This Binary System is Destined to Become a Kilonova

Kilonovae are extraordinarily rare. Astronomers think there are only about 10 of them in the Milky Way. But they’re extraordinarily powerful and produce heavy elements like uranium, thorium, and gold.

Usually, astronomers spot them after they’ve merged and emitted powerful gamma-ray bursts (GRBs.) But astronomers using the SMARTS telescope say they’ve spotted a kilonova progenitor for the first time.

A kilonova explosion occurs when two neutron stars—or a neutron star and a black hole—merge. Neutron stars are the stellar remnants of massive stars that explode as supernovae. They’re the smallest and densest astronomical objects we know of.

Astronomers spotted the progenitor kilonova stars about 11,400 light-years away. They’re named CPD-29 2176 and were first spotted with NASA’s Swift observatory. More observations with the SMARTS 1.5-meter Telescope at the Cerro Tololo Inter-American Observatory in Chile revealed more data.

The findings are in a paper titled “A high-mass X-ray binary descended from an ultra-stripped supernova.” It’s published in the journal Nature. The lead author is Noel D. Richardson, an assistant professor in the Physics and Astronomy Department at Embry-Riddle Aeronautical University.

CPD-29 2176 isn’t a pair of neutron stars, not yet. One of them is a neutron star, and the other is a massive star on its way to exploding as a supernova and leaving a neutron star behind. The stage is set for a kilonova about one million years from now, probably later.

But for the pair of neutron stars to merge as a kilonova in the future, the second star has to explode as a particular type of supernova called an ultra-stripped supernova. One of the reasons that kilonovae are so rare is that ultra-stripped supernovae are so rare. And if that’s not rare enough, the existing neutron star also had to explode as an ultra-stripped supernovae.

When a typical supernova (SN) explodes, it releases a tremendous amount of energy. The explosion can kick its neutron star companion out of the system, eliminating the pathway to a potential kilonova. Eventually, the SN will leave a neutron star behind, but it’ll be alone, and there’ll be no opportunity for two neutron stars to merge and explode as a kilonovae.

But an ultra-stripped supernova (USSN) is different. Ultra-stripped means the SN has experienced extreme mass loss prior to exploding. The mass is lost to its stellar companion, and without that mass, the SN explosion isn’t powerful enough to kick out its companion when the SN explodes. These are important details because most stars massive enough to explode as SN exist in binary pairs.

The interactions between the pair of stars prior to one exploding as a SN are critical to any eventual kilonova. Changes in mass, stellar rotation, and nuclear burning all determine the eventual core mass of the SN. Under the right but rare conditions, it creates an ultra-stripped supernova.

This is what’s happening in CPD-29 2176, and the researchers doubt the SN will have enough energy when it explodes to eject its neutron star companion. Not only does the current massive star need to explode as a USSN, but the existing neutron star did, too, or else when it exploded as an SN, it would’ve kicked out its stellar companion. So two USSNs are necessary.

“The current neutron star would have to form without ejecting its companion from the system. An ultra-stripped supernova is the best explanation for why these companion stars are in such a tight orbit,” said lead author Richardson. “To one day create a kilonova, the other star would also need to explode as an ultra-stripped supernova so the two neutron stars could eventually collide and merge.” This explains why kilonovae are so rare. Mass stripping and weakened SN explosions are prerequisites.

The researchers explained how the system developed so far and what will likely happen in the future.

First, two massive blue stars form in a binary pair. Stars are never the same size; one is always more massive. As the more massive one approaches the end of its life and swells up, the smaller companion is able to siphon off some of the larger star’s material and strip off a significant amount of its outer atmosphere. Then the larger star explodes as an ultra-stripped supernova, but without enough explosive power to kick out its companion, it leaves behind a neutron star.

The next stage is where CPD-29 2176 is now. There’s the neutron star and the larger star that hasn’t exploded yet. The neutron star is siphoning off the star’s outer layers, causing significant mass loss. The tables are turned.

