A white dwarf's strong gravity can cause the asteroids and rocky planets that orbit it to be ripped apart.
A white dwarf's strong gravity can cause the asteroids and rocky planets that orbit it to be ripped apart. Photo by Mark Garlick/markgarlick.com
Many rocky worlds that look similar to Earth orbit other stars are known. They are small, faint, and difficult to observe. This makes it hard to find and learn about them.
Some astronomers borrowed an archaeologist's idea and found their gravesites.
This idea is keyed by an object called a "white dwarf". The basic idea is that when stars such as the Sun exhaust their nuclear fuel, they expand into a red giant and blow off their outer layers. This exposes their core to the rest of the universe. The core, which is about half the star's original mass but about the same density as the Earth, is extremely hot and small. We call it a white dwarf.
Sirius B, the nearest white dwarf to Earth, has twice the mass of our Sun, but half the size of the Earth. The Sun is more than 100 times larger than Earth. Credit: NASA and ESA
Most people are familiar with the concept that heavier/denser stuff sinks while lighter stuff floats in liquids or gases. This is due to gravity. White dwarfs have extremely strong gravity. The interior of white dwarfs is flooded with heavy elements, which causes them to sink quickly into the interior. This leaves behind lighter elements like hydrogen and helium.
Nearly 75% appear to have surfaces made of hydrogen and helium, which is about 75% of all white dwarfs. Rest of the white dwarfs have "pollutants", which are heavier elements that are floating about. These should sink very quickly into the white dwarf so that they can be seen at all, it must be that there is a supply of them actively falling onto white dwarf. Where is the source?
These elements include magnesium, calcium and silicon. Knowing that stars like the Sun have planets, it is obvious that these elements come from rocky planets or asteroids that once orbited them.
Kepler-10c is an enormous world twice as large as Earth, but it has a thick atmosphere and is rocky. Photo by David Aguilar/Harvard-Smithsonian Center for Astrophysics
How? The tidal force can cause the planet to fall apart if a rocky planet is too close to the white dwarf. This can happen as a result of the star's death or the passage of other nearby stars. The debris falls onto the star, and voilà: Pollution.
This technique has been used before to detect the remains planets. In fact, the first evidence humanity had of planets other than our solar system came from observations made in 1917 by a nearby white dwarf. The data showing iron, magnesium, and calcium in the star were not understood at the time. However, we now know that the star died and ate its planets. It is a zombie.
This is already very impressive, but it gets even better: We can use the amounts of these elements to determine a lot more about the semi-digested planet remains and thus, something about the planets before their death.
These are the exact measurements that astronomers made. The 23 white dwarfs were located within 650 light-years of the Sun. All of them have precise measurements of the amount of calcium, magnesium and silicon. The relative abundances of these elements was then compared to the ratios of silicon to magnesium and calcium to iron in white dwarfs. These ratios can be compared with the ratios in our solar system to determine if alien planetary remains are any like what we see locally.
Astronomers were shocked to discover that they don't. They don't.
A white dwarf's strong gravity can cause the asteroids and rocky planets that orbit it to be ripped apart. Photo by Mark Garlick/markgarlick.com
The elemental abundances of the white dwarfs are not what they see when looking at the numbers for Earth, Mars, Earth and many meteorites that were brought to Earth from different asteroids in our solar system. Some meteorites may have a lower magnesium-to-silicium ratio than others, while some have a higher ratio. However, very few of these elements are comparable to those seen in dead stars.
This strongly suggests that exoplanets, or alien worlds, are very mineralogically different from Earth. We would see similar ratios if they were like us. They aren't because they don't have the same characteristics as us.
What makes them different? It's a great question. The formation of planets occurs along with the stars. Astronomers looked at the ratios between these elements in 4,000 stars nearby like the Sun and came up empty. Even though they have very similar building materials, they are subject to change as they age.
A few ratios observed by astronomers suggest that elements found in polluted white dwarfs may have come from the crusts on rocky planets. This would be a significant conclusion because crust dynamics is strongly dependent on plate tectonics which is unique to Earth in our solar system.
Astronomers disagree with this assertion in their new work. The astronomers see iron, silica, and magnesium ratios that are much closer to the mantles than crusts. This makes sense, as the Earth's crust only accounts for 1% of its total weight. However, the mantle contains 67%. Therefore, you would expect to find far more planetary material in these white dwarfs that crust. This is disappointing, but it's consistent with what we know.
Given these strange elemental ratios, they also modelled the conditions on planetary mantles and discovered that these planets would contain minerals that were very different from those found on Earth. Some of these minerals are so unusual that new classifications had to be created.
This is a very interesting way to look at rocky exoplanets. It implies that, if we can examine extant planets, still orbiting their Sun-like host stars, we will discover that the term "alien" does indeed apply to them.
