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Is the star RZ Piscium eating its planets?

Astronomers knew this star was “winking,” or dimming erratically. They thought it might be young. Now they think RZ Piscium might be more evolved than our sun, and hungry.

A team of U.S. astronomers said on December 21, 2017 that they’ve found evidence suggesting the strange, unpredictable dimming episodes of the star RZ Piscium might be caused by vast orbiting clouds of gas and dust, the remains of one or more destroyed planets. This star is located about 550 light-years away, in the direction to the constellation Pisces the Fish. Its erratic dimming episodes can last as long as two days, during which time the star becomes as much as 10 times fainter. Kristina Punzi – a doctoral student at the Rochester Institute of Technology (RIT) in New York and lead author of a paper about this star published in the peer-reviewed Astronomical Journal, said in a statement:

Our observations show there are massive blobs of dust and gas that occasionally block the star’s light and are probably spiraling into it. Although there could be other explanations, we suggest this material may have been produced by the break-up of massive orbiting bodies near the star.

The evidence for the dust is pretty clear. RZ Piscium produces far more energy at infrared wavelengths than emitted by stars like our sun, indicating a disk of warm dust surrounding the star. In fact, said the statement from these astronomers:

…about 8 percent of its total luminosity is in the infrared, a level matched by only a few of the thousands of nearby stars studied over the past 40 years. This implies enormous quantities of dust.

These and other observations led some astronomers to conclude that RZ Piscium is a young sun-like star surrounded by a dense asteroid belt, where frequent collisions grind the rocks to dust.

But, according to Punzi and colleagues, the evidence wasn’t clearcut. Perhaps RZ Piscium wasn’t young at all, but older. These astronomers said:

An alternative view suggests the star is instead somewhat older than our sun and just beginning its transition into the red giant stage. A dusty disk from the star’s youth would have dispersed after a few million years, so astronomers needed another source of dust to account for the star’s infrared glow. Because the aging star is growing larger, it would doom any planets in close orbits, and their destruction could provide the necessary dust.

So which is it, a young star with a debris disk or a planet-smashing stellar senior? According to the research by Punzi and her colleagues, RZ Piscium is a bit of both. Thanks to 11 hours of observing with ESA’s XMM-Newton space observatory, Punzi’s team found that RZ Piscium has a total X-ray output roughly 1,000 times greater than our sun’s, indicating it is a young star.

Meanwhile, ground-based observations – in particular, a measurement of the amount of the element lithium in this star – indicate the star is about 30 to 50 million years old, too old to be surrounded by so much gas and dust. Team member Ben Zuckerman, an astronomy professor at the University of California, Los Angeles, said:

Most sun-like stars have lost their planet-forming disks within a few million years of their birth. The fact that RZ Piscium hosts so much gas and dust after tens of millions of years means it’s probably destroying, rather than building, planets.

Ground-based observations also showed substantial amounts of gas in the RZ Piscium system. Based on the temperature of the dust, around 450 degrees F (230 degrees C), the researchers think most of the debris is orbiting about 30 million miles (50 million km) from the star. Co-author Carl Melis, an associate research scientist at the University of California, San Diego, said:

While we think the bulk of this debris is about as close to the star as the planet Mercury ever gets to our sun, the measurements also show variable and rapidly moving emission and absorption from hydrogen-rich gas. Our measurements provide evidence that material is both falling inward toward the star and also flowing outward.

The astronomers say it’s possible the star’s gravitational tides may be stripping material from a close substellar companion or giant planet, producing intermittent streams of gas and dust, or that the companion is already completely dissolved.

Another possibility is that one or more massive gas-rich planets in the system underwent a catastrophic collision in the astronomically recent past.

Read more about RZ Piscium from NASA Goddard

Bottom line: Astronomers knew the star was “winking,” or dimming erratically. They thought it was a young star. But RZ Piscium might be more evolved than our sun, and it might be destroying and consuming its planets.

Source: Is the Young Star RZ Piscium Consuming Its Own (Planetary) Offspring?

Deborah Byrd

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