This infographic illustrates the evolution of the star system CPD-29 2176, the first confirmed kilonova progenitor. Stage 1, two massive blue stars form in a binary star system. Stage 2, the larger of the two stars nears the end of its life. Stage 3, the smaller of the two stars siphons off material from its larger, more mature companion, stripping it of much of its outer atmosphere. Stage 4, the larger star forms an ultra-stripped supernova, the end-of-life explosion of a star with less of a “kick” than a more normal supernova. Stage 5, as currently observed by astronomers, the resulting neutron star from the earlier supernova begins to siphon off material from its companion, turning the tables on the binary pair. Stage 6, with the loss of much of its outer atmosphere, the companion star also undergoes an ultra-stripped supernova. This stage will happen in about one million years. Stage 7, a pair of neutron stars in close mutual orbit now remain where once there were two massive stars. Stage 8, the two neutron stars spiral into toward each other, giving up their orbital energy as faint gravitational radiation. Stage 9, the final stage of this system as both neutron stars collide, producing a powerful kilonova, the cosmic factory of heavy elements in our Universe. Image Credit: CTIO/NOIRLab/NSF/AURA/P. Marenfeld  

Sometime about a million years in the future, the remaining star will have lost much of its mass and will explode as an ultra-stripped supernovae. It won’t be powerful enough to kick out its neutron star companion. It’ll leave a neutron star behind, and the pair of neutron stars will orbit each other until they spiral inward and eventually merge.

“For quite some time, astronomers speculated about the exact conditions that could eventually lead to a kilonova,” said NOIRLab astronomer and co-author André-Nicolas Chené. “These new results demonstrate that, in at least some cases, two sibling neutron stars can merge when one of them was created without a classical supernova explosion.”

The odds against this happening are almost overwhelming. But since kilonovae do exist, circumstances must line up to produce them. So every time we witness a kilonova, we’re witnessing a one-in-ten-billion event.

“We know that the Milky Way contains at least 100 billion stars and likely hundreds of billions more. This remarkable binary system is essentially a one-in-ten-billion system,” said Chené. “Prior to our study, the estimate was that only one or two such systems should exist in a spiral galaxy like the Milky Way.”

There’s more to kilonovae than gravitational waves and a massive explosion. These events are also a source of the Universe’s heavy elements. So studying them not only reveals details about the events leading up to them but it also helps untangle the history of nucleosynthesis.

This figure from the study shows the stellar radii (blue for the secondary star and red for the primary star) and the orbital radius in orange. The primary star’s supernova event is shown as a vertical dashed line. Before exploding as an ultra-stripped supernova, the primary star’s radius grew, then shrank as the secondary star siphoned off some of its mass. Eventually, the same thing will happen to the secondary star. Image Credit: Richardson et al. 2023.

But humanity will have to survive an awfully long time to see this kilonova event. It could take over a million years for the star to explode as an ultra-stripped supernova. And when it does, the two neutron stars will have to be close enough together before a kilonova can occur. That’s a lot of time and a lot of circumstances.

Now that astronomers have spotted one of these potential kilonova progenitors, they might be in a better position to find more. Along the way, they’ll learn more about ultra-stripped supernovae.

“This system reveals that some neutron stars are formed with only a small supernova kick,” said Richardson. “As we understand the growing population of systems like CPD-29 2176, we will gain insight into how calm some stellar deaths may be and if these stars can die without traditional supernovae.”

More:

• Press Release: First Kilonova Progenitor System Identified
• Research: A high-mass X-ray binary descended from an ultra-stripped supernova
• Universe Today: Astronomers See Strontium in the Kilonova Wreckage, Proof that Neutron Star Collisions Manufacture Heavy Elements in the Universe

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The First Stars May Have Weighed More Than 100,000 Suns

The universe was simply different when it was younger. Recently astronomers have discovered that complex physics in the young cosmos may have led to the development of supermassive stars, each one weighing up to 100,000 times the mass of the Sun.

We currently have no observations of the formation of the first stars in the universe, which is thought to have taken place when our cosmos was only a few hundred million years old. To understand this important epoch, astronomers turn to sophisticated computer simulations to test out models of how the first stars formed.

Over the years astronomers have wrestled with the key question of what is the typical size of the first stars. Some early estimates predicted that the first stars could be hundreds of times more massive than the Sun, while later simulation suggested that they would be more normally sized. 

Recently a team of researchers have put together a new round of simulations and come to a very surprising conclusion. Their simulations specifically looked at a phenomenon known as cold accretion. To build large stars you have to pull a lot of material into a very small volume very quickly. And you have to do it without raising the temperature of the material, because warmer material will prevent itself from collapsing. So you need some method of removing heat from material as it collapses very quickly.

