Tag: white dwarfs

The Planet Larger Than Its Star

Artist animation of WD 1856 b orbiting the white dwarf. Due to the tiny size of the white dwarf and close orbit of the planet, the animation is to scale. The slightly inclined orbit means that the planet does not entirely block the white dwarf’s light as it transits (Tasker).

It has been an exciting month for planets. Just days after the announcement of a detection of phosphine in the clouds of Venus, another first in planet discoveries was declared. The new find is the first planet observed to be orbiting a white dwarf; a dead star that is much smaller than the planet it hosts.

Planet WD 1856+534 b was first spotted by the NASA’s Transiting Exoplanet Survey Satellite (TESS) and confirmed with a series of observations from ground-based telescopes. The results showed the light from a white dwarf being periodically dimmed by a staggering 56% for brief 8 minutes.

For comparison, one of the easiest exoplanet types to detect is a hot Jupiter that would typically cause a 1% dip in brightness of its star over a period of a few hours.

This suggested a Jupiter-sized planet was closely orbiting a white dwarf that was similar in size to the Earth. Light from the white dwarf is obscured each time the planet passes in front of (or transits) the dead star’s surface on its orbit. Interestingly, the light dip is shaped like the letter V, showing a gentle gradient decreasing and rising from the maximum occultation. The lack of a sharp drop in brightness implied the planet’s orbit was slightly inclined so that it grazed the white dwarf’s surface and only obscured part of the much smaller star.

Light dip (transit) observations of WD 1856 observed with the Gran Telescopio Canarias (GTC) in visible light. The red curve is the best-fitting models. The V-shape suggests the planet is grazing the white dwarf and does not obscure it completely (Vanderburg et al. 2020, Figure 1a).

Although certainly unusual, WD 1856 b is not the first planet known to orbit a smaller star. The first extrasolar planets to be discovered orbit another type of stellar remnant known as a neutron star. While white dwarfs typically have sizes similar to a terrestrial planet, neutron stars have city-sized diameters of order 10 km.

The fact both these cases involve dead stars is no coincidence. In order to orbit, the mass of the planet must be much less than that of the star.… Read more

Asteroid Remains Around Dead Stars Reveal the Likely Fate of Our Solar System

Artist concept of an asteroid breaking up. (NASA/JPL-Caltech)

June 30th has been designated “Asteroid Day” to promote awareness of these small members of our solar system. But while asteroids are often discussed in the context of the risk they might pose to the Earth, their chewed up remains around other stars may also reveal the fate of our solar system.

It is 6.5 billion years into our future. The sun has fused hydrogen into a core of heavier helium. Compressed by its own gravity, the helium core releases heat and the sun begins to swell. It is the end of our star’s life, but what will happen to the solar system?

While very massive stars end their element-fusing days in a colossal explosion known as a supernovae, the majority of stars in our galaxy will take a less dramatic exit.

Our sun’s helium core will fuse to form carbon but there is not enough mass to achieve the crushing compression needed for the creation of heavier elements. Instead, the outer layers of the dying star will be blown away to leave a dense remnant with half the mass of our current sun, but squeezed down to the size of the Earth. This is a white dwarf; the most common of all stellar ends.

The life cycle of our sun

The white dwarf rapidly cools to become a dim twinkle in the sky. Within a few million years, our white dwarf will be less luminous that the sun today. Within 100 million years, it will be dimmer by a factor of 100. But examination of white dwarfs in our galaxy reveals this gentle dimming of the lights is not as peaceful as first appears.

The remnants of stars too light to fuse carbon, white dwarfs have atmospheres that should be thin shells of residue hydrogen and helium. Instead, observations have detected 20 different heavy elements in this envelope of gases that include rock-forming elements such as silicon and iron and volatiles such as carbon and nitrogen.

Infrared observations of over forty white dwarfs have additionally revealed compact dusty discs circling the dead stars. Sitting within the radius of a regular star, these could not have formed before the star shrank into a white dwarf. These must be the remains of what occurred as the star morphed from a regular fusion burner into a white dwarf.

This grizzly tale begins with the star’s expansion.… Read more

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