothesis could explain the observations, Lu says. “That may be a good clue.”


Key elements of the original article appear below, using the full title and authors to insure permission:

Periodic Repeating Fast Radio Bursts: Interaction between a Magnetized Neutron Star and Its Planet in an Eccentric Orbit

by Abdusattar Kurban (阿布都沙塔尔·库尔班)1,2,3,4 , Yong-Feng Huang (黄永锋)2,5 , Jin-Jun Geng (耿金军)6 , Bing Li
(李兵)7,8 , Fan Xu (许帆)2 , Xu Wang (王旭)2 , Xia Zhou (周霞)1,3,4 , Ali Esamdin (艾力·伊沙木丁)1 , and Na Wang (王娜)1,3,4
1 Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, Xinjiang, Peopleʼs Republic of China; 2 School of Astronomy and Space Science, Nanjing University, Nanjing 210023, Peopleʼs Republic of China;
3 Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Urumqi 830011, Xinjiang, Peopleʼs Republic of China
4 Xinjiang Key Laboratory of Radio Astrophysics, Urumqi 830011, Xinjiang, Peopleʼs Republic of China
5 Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210023, Peopleʼs Republic of China 6 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, Peopleʼs Republic of China
7 Key Laboratory of Particle Astrophysics, Chinese Academy of Sciences, Beijing 100049, Peopleʼs Republic of China
8 Particle Astrophysics Division, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, Peopleʼs Republic of China Received 2021 February 8; revised 2022 February 11; accepted 2022 February 14; published 2022 March 29

Fast radio bursts (FRBs) are mysterious transient phenomena. The study of repeating FRBs may provide useful information about their nature due to their redetectability. The two most famous repeating sources are FRBs 121102 and 180916, with a period of 157 days and 16.35 days, respectively. Previous studies suggest that the periodicity of FRBs is likely associated with neutron star (NS) binary systems. Here we introduce a new model which proposes that periodic repeating FRBs are due to the interaction of a NS with its planet in a highly elliptical orbit. The periastron of the planet is very close to the NS so that it would be partially disrupted by tidal force every time it passes through the periastron. Fragments generated in the process could interact with the compact star through the Alfvén wing mechanism and produce FRBs. The model can naturally explain the repeatability of FRBs, with a period ranging from a few days to several hundred days, but it generally requires that the eccentricity of the planet’s orbit should be large enough. Taking FRBs 121102 and 180916 as examples, it is shown that the main features of the observed repeating behaviors can be satisfactorily accounted for.

Here we will mainly focus on the periodic repeating activities of FRBs. The most famous periodic repeating sources are FRB 121102 and FRB 180916. FRB 121102 has a period of 157 days (Rajwade et al. 2020), and FRB 180916 has a period of 16.35 days (Chime/Frb Collaboration et al. 2020). Two kinds of models, the single-star model and binary model, have been proposed to interpret the periodic repeatability of these FRBs. The single-star models are mainly concerned with the precession of neutron stars (NSs; Levin et al. 2020; Sob’yanin 2020; Yang & Zou 2020; Zanazzi & Lai 2020), while the binary models associate FRBs with the interaction between the two objects in NS binary systems (Mottez & Zarka 2014; Dai et al. 2016; Zhang 2017, 2018; Dai 2020; Dai & Zhong 2020; Geng et al. 2020; Gu et al. 2020; Ioka & Zhang 2020; Lyutikov et al. 2020; Mottez et al. 2020; Decoene et al. 2021; Du et al. 2021). Usually, the precession period of a NS is unlikely to be as long as 16.35 days(Chime/Frb Collaboration et al. 2020). Additionally, the fixed emission region of FRBs in the precession models has not yet been properly addressed (Xiao et al. 2021). Various observational facts imply that binary models are more likely favored by the periodicity of FRBs. The binary-interaction models can be further categorized into two main classes: wind-like models and accretion/collision-like models. The wind-like models include the binary comb mechanism (Zhang 2017, 2018; Ioka & Zhang 2020), mild pulsars in tight O/B-star binaries (Lyutikov et al. 2020), small bodies orbiting around a pulsar or a magnetar (Mottez & Zarka 2014; Mottez et al. 2020; Voisin et al. 2021), and Kozai–Lidov feeding of NSs in binary systems (Decoene et al. 2021). The accretion/collision-like models include the collision between a magnetized NS and an asteroid belt (Dai et al. 2016; Smallwood et al. 2019; Dai 2020; Dai & Zhong 2020), accretion of strange stars from low-mass companion stars (Geng et al. 2021), and NS–white dwarf (WD) interactions (Gu et al. 2016, 2020). FRBs and their counterparts in other wavelengths have been studied by Yang & Zhang (2021), Yang (2021), and by many other authors. As suggested earlier by a few authors, collisions between small bodies and a NS can generate transient events such as gamma-ray bursts (Campana et al. 2011), glitch/antiglitches and X-ray bursts (Huang & Geng 2014; Yu & Huang 2016), and FRBs (Geng & Huang 2015; Dai et al. 2016).