Earlier simulations had found the appearance of dense pockets within early galaxies that cool off rapidly from emitting radiation, but did not have the resolution needed to follow their further evolution. The new research takes it a step further by examining how the cold dense pockets that initially form in the early universe behave. 

This simulations revealed that large flows of cold, dense matter can strike an accretion disk at the center of giant clumps of matter. When that happens a shockwave forms. That shockwave rapidly destabilizes the gas and triggers the instant collapse of large pockets of matter.

Those large pockets can be tens of thousands times more massive than the Sun, and in some cases even 100,000 times more massive than the Sun. With nothing to stop their collapse, they immediately form gigantic stars, known as supermassive stars.

The astronomers do not yet know if supermassive stars formed in the early universe. They hope that future observations with the James Webb Space Telescope will reveal clues as to the formation of the first stars and galaxies and determine if these monsters appeared in the infant universe.

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Drag Sail Success! This Satellite Won't Turn Into Space Junk

The European Space Agency successfully tested a solar-sail-type device to speed up the deorbit time for a used cubesat carrier in Earth orbit.  The so-called breaking sail, the Drag Augmentation Deorbiting System (ADEO) was deployed from an ION satellite carrier in late December 2022. Engineers estimate the sail will reduce the time it takes for the carrier to reenter Earth’s atmosphere from 4-5 years to approximately 15 months.

The sail is one of many ideas and efforts to reduce space junk in Earth orbit.   

“We want to establish a zero debris policy, which means if you bring a spacecraft into orbit you have to remove it,” said Josef Aschbacher, ESA Director General.

Last year, China successfully deployed a similar sail from a Long March 2 rocket.

ESA says the sail provides a passive method of deorbiting by increasing the atmospheric surface drag effect and causing an accelerated decay in the satellite’s orbital altitude. “The satellite will eventually burn-up in the atmosphere, providing a quicker, residue-free method of disposal.”

The camera view from a satellite ??as it deploys a sail?? – intended to help it burn up in the atmosphere faster ? to keep space clear. This 3.6-m-area Drag Augmentation Deorbiting System, or ADEO, was developed by @HPS_GmbH ??, supported by #ESATech https://t.co/fDJE8ykIIx pic.twitter.com/2ZIozXAIog

— ESA Technology (@ESA_Tech) February 1, 2023

This test was the final in-flight qualification flight needed to provide the technological proof-of-concept. A smaller 2.5 square meter sail was fitted onto the upper stage of the Electron launch vehicle in 2018. Previously, the ADEO sail was tested on several parabolic flights from 2019 to 2022. This larger orbital version was deployed from the Italian space company D-ORBIT’s ION Satellite Carrier, which carries and deploys cubesats and weighs 160 kg (350 lb).

Artist impression of the Drag Augmentation Deorbiting System (ADEO) breaking sail. Credit: ESA

The sail membrane has a surface area of 3.6 square meters and is made of an aluminum-coated polyamide. It was packed inside an impressively small container, 10 x 10 x 10 cm.  ESA says the sail’s size can be scaled up for medium and large size satellites, or multiple sails can be used on larger pieces such as an upper rocket stage. Various sizes could be tailor-made, depending on the initial orbit, satellite mass and required de-orbiting time. The largest variation could be as large as 100 square meters, which could take up to 45 mins to deploy. The smallest sail available is just 3.5 square meters and deploy in just 0.8 seconds.

For the orbital test, cameras on ION captured the deployment of ADEO. ESA determined the satellite carrier immediately slowed to begin its deorbit process.

Read more about the test on ESA’s website.

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Good News! Webb is Fully Operational Again

The James Webb Space Telescope is back to full science operations. One of the telescope’s instruments, the Near Infrared Imager and Slitless Spectrograph (NIRISS) had been offline since January 15 due to a communications error. But engineers worked through the problem and were able to return the instrument to full operations. 

“NASA and CSA [Canadian Space Agency] partnered to approach the problem as technically possible, using a detailed consideration of all areas of operation of the instrument,” said Julie Van Campen, Webb Integrated Science Instrument Module (ISIM) systems engineer at NASA’s Goddard Space Flight Center, in a blog post update.

