The X-Files of Astronomy: The Undead and Its Ghost Planets

Thanks to ever better technology, innovative approaches and international cooperation, astronomy is flourishing. But while many observations help refine or sort out theories, there are always discoveries that just don’t seem to fit. Mysterious signals, alleged violations of the laws of nature and – as yet – phenomena that cannot be explained. The public then likes to discuss whether there are traces of extraterrestrial intelligence, scientists know that in the end there is almost always a natural explanation. But everywhere the imagination is stimulated.

In a series of articles on heise online in the coming weeks we will present some of these astronomical anomalies from a recently presented collection and explain why all attempts to explain them have failed so far.

In astronomy there are always observations that cannot be explained at first. While some suspect extraterrestrials behind it, others expect new insights into the nature of the universe. They are always exciting. heise online takes a look at some of these up to now inexplicable anomalies.

Can planets survive if their star becomes a supernova? In answering this question, it helps to look at the course of a classic supernova – the “core collapse supernova”. At the end of their life, heavier and heavier elements are fused in increasingly shorter succession in the center of massive stars. When there is no longer enough fuel for a nuclear reaction and its radiation pressure decreases, the weight of the star compresses its interior and heats it up until the next higher fusion stage ignites, which continues to stabilize the star with its radiation pressure.

The end of the flagpole is finally reached with iron – the fusion of iron no longer releases energy, but on the contrary binds energy and the stabilizing radiation pressure suddenly disappears. While in stars up to less than 10 solar masses the nucleus of up to 1.4 solar masses is stabilized by the degenerative pressure of the electrons and ends up as a white dwarf, that of more massive stars is no longer sufficient: the electrons are pressed into the atomic nuclei and combine with release from tons of neutrinos with the protons to neutrons, leaving nothing but neutrons of the atoms.

The neutrons take up much less space than the iron atoms before, and the iron core, around 10,000 km in diameter, suddenly collapses under its weight to form a ball of neutrons with a diameter of 20-30 kilometers – a neutron star. Stars with more than about 25 solar masses collapse into a black hole, but that’s not what we’re talking about here.

Back to the neutron star that has just formed: the star matter falling unsteadily onto it in the cavity that has just formed is compressed and heated so strongly by the returning shock wave that a large part of the star ignites thermonuclearly – a gigantic hydrogen bomb. In a supernova, a star sets an energy of 10 within seconds44 Watt seconds free. That is roughly the amount of energy that our sun will produce within 10 billion years at current luminosity.

The physicist Randall Monroe has on his xkcd side of all thingsthat the brightness of a supernova at a distance from the earth to the sun is around a billion times brighter than a hydrogen bomb that explodes right in front of the eye – which is due to the fact that a hydrogen bomb only fuses a few kilograms of hydrogen while it is the supernova 2 to 5 * 1031 Kilograms are. Even after the explosion, the star shines for weeks with ten billion times the luminosity of the sun because of the ejected fusion products some tens of thousands of earth masses of radioactive nickel-56, which breaks down to iron-56 via cobalt-56.

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The spiral galaxy NGC 1288 captured with ESO’s Very Large Telescope: All white, sharply delimited points in the image are foreground stars of the Milky Way – except for those to the left of the center of the galaxy. It is the supernova SN 2006dr, which lit up on June 17, 2006 in the galaxy 200 million light years away. She was of type Ia.

(Image: ESO, CC BY 4.0)

As a result of the explosion, the previous star (Progenitor) loses a large part of its mass, which is thrown into space at thousands of kilometers per second. Often the explosion occurs asymmetrically and the remaining neutron star receives a kick of up to hundreds of kilometers per second from the directed recoil.

Could planets survive something like this? Certainly not. If they are not melted and blown away, the star with its ejected matter loses a large part of its mass – the formerly more than ten solar masses leave behind a neutron star of 1.4 to about 2.8 solar masses, whose escape speed is less than the orbit speed of the previous star . So presumed planetary remnants would have to fly away in all directions. At least that’s what people thought 30 years ago.

Cut – we are in the early 1990s. No planet outside of the solar system has yet been discovered. It was technically the first time that it was able to detect exoplanets, and it was hoped that they could be detected using high-resolution Echelle spectrographto prove the new CCD sensor technology and computer-aided image processing in the spectral lines of a fixed star. Due to the mass of the orbiting planets, the star would be forced into a tumbling motion around the center of gravity of the system, the barycenter, which should manifest itself as a tiny periodic shift of its spectral lines due to the Doppler effect.

But before the Swiss astronomers Michel Mayor and Didier Queloz, who were awarded the Nobel Prize last year, made the first discovery of an exoplanet around an ordinary star in 1995, uninvited guests stormed the premiere stage and caused long faces and great perplexity. The Polish-Canadian team of radio astronomers Alexander Wolszczan and Dale Frail had already reported in the journal Nature on January 9, 1992 that they had succeeded in using the 300 meter Arecibo radio telescope to prove that they discovered on February 2, 1990, 2300 light-years away pulsar PSR B1257 + 12 in the constellation Virgo is orbited by at least two exoplanets.

Pulsars – discovered by Jocelyn Bell in 1967, for which her doctoral supervisor won the Nobel Prize in 1974 – are neutron stars that emit regular pulses of radio waves with the precision of an atomic clock. They have a strong magnetic field whose axis is tilted against the axis of rotation and which accelerates charged particles in their surroundings. Accelerated charge produces radio waves. Every time, as a result of the rotation, the magnetic axis briefly sweeps the line of sight to earth, this radio radiation hits us, similar to the way the circling headlight cone of a lighthouse periodically hits the observer.

Typically, the pulse rate is on the order of a second or less. However, some neutron stars rotate extremely quickly – down to milliseconds. The fastest known rotates 716 times per second and consequently sends a pulse towards earth every 1.4 milliseconds. It is considered certain that the millisecond pulsars do not turn so fast just because of the rapid rotation of their progenitor, but that they have caught matter that has overflowed from a companion star.

This matter first collects in a rotating accretion disk around the pulsar, where it has to break down energy through friction, heating and radiation in order to gradually spiral down onto the neutron star. The angular momentum is retained and is transferred to the neutron star, which rotates increasingly faster in the direction of the incident material. In this way, millisecond pulsars were driven up to ever faster rotation. PSR B1257 + 12 is also such a millisecond pulsar, which sends its pulses 160.8 times per second towards earth. If you were to make its pulses audible, it would hum like a bassy humming top.

Wolszczan and Frail used the precise pulses to locate the signature of the planets in them. As with ordinary stars, the orbiting planets force the pulsar to tumble around the barycenter. This is also revealed here by the Doppler effect: Instead of rocking spectral lines, Wolszczan and Frail measured how the pulses of the pulsar slowed down slightly when it moved away from the earth on its orbit and accelerated when it approached it again, i.e. the Pulse rate was modulated.

A more precise analysis of the modulation provided the superimposed oscillations of two planets, from whose period the orbital times and from whose amplitudes the respective mass of the planets in relation to that of the pulsar followed. The symmetry of the vibrations suggested almost perfectly circular orbits. According to this, the planets had (according to today’s figures) 4.13 and 3.82 earth masses and their distances from the pulsar were 0.36 AU (astronomical unit, i.e. the mean distance between the earth and the sun) and 0.46 AU. For comparison: Mercury, the innermost planet of the sun, orbits it at a mean distance of 0.39 AU. The orbital times of the pulsar planets were 66.5 and 98.2 earth days.

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