From The University of California-Berkeley: “Brightest stars in the night sky can strip planets to their rocky cores” 

From The University of California-Berkeley

8.12.22
Robert Sanders
rlsanders@berkeley.edu

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Artist’s concept of a Neptune-sized planet, left, around a blue, A-type star. UC Berkeley astronomers have discovered a hard-to-find gas giant around one of these bright, but short-lived, stars, right at the edge of the hot Neptune desert where the star’s strong radiation likely strips any giant planet of its gas. (Image credit: Steven Giacalone, UC Berkeley)

Over the last 25 years, astronomers have found thousands of exoplanets around stars in our galaxy, but more than 99% of them orbit smaller stars — from red dwarfs to stars slightly more massive than our sun, which is considered an average-sized star.

Few have been discovered around even more massive stars, such as A-type stars — bright blue stars twice as large as the sun — and most of the exoplanets that have been observed are the size of Jupiter or larger. Some of the brightest stars in the night sky, such as Sirius and Vega, are A-type stars.

University of California-Berkeley astronomers now report a new, Neptune-sized planet — called HD 56414 b — around one of these hot-burning, but short-lived, A-type stars and provide a hint about why so few gas giants smaller than Jupiter have been seen around the brightest 1% of stars in our galaxy.

Current exoplanet detection methods most easily find planets with short, rapid orbital periods around their stars, but this newly found planet has a longer orbital period than most discovered to date. The researchers suggest that an easier-to-find Neptune-sized planet sitting closer to a bright A-type star would be rapidly stripped of its gas by the harsh stellar radiation and reduced to an undetectable core.

While this theory has been proposed to explain so-called hot Neptune deserts around redder stars, whether this extended to hotter stars — A-type stars are about 1.5 to 2 times hotter than the sun — was unknown because of the dearth of planets known around some of the galaxy’s brightest stars.

“It’s one of the smallest planets that we know of around these really massive stars,” said UC Berkeley graduate student Steven Giacalone. “In fact, this is the hottest star we know of with a planet smaller than Jupiter. This planet’s interesting first and foremost because these types of planets are really hard to find, and we’re probably not going to find many like them in the foreseeable future.”

Hot Neptune desert

The discovery of what the researchers term a “warm Neptune” just outside the zone where the planet would have been stripped of its gas suggests that bright, A-type stars may have numerous unseen cores within the hot Neptune zone that are waiting to be discovered through more sensitive techniques.

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Astronomers have found thousands of exoplanets (black dots) around stars in the Milky Way galaxy, but few Neptune-sized planets have been discovered in short-period orbits around their stars, creating what astronomers call a Hot Neptune desert (pink region, representing planets with radii 3-10 times that of Earth with orbital periods under 3 days). A new-found Neptune-sized planet (yellow star) suggests that they don’t survive long enough to detect. The planets on this chart were discovered when they crossed in front of or transited their star, dimming its light. Current techniques are limited to finding planets in close, short-period orbits, less than about 100 days. (Graphic by Steven Giacalone, using data courtesy of NASA)

“We might expect to see a pileup of remnant Neptunian cores at short orbital periods” around such stars, the researchers concluded in their paper.

The discovery also adds to our understanding of how planetary atmospheres evolve, said Courtney Dressing, UC Berkeley assistant professor of astronomy.

“There’s a big question about just how do planets retain their atmospheres over time,” Dressing said. “When we’re looking at smaller planets, are we looking at the atmosphere that it was formed with when it originally formed from an accretion disk? Are we looking at an atmosphere that was outgassed from the planet over time? If we’re able to look at planets receiving different amounts of light from their star, especially different wavelengths of light, which is what the A stars allow us to do — it allows us to change the ratio of X-ray to ultraviolet light — then we can try to see how exactly a planet keeps its atmosphere over time.”

Giacalone and Dressing reported their discovery in a paper accepted by The Astrophysical Journal Letters.

According to Dressing, it’s well-established that highly-irradiated, Neptune-sized planets orbiting less massive, sun-like stars are rarer than expected. But whether this holds for planets orbiting A-type stars is not known because those planets are challenging to detect.

And an A-type star is a different animal from smaller F, G, K and M dwarfs. Close-in planets orbiting sun-like stars receive high amounts of both X-ray and ultraviolet radiation, but close-in planets orbiting A-type stars experience much more near-ultraviolet radiation than X-ray radiation or extreme ultraviolet radiation.

“Determining whether the hot Neptune desert also extends to A-type stars provides insight into the importance of near-ultraviolet radiation in governing atmospheric escape,” she said. “This result is important for understanding the physics of atmospheric mass loss and investigating the formation and evolution of small planets.”