The instrument, built by CSA, was returned to full operations on January 31. The problem began when a communications delay within the science instrument caused its flight software to time out. Engineers determined the cause of the issue was a hit by a galactic cosmic ray, a form of high-energy radiation from outside our solar system that can sometimes disrupt electrical systems.

FGS/NIRISS was built by the Canadian Space Agency. Credit: CSA

Van Campen said encountering cosmic rays is a normal and expected part of operating any spacecraft, and that this cosmic ray event affected logic in the solid-state circuitry of NIRISS electronics known as the Field Programmable Gate Array. JWST engineers determined that rebooting the instrument would bring it back to full functionality.

“They analyzed all possible methods to safely recover the electronics. When performing the operation, reviews were held at each intermediate step,” she said. “We are now happy to report that Webb’s NIRISS instrument is back online, and is performing optimally.”

NIRISS has two capabilities: Wide-Field Slitless Spectroscopy, which involves capturing the overall spectrum of a wide field of view such as a field of stars, part of a nearby galaxy, or many galaxies at once. Its Single-Object Slitless Spectroscopy capabilities involves capturing the spectrum of a single bright object like a star in a field of view.

While JWST has been performing magnificently, this isn’t the first technical issues that engineers have had to deal with. It went into safe mode from December 7-20, 2022 due to a software fault in the telescope’s attitude control system. The telescope’s MIRI (Mid-Infrared Instrument) was also briefly non-operational last September due to increased friction in one of MIRI’s mechanisms in the Medium-Resolution Spectroscopy (MRS) mode. Engineers were able to remotely repair and come up with work-arounds for all issues so far.

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Astronomers Come Closer to Understanding How Mercury Formed

Simulations of the formation of the solar system have been largely successful. They are able to replicate the positions of all the major planets along with their orbital parameters. But current simulations have an extreme amount of difficulty getting the masses of the four terrestrial planets right, especially Mercury. A new study suggests that we need to pay more attention to the giant planets in order to understand the evolution of the smaller ones.

Of all the rocky inner planets of the solar system, Mercury is the strangest. Not only does it have the lowest mass, but compared to its size it has the biggest core. This presents a major challenge for planet formation simulations, because it’s difficult to build such a large core without growing a proportionally larger planet along with it.

A team of astronomers recently investigated several possibilities to explain Mercury’s strange properties by performing simulations of the formation of the solar system. In the earliest days of the solar system, instead of a neat series of planets we instead had a protoplanetary disk made of gas and dust. Embedded in that disk were dozens of planetesimals which would eventually collide and merge and grow to become planets.

Astronomers believe that the inner edge of the protoplanetary disc was probably relatively lacking in material. Also in that young system the giant planets did not appear in their present day orbits. Instead they migrated from where they initially formed to their current positions. As those giant planets moved they destabilized the inner disk, potentially removing even more material.

Putting these ideas together, the astronomers were able to build a formation history of Mercury. Originally the inner protoplanetary disk contained a lot of planetesimals, but as the giant planets moved and migrated they pulled away a lot of the planet-building material with them. The remaining planetesimals collided together in a series of frequent collisions, which resulted in a lot of heavy metals being dumped into the innermost planet, creating the large core of Mercury.

While the models were able to capture the core size of Mercury, the simulations still couldn’t get the overall mass of the planet right. The simulations generally produced a Mercury that was two to four times more massive than it really is.

It remains an open question as to how Mercury came to be. The astronomers suspect we need to pay more careful consideration to the chemical properties of the protoplanetary disk, especially focusing on how dust grains can stick together and survive the intense radiation environment at Mercury’s orbit.

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Rolls-Royce Reveals a Nuclear Reactor That Could Provide Power on the Moon

For space agencies and the commercial space industry, the priorities of the next two decades are clear. First, astronauts will be sent to the Moon for the first time since the Apollo Era, followed by the creation of permanent infrastructure that will allow them to say there for extended periods. Then, the first crewed missions will be sent to Mars, with follow-up missions every 26 months, culminating in the creation of surface habitats (and maybe a permanent base). To meet these objectives, space agencies are investigating next-generation propulsion, power, and life support systems.

