UCL: Evidence of Rocky Planet Formation around Binary Star

[bystander]

First evidence of rocky planet formation in Tatooine system
University College, London | 2017 Feb 27

[attachment=0]FOR-WEB-SDSS-1557-V5c.jpg[/attachment]

---

Evidence of planetary debris surrounding a double sun, ‘Tatooine-like’ system has been found for the first time by a UCL-led team of researchers.

Published today in Nature Astronomy and funded by the Science and Technology Facilities Council and the European Research Council, the study reports on the remains of shattered asteroids orbiting a double sun consisting of a white dwarf and a brown dwarf roughly 1000 light-years away in a system called SDSS 1557.

The discovery is remarkable because the debris appears to be rocky and suggests that terrestrial planets like Tatooine – Luke Skywalker’s home world in Star Wars – might exist in the system. To date, all exoplanets discovered in orbit around double stars are gas giants, similar to Jupiter, and are thought to form in the icy regions of their systems.

In contrast to the carbon-rich icy material found in other double star systems, the planetary material identified in the SDSS 1557 system has a high metal content, including silicon and magnesium. These elements were identified as the debris flowed from its orbit onto the surface of the star, polluting it temporarily with at least 1017 g (or 1.1 trillion US tons) of matter, equating it to an asteroid at least 4 km in size. ...

Scientists find evidence of Star Wars-like planetary system
University of Sheffield | 2017 Feb 27

A Circumbinary Debris Disk in a Polluted White Dwarf System - J. Farihi, S. G. Parsons, B. T. Gänsicke

• arXiv.org > astro-ph > arXiv:1612.05259 > 15 Dec 2016

[Ann]

This is fascinating, to be sure. Personally I'm more interested in white dwarfs and pulsars having planets than in Tatooine-style binaries having planets. Isn't it fascinating that several white dwarfs have atmospheres polluted by the debris of just-shattered planetary bodies?

University of Warwick wrote:

University of Warwick astrophysicists have pinpointed four white dwarfs surrounded by dust from shattered planetary bodies which once bore striking similarities to the composition of the Earth.
...
The atmosphere of a white dwarf is made up of hydrogen and/or helium, so any heavy elements that come into their atmosphere are dragged downwards to their core and out of sight within a matter of days by the dwarf’s high gravity. Given this, the astronomers must literally be observing the final phase of the death of these worlds as the material rains down on the stars at rates of up to 1 million kilograms every second.

Personally I'm even more fascinated by pulsar PSR B1257+12.

Wikipedia wrote:

The pulsar has a planetary system with three known extrasolar planets, named "Draugr" (PSR B1257+12 A), "Poltergeist" (PSR B1257+12 B) and "Phobetor" (PSR B1257+12 C), respectively. They were both the first extrasolar planets and the first pulsar planets to be discovered; A and B in 1992 and C in 1994. A is the lowest mass planet yet discovered by any observational technique, with somewhat less than twice the mass of Earth's moon.

Wow!!! These pulsar planets were the first extrasolar planets ever to be discovered!!!

The planets are believed to be the result of a second round of planetary system formation as a result of two white dwarfs merging with each other into a pulsar and a resulting disk of material in orbit around the star.[16] Other scenarios include unusual supernova remnants or a quark-nova.[19] However, the white dwarf–white dwarf merge model seems to be the most likely cause of the formation of the planets.

Wow!!! These planets were likely formed not from a protoplanetary disk around a baby sun, but from a debris disk formed when two white dwarfs collided!! If so, then these planets were never in orbit around a normal star like the Sun, they are the "children" of the destruction of two white dwarfs!!

Amazing! Ever since I read about this, I have been wondering if there are more planets being born not from the birth cries but from the death throes of stars. For example, it is known that stars produce huge amounts of dust during some phases of their red giant lifetime (see for example here and here). Could some of that dust form planets? And what about the huge amounts of dust produced by supernova explosions?

Ann

[BDanielMayfield]

Isn't it fascinating that several white dwarfs have atmospheres polluted by the debris of just-shattered planetary bodies?

I find it fascinating that there are so many systems being discovered with close in planets. This suggests that it is common for planets to wander in toward their stars over time. Get too close, and tides rip the planet to pieces, so this is a natural consequence of planets being pulled in by gravity.

Why is it common for planets to have wandered in? In the case of white dwarf stellar remnants consider what's already happened to the system. The star has likely bloated up as a red giant and/or thrown off material in a planetary nebula producing phases. At these times old planets that formed normally back when the star was young will experience drag, so their orbits will decay, like our satellites in low Earth orbit do.

University of Warwick wrote:

University of Warwick astrophysicists have pinpointed four white dwarfs surrounded by dust from shattered planetary bodies which once bore striking similarities to the composition of the Earth.
...
The atmosphere of a white dwarf is made up of hydrogen and/or helium, so any heavy elements that come into their atmosphere are dragged downwards to their core and out of sight within a matter of days by the dwarf’s high gravity. Given this, the astronomers must literally be observing the final phase of the death of these worlds as the material rains down on the stars at rates of up to 1 million kilograms every second.

Personally I'm even more fascinated by pulsar PSR B1257+12.

Wikipedia wrote:

The pulsar has a planetary system with three known extrasolar planets, named "Draugr" (PSR B1257+12 A), "Poltergeist" (PSR B1257+12 B) and "Phobetor" (PSR B1257+12 C), respectively. They were both the first extrasolar planets and the first pulsar planets to be discovered; A and B in 1992 and C in 1994. A is the lowest mass planet yet discovered by any observational technique, with somewhat less than twice the mass of Earth's moon.