The planet HD 56414 b was detected by NASA’s TESS mission as it transited its star, HD 56414.

Dressing, Giacalone and their colleagues confirmed that HD 56414 was an A-type star by obtaining spectra with the 1.5-meter telescope operated by the Small and Moderate Aperture Research Telescope System (SMARTS) Consortium at Cerro Tololo in Chile.

The planet has a radius 3.7 times that of Earth and orbits the star every 29 days at a distance equal to about one-quarter the distance between Earth and the sun. The system is roughly 420 million years old, much younger than our sun’s 4.5-billion-year age.

The researchers modeled the effect that radiation from the star would have on the planet and concluded that, while the star may be slowly whittling away at its atmosphere, it would likely survive for a billion years — beyond the point at which the star is expected to burn out and collapse, producing a supernova.

Giacalone said that Jupiter-sized planets are less susceptible to photoevaporation because their cores are massive enough to hold onto their hydrogen gas.

“There’s this balance between the central mass of the planet and how puffy the atmosphere is,” he said. “For planets the size of Jupiter or larger, the planet is massive enough to gravitationally hold on to its puffy atmosphere. As you move down to planets the size of Neptune, the atmosphere is still puffy, but the planet is not as massive, so they can lose their atmospheres more easily.”

Giacalone and Dressing continue to search for more Neptune-sized exoplanets around A-type stars, in hopes of finding others in or near the hot Neptune desert, to understand where these planets form in the accretion disk during star formation, whether they move inward or outward over time, and how their atmospheres evolve.

The work was supported by a FINESST award from NASA (80NSSC20K1549) and the David and Lucile Packard Foundation (2019-69648).

Science paper:
The Astrophysical Journal Letters

See the full article here .

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The University of California-Berkeley is a public land-grant research university in Berkeley, California. Established in 1868 as the state’s first land-grant university, it was the first campus of the University of California system and a founding member of the Association of American Universities . Its 14 colleges and schools offer over 350 degree programs and enroll some 31,000 undergraduate and 12,000 graduate students. Berkeley is ranked among the world’s top universities by major educational publications.

Berkeley hosts many leading research institutes, including the Mathematical Sciences Research Institute and the Space Sciences Laboratory. It founded and maintains close relationships with three national laboratories at The DOE’s Lawrence Berkeley National Laboratory, The DOE’s Lawrence Livermore National Laboratory and The DOE’s Los Alamos National Lab, and has played a prominent role in many scientific advances, from the Manhattan Project and the discovery of 16 chemical elements to breakthroughs in computer science and genomics. Berkeley is also known for student activism and the Free Speech Movement of the 1960s.

Berkeley alumni and faculty count among their ranks 110 Nobel laureates (34 alumni), 25 Turing Award winners (11 alumni), 14 Fields Medalists, 28 Wolf Prize winners, 103 MacArthur “Genius Grant” recipients, 30 Pulitzer Prize winners, and 19 Academy Award winners. The university has produced seven heads of state or government; five chief justices, including Chief Justice of the United States Earl Warren; 21 cabinet-level officials; 11 governors; and 25 living billionaires. It is also a leading producer of Fulbright Scholars, MacArthur Fellows, and Marshall Scholars. Berzerkeley alumni, widely recognized for their entrepreneurship, have founded many notable companies.

Berkeley’s athletic teams compete in Division I of the NCAA, primarily in the Pac-12 Conference, and are collectively known as the California Golden Bears. The university’s teams have won 107 national championships, and its students and alumni have won 207 Olympic medals.

Made possible by President Lincoln’s signing of the Morrill Act in 1862, the University of California was founded in 1868 as the state’s first land-grant university by inheriting certain assets and objectives of the private College of California and the public Agricultural, Mining, and Mechanical Arts College. Although this process is often incorrectly mistaken for a merger, the Organic Act created a “completely new institution” and did not actually merge the two precursor entities into the new university. The Organic Act states that the “University shall have for its design, to provide instruction and thorough and complete education in all departments of science, literature and art, industrial and professional pursuits, and general education, and also special courses of instruction in preparation for the professions”.

Ten faculty members and 40 students made up the fledgling university when it opened in Oakland in 1869. Frederick H. Billings, a trustee of the College of California, suggested that a new campus site north of Oakland be named in honor of Anglo-Irish philosopher George Berkeley. The university began admitting women the following year. In 1870, Henry Durant, founder of the College of California, became its first president. With the completion of North and South Halls in 1873, the university relocated to its Berkeley location with 167 male and 22 female students.