This includes solar-electric propulsion (SEP), where solar energy is used to power extremely fuel-efficient Hall-Effect thrusters. Similarly, they are looking into nuclear thermal propulsion (NTP) and compact nuclear reactors, allowing for shorter transit times and providing a steady power supply for Lunar and Martian habitats. Beyond NASA, the UK Space Agency (UKSA) has partnered with Rolls-Royce to develop nuclear systems for space exploration. In a recent tweet, the international auto and aerospace giant provided a teaser of what their “micro-reactor” will look like.

Thermoelectric generators have been integral to long-range space exploration for decades. The first missions to rely on them include the Viking 1 and 2 orbiters and landers that were the first to explore the surface of Mars. The Voyager 1 and 2 probes, currently in interstellar space, also relied on thermoelectric reactors that allowed them to remain in operation for more than 45 years. In recent decades, multi-mission radioisotope thermoelectric generators (MMRTG) have enabled missions like the New Horizons probe and the Curiosity and Perseverance rovers.

Looking toward the future of space and the exploration goals of NASA, the ESA, China, and others, researchers are considering nuclear technologies that have been thoroughly tested since the early space age – like the Nuclear Engine for Rocket Vehicle Application (NERVA). More recent efforts have led to programs like NASA’s Kilopower Reactor Using Stirling TechnologY (KRUSTY) and the NASA/DARP effort to realize a spacecraft that would rely on nuclear-thermal propulsion (NTP). Not to be left behind, the UKSA (an integral part of the ESA) has partnered with Britain’s chief aerospace developer.

The partnership was announced in December 2021, with Rolls-Royce stating that they had signed a contract with the UKSA to study nuclear power options for future space missions. The resulting technology will provide propulsion and power systems for long-duration missions far from Earth, where solar power is not always an option. This includes the South Pole-Aitken Basin, where NASA, the ESA, China, and Russia are all planning on building surface habitats in the coming years. In this region, a single “lunar night” lasts fourteen days, followed by another fourteen days of perpetual sunlight.

During a Martian year (which lasts roughly 687 Earth days), the distance between Mars and the Sun ranges from 1.38 to 1.66 times the distance between the Earth and the Sun. As a result, Mars receives about half the energy Earth does, and seasonal dust storms can lead to heavily-overcast skies that can play havoc with solar panels. Some examples include the Opportunity rover, which remained in continuous operation on Mars for 15 years until a global dust storm in 2018 ended the mission. More recently, the InSight lander ceased operations due to the build-up of dust on its solar panels.

Another issue with sending crewed missions to Mars is the transit times involved. The current mission architecture for NASA and the China National Space Agency (CNSA) is to launch missions every 26 months to coincide with Mars and Earth being at their closest points in their orbit (aka. a Mars Opposition). Using conventional technology, these missions will take (at minimum) six months to reach the Red Planet. During that time, the crews will be exposed to elevated levels of solar and cosmic radiation and living in microgravity.

A Rolls-Royce Micro-Reactor is designed to use an inherently safe and extremely robust fuel form. Each uranium particle is encapsulated in multiple protective layers that act as a containment system, allowing it to withstand extreme conditions.https://t.co/OOc9kBGXDx pic.twitter.com/wkXmZgzhrs

— Rolls-Royce (@RollsRoyce) January 27, 2023

As per the agreement, Rolls-Royce is developing a “micro-reactor” to enable nuclear propulsion and surface base power. The concept was unveiled in October 2021 at the International Astronautical Congress (IAC) conference in Dubai. As they described in a press release, the system would be capable of providing power in the “watts to megawatts” range, and the technology would have applications in space and here at home. They further stated that they planned to have a prototype micro-reactor prepared for 2029. Abi Clayton, the Director for Future Programs at Rolls-Royce, said at the time:

“Alongside the micro-reactor technology, we are also providing our nuclear knowledge in the development of Radioisotope Power Systems, and the space opportunities of converting ‘decay heat’ into electrical energy via thermoelectric generators or moving parts. This is a very exciting time for the Future Programmes team and the development of nuclear power across Rolls-Royce.”

The early mockup shown in the tweet is the same design as the mini-reactor presented at the IAC 2021. This time, however, the company provided a few more details about how it will operate, writing, “A Rolls-Royce Micro-Reactor is designed to use an inherently safe and extremely robust fuel form. Each uranium particle is encapsulated in multiple protective layers that act as a containment system, allowing it to withstand extreme conditions.”