Wow!!! These pulsar planets were the first extrasolar planets ever to be discovered!!!

The planets are believed to be the result of a second round of planetary system formation as a result of two white dwarfs merging with each other into a pulsar and a resulting disk of material in orbit around the star.[16] Other scenarios include unusual supernova remnants or a quark-nova.[19] However, the white dwarf–white dwarf merge model seems to be the most likely cause of the formation of the planets.

Wow!!! These planets were likely formed not from a protoplanetary disk around a baby sun, but from a debris disk formed when two white dwarfs collided!! If so, then these planets were never in orbit around a normal star like the Sun, they are the "children" of the destruction of two white dwarfs!!

Amazing! Ever since I read about this, I have been wondering if there are more planets being born not from the birth cries but from the death throes of stars. For example, it is known that stars produce huge amounts of dust during some phases of their red giant lifetime (see for example here and here). Could some of that dust form planets? And what about the huge amounts of dust produced by supernova explosions?

Ann

I'd be very skeptical about the suggestion that planets can form around a SN remnant from its own explosive debris. Think about it. The force of the blast ejects material at way beyond escape velocity. It seems more likely that any planets found orbiting SN remnants will be what's left over from the star's pre-SN system. Just an opinion though, could be very wrong.

Bruce

[Ann]

BDanielMayfield wrote:
I'd be very skeptical about the suggestion that planets can form around a SN remnant from its own explosive debris. Think about it. The force of the blast ejects material at way beyond escape velocity. It seems more likely that any planets found orbiting SN remnants will be what's left over from the star's pre-SN system. Just an opinion though, could be very wrong.

Bruce

Sounds reasonable, Bruce.

Ann

[BDanielMayfield]

Now, when we have two WDs merging into a pulsar (spinning neutron star), would there be a SN explosion? Need to read the article before sticking my head out further ...

[neufer]

10¹⁷ g = 10¹⁴ kg = 10¹¹ tonnes = 110 billion US tons

[Ann]

Now, when we have two WDs merging into a pulsar (spinning neutron star), would there be a SN explosion? Need to read the article before sticking my head out further ...

Not necessarily. It depends on the combined mass of the white dwarfs. I'm not sure exactly how massive a neutron star has to be to collapse under its own weight, but two white dwarfs merging could easily produce a neutron star weighing no more than 1.4 solar masses. That wouldn't produce a supernova.

Ann

EDIT: How utterly stupid of me! Of course a double white dwarf merger would lead to an SN1a supernova! But how strange - an SN1a supernova shouldn't lead to the creation of a pulsar. But of course the pulsar is there, otherwise the planets would have nothing to orbit!

Wikipedia wrote:

When a slowly-rotating[2] carbon-oxygen white dwarf accretes matter from a companion, it can exceed the Chandrasekhar limit of about 1.44 M☉, beyond which it can no longer support its weight with electron degeneracy pressure.[10] In the absence of a countervailing process, the white dwarf would collapse to form a neutron star, in an accretion-induced non-ejective process,[11] as normally occurs in the case of a white dwarf that is primarily composed of magnesium, neon, and oxygen.[12]
The current view among astronomers who model Type Ia supernova explosions, however, is that this limit is never actually attained and collapse is never initiated.

I'm still confused as to how the merger of two white dwarfs could produce a neutron star.

[BDanielMayfield]

10¹⁷ g = 10¹⁴ kg = 10¹¹ tonnes = 110 billion US tons

How cryptic.

Yes, two merging WDs would produce a SN, (Duh, shoulda known that.) But Ann, it looks like a second round of post SN planetary accretion can and does occur, apparently.

Here's a possible scenario to explain the PSR B1257+12 system; Two tightly bound WDs spiral in due to gravitational radiation. They get so close that the less massive of the pair gets tidally disrupted into an accretion ring. The remaining WD accretes so much mass that it pops as a SN, but the force is insufficient to totally disrupt the ring, so a pulsar + ring is produced. Post SN rocky planets accrete from the remains of the ring.

Bruce

[neufer]

10¹⁷ g = 10¹⁴ kg = 10¹¹ tonnes = 110 billion US tons

How cryptic.

First evidence of rocky planet formation in Tatooine system
University College, London | 2017 Feb 27

In contrast to the carbon-rich icy material found in other double star systems, the planetary material identified in the SDSS 1557 system has a high metal content, including silicon and magnesium. These elements were identified as the debris flowed from its orbit onto the surface of the star, polluting it temporarily with at least 10¹⁷g (or 1.1 trillion US tons) of matter, equating it to an asteroid at least 4 km in size. ...

[BDanielMayfield]

10¹⁷ g = 10¹⁴ kg = 10¹¹ tonnes = 110 billion US tons

How cryptic.

First evidence of rocky planet formation in Tatooine system
University College, London | 2017 Feb 27

In contrast to the carbon-rich icy material found in other double star systems, the planetary material identified in the SDSS 1557 system has a high metal content, including silicon and magnesium. These elements were identified as the debris flowed from its orbit onto the surface of the star, polluting it temporarily with at least 10¹⁷g (or 1.1 trillion US tons) of matter, equating it to an asteroid at least 4 km in size. ...

Ah, and you had me wary of becoming the butt of another joke, when all you ment to do was point out a math error. Even still, I suspect an Artful allusion may be in the offing.