Beginning in 1891, Phoebe Apperson Hearst made several large gifts to Berkeley, funding a number of programs and new buildings and sponsoring, in 1898, an international competition in Antwerp, Belgium, where French architect Émile Bénard submitted the winning design for a campus master plan.

20th century

In 1905, the University Farm was established near Sacramento, ultimately becoming the University of California-Davis. In 1919, Los Angeles State Normal School became the southern branch of the University, which ultimately became the University of California-Los Angeles. By 1920s, the number of campus buildings had grown substantially and included twenty structures designed by architect John Galen Howard.

In 1917, one of the nation’s first ROTC programs was established at Berkeley and its School of Military Aeronautics began training pilots, including Gen. Jimmy Doolittle. Berkeley ROTC alumni include former Secretary of Defense Robert McNamara and Army Chief of Staff Frederick C. Weyand as well as 16 other generals. In 1926, future fleet admiral Chester W. Nimitz established the first Naval ROTC unit at Berkeley.

In the 1930s, Ernest Lawrence helped establish the Radiation Laboratory (now DOE’s Lawrence Berkeley National Laboratory) and invented the cyclotron, which won him the Nobel physics prize in 1939. Using the cyclotron, Berkeley professors and Berkeley Lab researchers went on to discover 16 chemical elements—more than any other university in the world. In particular, during World War II and following Glenn Seaborg’s then-secret discovery of plutonium, Ernest Orlando Lawrence’s Radiation Laboratory began to contract with the U.S. Army to develop the atomic bomb. Physics professor J. Robert Oppenheimer was named scientific head of the Manhattan Project in 1942. Along with the Lawrence Berkeley National Laboratory, Berzerkeley founded and was then a partner in managing two other labs, The Doe’s Los Alamos National Laboratory (1943) and The DOE’s Lawrence Livermore National Laboratory (1952).

By 1942, the American Council on Education ranked Berkeley second only to Harvard University in the number of distinguished departments.

In 1952, the University of California reorganized itself into a system of semi-autonomous campuses, with each campus given its own chancellor, and Clark Kerr became Berkeley’s first Chancellor, while Sproul remained in place as the President of the University of California.

Berkeley gained a worldwide reputation for political activism in the 1960s. In 1964, the Free Speech Movement organized student resistance to the university’s restrictions on political activities on campus—most conspicuously, student activities related to the Civil Rights Movement. The arrest in Sproul Plaza of Jack Weinberg, a recent Berkeley alumnus and chair of Campus CORE, in October 1964, prompted a series of student-led acts of formal remonstrance and civil disobedience that ultimately gave rise to the Free Speech Movement, which movement would prevail and serve as precedent for student opposition to America’s involvement in the Vietnam War.

In 1982, the Mathematical Sciences Research Institute (MSRI) was established on campus with support from the National Science Foundation and at the request of three Berzerkeley mathematicians — Shiing-Shen Chern, Calvin Moore and Isadore M. Singer. The institute is now widely regarded as a leading center for collaborative mathematical research, drawing thousands of visiting researchers from around the world each year.

21st century

In the current century, Berkeley has become less politically active and more focused on entrepreneurship and fundraising, especially for STEM disciplines.

Modern Berkeley students are less politically radical, with a greater percentage of moderates and conservatives than in the 1960s and 70s. Democrats outnumber Republicans on the faculty by a ratio of 9:1. On the whole, Democrats outnumber Republicans on American university campuses by a ratio of 10:1.

In 2007, the Energy Biosciences Institute was established with funding from BP and Stanley Hall, a research facility and headquarters for the California Institute for Quantitative Biosciences, opened. The next few years saw the dedication of the Center for Biomedical and Health Sciences, funded by a lead gift from billionaire Li Ka-shing; the opening of Sutardja Dai Hall, home of the Center for Information Technology Research in the Interest of Society; and the unveiling of Blum Hall, housing the Blum Center for Developing Economies. Supported by a grant from alumnus James Simons, the Simons Institute for the Theory of Computing was established in 2012. In 2014, Berkeley and its sister campus, University of California-San Fransisco, established the Innovative Genomics Institute, and, in 2020, an anonymous donor pledged $252 million to help fund a new center for computing and data science.

Since 2000, Berkeley alumni and faculty have received 40 Nobel Prizes, behind only Harvard and Massachusetts Institute of Technology among US universities; five Turing Awards, behind only MIT and Stanford University; and five Fields Medals, second only to Princeton University. According to PitchBook, Berkeley ranks second, just behind Stanford University, in producing VC-backed entrepreneurs.

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