In space, catastrophic events happen to stars all the time. Some explode as supernovae, some are ruptured by black holes, and some suffer other fates. But when it comes to planets, stars turn the tables. Then it is the stars that are allowed to wreak havoc.
Expanding red giant stars consume and destroy planets that get too close, and a new study takes a closer look at the stellar engulfing process.
Stars like our sun eventually become red giants. Through nuclear fusion, they turn mass into energy (E=mc2, right?). Over their lifetime, they lose so much mass as energy that they eventually expand and turn red. For planets too close to these swollen spheres, this means the end. They are eventually devoured and completely destroyed.
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Much research has gone into the planetary engulfing process, and a new study calculated that one in ten evolved stars in the Milky Way will engulf Jupiter-mass planets.
The study is titled “Giant planet entanglement by evolved giant stars: light curves, asteroseismology, and survivability.” First author is Christopher O’Connor. O’Connor is a Ph.D. Student in the Department of Astronomy at Cornell University. The study has not yet been peer-reviewed.
The study focuses on two types of evolved stars that are closely related: Red Giant Branch Stars (RGB) and Asymptotic Giant Branch Stars (AGB). The two are very similar, and indeed, RGB stars can become AGB stars. The term evolved star is descriptive enough to cover both, and in this work the important thing is that both RGB stars and AGB stars have left the main sequence.
As these evolved stars lose mass, they expand, and at this stage any planets in the immediate vicinity are in danger. The star’s convective envelope swells, ensnaring the planet. This creates drag that causes the planet to spiral inward toward the star. Astronomers know this, and in this paper the authors examined the frequency of these events and how the stars respond to them.
They describe a sun-like star as a star with 1 to 2 solar masses. About 10% of these stars will envelop a planet between 1 and 10 Jupiter masses. For these mass ratios, the in spiral will require between 10 and 100 years or between 100 and 1000 orbits.
To determine these ranges and the star’s response, the researchers used an open-source astronomy software tool called MESA (Modules for Experiments in Stellar Astrophysics). “We use the Modules for Experiments in Stellar Astrophysics (MESA) software instrument to track the star’s response to the energy output while evolving the planet’s orbit,” they explain. MESA revealed how the differently evolved stars responded to engulfing planets of different masses.
While many astrophysical events play out over thousands, millions, or even hundreds of millions of years, the planetary engulfment is a much more rapid process. But before the planet and star make contact, they pull together two things: stellar expansion and orbital decay. This is the first phase of the entanglement, in which tidal friction causes the planet’s orbit to break up. The authors state that the tidal friction “is most likely due to turbulent dissipation in the star’s convective envelope.” At this point in the process, stellar corona drag and stellar wind are minimal.
Once the star and planet make contact, things change. Tidal friction takes a back seat to drag forces. The authors call this the “grazing” phase. “The ‘grazing’ hydrodynamic interaction between star and planet is complex and three-dimensional,” they write. Complexities in the grazing phase can include phenomena such as the ejection of matter from the star and optical and X-ray transients induced by tremors. But this study puts these phenomena aside for now. “We’re concentrating on them
later “inspirational” phase of engulfment, when planet is fully immersed in envelope,” they write.
When a planet is in the inspiration phase, it gives off heat to the star. The final part of this phase is called the late inspiration phase, and the heat supplied to the star during this phase is largely responsible for the star’s response. The mass of the planet is a determining factor in how much heat is deposited.
This figure from the paper shows the heat deposited in stars in the later inspiration phase. The RGBs and AGBs in the legend are modeled host stars with different masses. The x-axis shows the planet’s mass and the y-axis shows the amount of heat given off. The more massive the planet is, the more heat is deposited. Photo credit: O’Connor et al. 2023
The entanglements cause the star’s envelope to expand and contract, albeit not monotonically. A given mass envelope may expand and contract multiple times during the event. The researchers say the planet can be visualized as a local heat source in the envelope, and the source is moving toward the star’s center. This movement and other properties of the star produce differential expansion and contraction.
This research is consistent with previous research showing that planetary confinement leads to optical and infrared luminosity bursts. The magnitude and duration of these outbursts are largely determined by the mass of the planet and star, although other factors such as rotation may play a role. The researchers found that for all RGB stars and AGB stars enveloping planets up to five times the mass of Jupiter, the star brightens significantly in just a few years.
This figure from the study shows the changes in radius and size for one of the host stars modeled in the study. The top image shows how a star can expand and contract multiple times during the entanglement. The bottom panel shows how the star changes size. Photo credit: O’Connor et al. 2023
The researchers’ overall results show that for both types of evolved stars enveloping a planet on the low side of the range of up to three Jupiter masses, the changes in stellar structure are mild to moderate. The star’s brightness increases by up to an order of magnitude in just a few years. Brighter stars may experience a double peak.
For stars in the later stages of AGB, the entwined planet can cause a large disturbance in the star’s outer layers. It can trigger supersonic expansion of the star’s outer layers. In this case, the stars may resemble Luminous Red Novae (LRN) in that they produce bright, red, dusty flares.
Regardless of the type of star, the mass of the planet, and how the star responds to the entanglement, the fate of the planet is always the same: tidal disturbance.
This study has limited applicability to our solar system. Our sun is going to be a red giant in a few billion years, but unless something extremely disruptive happens before then, Jupiter is out of reach. Instead, the inner rocky planets are being devoured.
This study is based on simulations rather than observations, but the simulations could help astronomers identify reality when it happens. Engulfments are transient events, and some existing and future telescopes and observatories focus solely on transient and time-based astronomy. When the Vera Rubin Observatory comes online around August 2024, it will detect a variety of transient events, some of which will be evolved stars devouring Jupiter-mass planets.
The results of this study could help identify them.
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