Tidal disruption of minor planets/asteroids around WDs has also been extensively studied (Bear & Soker 2013; Vanderburg et al. 2015; Granvik et al. 2016). Recent simulations (Malamud & Perets 2020a, 2020b) have shown that a planet in a highly eccentric orbit around a WD could be tidally disrupted by tidal force, and materials in the inner side of the orbit would be accreted by the WD. Accreted clumps of such materials may be responsible for the pollution of a WD’s atmosphere by heavy elements (Vanderburg et al. 2015; Malamud & Perets 2020a, 2020b). Similar processes (disruption of a planet) can also occur in NS–planet systems if the initial parameters of the planetary system fulfill the tidal disruption condition (Brook et al. 2014). In fact, GRB 101225A may occur in this way (Campana et al. 2011). Much efforts have also been made to search for close-in exoplanets around pulsars (Geng & Huang 2015; Huang & Yu 2017; Kuerban et al. 2020).

In this study, we propose a new model to explain the periodic repeating properties of FRB sources. We argue that when a planet is in a highly eccentric orbit around a NS, it would be partially disrupted every time it passes through the pericenter. The major fragments generated during the disrup- tion will interact with the pulsar (rotating NS) wind to produce a series of FRBs. This model can naturally explain the periodic behavior of repeating FRBs. The structure of our paper is as follows. In Section 2, we present the basic framework of our model for repeating FRBs. In Section 3, the wind–clump interaction mechanism for FRBs is introduced. In Section 4, the periodicity and active window are described in view of the model. In Section 5, we estimate the evaporation timescale for a planet in an elliptical orbit. In Section 6, we address the possible existence of pulsar planets in highly eccentric orbits. Finally, Section 7 presents our conclusions and some brief discussion.

We demonstrated that to account for the observed repeating FRB periods ranging from tens of days to over one hundred days, a highly elliptical planet orbit with e0.9 is needed. It is a natural question that whether such highly elliptical orbits are possible or not for planets. Here we present some discussion on this issue.

Since the discovery of the first extrasolar planet around PSR 1257+12 (Wolszczan & Frail 1992), about 4700 exoplanets (as of 2021 April 27) have been discovered (see Extrasolar Planets Encyclopaedia—EU9; Schneider et al. 2011). Among them, more than 10 objects are pulsar planet candidates. Although the eccentricities of these pulsar planet candidates are generally very small, high-eccentricity pulsar binaries have been discovered (see references in the databases “Pulsars in globular clusters”10 and The ATNF pulsar catalog11; Manchester et al. 2005). Additionally, a few planets with large eccentricities orbiting around other types of stars have also been detected (see the EU database). Good examples for these include HD 20782 b (e = 0.97 ± 0.01), HD 80606 b (e = 0.93366 ± 0.00043), HD 7449 A b (e=0.92±0.03), and HD 4113 A b (e = 0.903 ± 0.005). The existence of these special planets indicates that the formation of high-eccentricity planetary systems around compact objects should also be possible. Planets with a large eccentricity could be formed around a NS through at least three channels. First, a free-floating planet (FFP) can be captured by a NS when they are in a close encounter. Second, exchange/snatch of a planet may happen between a NS and a nearby main-sequence planetary system. Third, the Kozai–Lidov effect in a multibody system may give birth to a high-eccentricity planet.

Here, we roughly calculate the population of highly eccentric planetary systems in the the Milky Way. It is estimated that there are 100–400 billion stars in our Galaxy (see the Universe Today12 and NASA13 websites). A study based on microlen- sing observations suggests that each star hosts 1.6 planets on average (Cassan et al. 2012). Taking 200 billion as the rough number of stars, then there would be about 320 billion planets in the the Milky Way. Since about 10%–50% of primordial planetary systems experience various dynamical encounters and produce FFPs, as mentioned above (Kremer et al. 2019), it is expected that there should be 20–100 billion FFPs in the whole Galaxy. More than 85% of the stars in the Galactic disk are in a mass range of 0.1 Me <M<2 Me. About 1% of them are expected to experience at least one capture process during their lifetime (Goulinski & Ribak 2018). This allows us to estimate that there are 1.7 billion captures and 99.1% (1.68 billion) of them give birth to planets in a highly eccentric orbit with e > 0.85. Currently, four highly eccentric (e > 0.9) planets have been confirmed among the observed 4700 planets, corresponding to a fraction of 0.085%. Using this ratio as a reference, it can be estimated that the number of highly eccentric (e > 0.9) planetary systems in our Galaxy is ∼170 million. From the above analysis, we can see that highly eccentric planetary systems are copious in the the Milky Way. However, it is not easy to detect them due to various observational biases. For these planets, the evaporation again can be safely omitted since the timescale is usually much more than 107 yr.