Other teasers, like the many videos and artist impressions featured on the Rolls-Royce Space website, show the many applications and roles they hope this technology will have. These include reactors that would power surface habitats on the Moon and Mars (to which they include resource acquisition and use) and fast-transit nuclear spacecraft that will explore beyond the Earth-Moon system and even beyond Mars. Other potential applications include hypersonic space planes, small satellites, and on-orbit assembly.

Artist’s impression of the future of space exploration enabled by “micro-reactor” technology. Credit: Rolls-Royce Space

While the details of the micro-reactor are still limited, it is clear that the UKSA and Rolls-Royce are intent on being an active part of the future of space exploration and the commercialization of space. Said Amanda Solloway MP, Parliamentary Under Secretary of State and Minister for Science, Research and Innovation:

“As we build back better from the pandemic, it is partnerships like this between business, industry and government that will help to create jobs and bring forward pioneering innovations that will advance UK spaceflight. Nuclear power presents transformative possibilities for space exploration and the innovative study we are conducting with Rolls-Royce on this could help to propel our next generation of astronauts into space faster and for longer, significantly increasing our knowledge of the universe.”

Further Reading: Rolls-Royce Space

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Astronomers Detect a Second Planet Orbiting Two Stars

Planets orbiting binary stars are in a tough situation. They have to contend with the gravitational pull of two separate stars. Planetary formation around a single star like our Sun is relatively straightforward compared to what circumbinary planets go through. Until recently, astronomers weren’t sure they existed.

Astronomers rarely find binary stars with planets orbiting them. It may be because they’re rare, or it may be because they’re difficult to detect, likely both. Now a team of researchers has found a binary star with more than one planet. This is only the second instance of a multiplanet, binary star system.What does it tell us about these types of solar systems?

The system is called TOI-1338 and is a binary star about 1300 light-years away in the constellation Pictoris. TOI 1338 A is a main sequence star of 1.12 solar masses, and TOI 1338 B is an M-dwarf (red dwarf) of 0.3 solar masses. The star system is about 4.4 billion years old.

A summer intern at NASA’s Goddard Space Flight Center found the first planet around the binary in 2017. TOI 1338 b is a circumbinary planet with about 33 Earth masses and is in between Saturn and Neptune in size. It’s on a 95-day orbit around the binary stars.

This is an artist’s illustration of TOI-1338b, the first planet found around the binary star. An intern found it in 2017, and now astronomers have found its sibling, TOI-1338/BEBOP-1c. Image Credit: By NASA – https://www.youtube.com/watch?v=8FrlhrtVEW8&t=16s, Public Domain, https://ift.tt/eMH8WdJ

Circumbinary planets are hard to find in the data because the stars can eclipse each other, making planetary transits difficult to discern. Their transits can also be irregular, and they can transit in front of only one of the binary stars. TOI 1338 b’s transits occur irregularly, between every 93 and 95 days, making it non-periodic. And since both stars are moving, the depth of the transit varies.

Because of TOI 1338 b’s inclination, from our perspective, it will stop transiting in front of its star in November 2023. Then in about 2031, we’ll see the transits again.

The angle of TOI 1338 b’s orbit around the stars changes over time, so after 2023, there will be an eight-year gap in transits from our point of view. This gap leads astronomers to believe that there are many other circumbinary planets out there, but we have to be observing at the right time to find them. Image Credit: NASA Goddard Space Flight Center.

Now astronomers have found a second planet orbiting TOI 1338. It’s called TOI-1338/BEBOP-1c, and they found it using the radial velocity method rather than the transit method. The name BEBOP comes from an observing project. “To increase the number of known circumbinary planets and to provide accurate masses for systems discovered with the transit method, we initiated a radial-velocity observing survey dedicated to circumbinary planet detection called Binaries Escorted By Orbiting Planets (BEBOP),” the authors explain in their paper.

The researchers reported their findings in a paper titled “The First Circumbinary Planet Discovered with Radial Velocities.” It’s been accepted for publication in Nature Astronomy and is available on arxiv.org. The lead author is Matthew R. Standing, a Ph.D. student at the School of Physics and Astronomy, University of Birmingham, UK.

The new planet is a gas giant of about 65 Earth masses. It’s on a wider orbit than TOI 1338 b and has an orbital period of about 215 days. Astronomers discovered it using radial-velocity data collected with the HARPS and ESPRESSO spectrographs. This discovery marks the first time astronomers have found a circumbinary planet using radial velocity, and the system is only the second multiple-planet circumbinary system found.