In this study, we aimed to explain the periodic repeatability of FRBs by considering a NS (neutron star)–planet interaction model. In our framework, a planet moves around its host NS in a highly eccentric orbit. The periastron of the planet satisfies a special condition rtd rp 2 rtd, so that the crust of the planet will be partially disrupted every time it pass through the periastron. Fragments with the size of a few kilometers are produced in the process. During this process, the fragments interact with the pulsar wind via the Alfvén wing mechanism* to give birth to FRBs. The periods of repeating FRBs correspond to the orbital periods of the planets. To account for the observed periods of ∼10–100 days, an orbital eccentricity larger than ∼0.9 is generally required. It is shown that the basic features of two well-known repeating sources, FRBs 121102 and 180916, can be satisfactorily interpreted by the model.

It is interesting to note that the interaction of small bodies with NSs has already been studied to interpret repeating FRBs, but generally in a very different framework. For example, Dai et al. (2016) explained repeating FRBs as due to the multiple collisions that happen when a NS travels through an asteroid belt. Decoene et al. (2021) even suggested a three-component scenario which involves a NS, an asteroid belt around it, and a third outer companion. In their model, the outer companion can be a black hole, a NS, a WD, or a main-sequence star. While our model is in principle different, we would like to point out that some ingredients in the above models may also play a role in our model. For example, when the fragments finally arrive at the NS and collide with it, FRBs may be produced via the NS–asteroid collision mechanism (Geng & Huang 2015; Dai et al. 2016). Yet, the time needed for the clumps to fall into the NS is highly uncertain and still needs to be further studied. Note that the disruption distance of rocky planets is ∼1011 cm (Mottez et al. 2013a, 2013b). At this distance, the evaporation takes a 4 time of only ∼10 yr (Kotera et al. 2016). However, the ellipticity of the orbit can prolong the evaporation timescale by several orders of magnitude, to 107 yr. Therefore, the evaporation does not affect our model significantly.


* There are two mechanisms of momentum transfer between the solar wind and the planetary ions. The first one was proposed by Alfvén in 1957 to explain the formation of cometary tails. Above the ionopause there is still a “corona” of neutral gas of planetary origin. These atoms are being continuously ionized by solar ultraviolet radiation and direct impact of the solar wind electrons. When ionized, the new ion is “picked-up”, i.e. accelerated by the convection electric field of the solar wind −VSW × BSW and dragged into the bulk motion of the solar wind following a cycloidal trajectory. The additional momentum transferred to these pick-up ions is extracted from the ambient bulk flow, and tends to slow it down. This process is called “mass loading”.

Alfvén waves have been invoked as an important mechanism of particle acceleration in stellar winds of cool stars. After their identification in the solar wind they started to be studied in winds of stars located in different regions of the HR (Hertzsprung-Russell) diagram**. Here some characteristics of these waves and we present a direct application in the acceleration of late-type stellar winds.

A: Draping of magnetic field lines around perfectly conductive sphere in case of stationary plasma; B: same but when magnetized plasma flow from the right to the left; C: same but with mass-loading by continuously ionizing exospheric atoms. (Adopted from Luhmann et al. 2004)

Note the formation of the Alfvén wing mechanism in C above, which demonstrates mass loading by continuously ionizing exospheric atoms.

(from: Current Systems in Planetary Magnetospheres and Ionospheres, Article in Space Science Reviews · May 2010)


See: DOI: 10.1007/s11214-010-9629-z


** Hertzsprung-Russell diagram (HR diagram) is one of the most important tools in the study of stellar evolution. Developed independently in the early 1900s by Ejnar Hertzsprung and Henry Norris Russell, it plots the temperature of stars against their luminosity (the theoretical HR diagram), or the colour of stars (or spectral type) against their absolute magnitude (the observational HR diagram, also known as a colour-magnitude diagram).

Depending on its initial mass, every star goes through specific evolutionary stages dictated by its internal structure and how it produces energy. Each of these stages corresponds to a change in the temperature and luminosity of the star, which can be seen to move to different regions on the HR diagram as it evolves. This reveals the true power of the HR diagram – astronomers can know a star’s internal structure and evolutionary stage simply by determining its position in the diagram.

The Hertzsprung-Russell diagram the various stages of stellar evolution. By far the most prominent feature is the main sequence (grey), which runs from the upper left (hot, luminous stars) to the bottom right (cool, faint stars) of the diagram. The giant branch and supergiant stars lie above the main sequence, and white dwarfs are found below it. Credit: R. Hollow, CSIRO.
(Note the location of the Sun, above, on the Main Sequence {V] above.)


The article shows that the observed repeating FRB periods, ranging from tens of days to over one hundred days, requires a highly elliptical planet orbit with e0.9***. In the study, "Periodic Repeating Fast Radio Bursts: Interaction between a Magnetized Neutron Star and Its Planet in an Eccentric Orbit" the periodic repeatability of FRBs by considering a NS (neutron star)–planet interaction model is explained.

*** Eccentricity of the ellipse:

The ratio of distances from the center of the ellipse from either focus to the semi-major axis of the ellipse is defined as the eccentricity of the ellipse.

The eccentricity of ellipse, e = c/a

Where c is the focal length and a is length of the semi-major axis.

Eccentricity (e) is measured as the elongation of ellipse. The value of ‘e’ lies between 0 and 1, for an ellipse.



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