This graphic from the research shows the TOI-1338 system in detail. Planet c has a much wider orbit than planet b, and neither is in the system’s habitable zone. Image Credit: Standing et al. 2023.

Astronomers are very interested in circumbinary planets. They’ve been common in science fiction but weren’t confirmed until the Kepler mission found the first one. It’s called Kepler-16b, and it’s an oddball in its own way. It’s inside the radius that astronomers thought was the inner limit for planets in binary star systems. Kepler-16b has no sibling planets.

Now we know of 12 circumbinary planets, and two of them are in multiplanet systems. The first multiplanet circumbinary system astronomers found is called Kepler-47, and it hosts three known explanets. The BEBOP observing program is designed to uncover more circumbinary planets and find out more about them. Its main goal is to find more of them, and it’ll do that by overcoming some of Kepler’s observational biases.

Binary star systems are far more complicated than single-star systems like ours. Binary stars disrupt planet formation in ways that more predictable single-star systems don’t. The dual stars create harsh conditions in the protoplanetary environment. Astronomers used to think that planets in these systems would be subjected to catastrophic collisions or be flung out of their systems by gravitational perturbations. But all these recent discoveries show that’s not necessarily true. By finding more circumbinary planets and characterizing their similarities with and differences from single-star planets, astronomers will learn a lot about how planets form and migrate.

One of the difficulties in studying circumbinary planets is determining their masses. BEBOP was designed to not only find planets but to measure their masses more accurately. That’s critical because knowing their masses helps determine which ones are puffy, with extended atmospheres suitable for atmospheric spectroscopy. BEBOP not only found the second planet, but it measured TOI-1338’s inner planet’s mass more accurately.

“If we are to unveil the mysteries of circumbinary Tatooine-like exo-atmospheres, the TOI-1338/BEBOP-1 system provides a new hope.”

From “The First Circumbinary Planet Discovered with Radial Velocities.”

Finding another multi-planet circumbinary system and determining their masses is an important discovery. While these systems up-end some parts of the models for how planets form, they’ll ultimately make our models more accurate.

This figure from the study illustrates what the astronomers found in TOI-1338. The red inset graph shows the magnified 14.6-day binary period associated with the stars, and the blue inset graph shows the magnified 215.5-day period of TOI1338/BEBOP-1c. Image Credit: Standing et al. 2023.

The researchers say that at some point, TOI-1338/BEBOP-1c is guaranteed to transit the primary star, but they can’t say when. That’s in spite of the misalignment between the planet and the star. “It may seem counterintuitive at first that a planet-binary misalignment makes transitability more likely,” they write. That’s because the planet’s sky inclination oscillates around the binary’s sky inclination, according to the authors, and eventually, the planet’s inclination will approach 90^o. That means “… the vast majority of circumbinary planets orbiting eclipsing binaries will eventually transit.”

The team also examined the issue of other planets around the binary star. None have been detected yet, but they may yet be. While they can’t say for sure if there are additional planets, they calculated and graphed the limitations on any potential detections.

This density plot from the research shows the detection limits of the researchers’ method. It’s pretty complicated, but it basically shows that their method is “…sensitive to additional sub-Saturn mass planets for periods out to 2000 days, while we are sensitive to Neptune mass planets near the instability limit.” Image Credit: Standing et al. 2023.

One of the problems with studying circumbinary planets around binary stars is that most of the ones we know of are too faint. That means that for most of them, including the new planet TOI-1338/BEBOP-1c, there’s no opportunity to employ spectroscopy to probe their atmospheres. But its previously discovered sibling, TOI-1338b, might be illuminated enough. “Therefore,” the researchers write, “despite the challenges it may present, TOI-1338/BEBOP-1b is our only possibility to shed light on the atmospheric make-up of circumbinary planets.”

“Of the now 15 known circumbinary exoplanets, TOI-1338/BEBOP-1b is the only one for which James Webb Space Telescope transmission spectroscopy can currently be pursued. If we are to unveil the mysteries of circumbinary Tatooine-like exo-atmospheres, the TOI-1338/BEBOP-1 system provides a new hope,” the authors write in their paper.

Tattoine. Image Credit: Lucasfilm/Twentieth Century Fox.

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