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Watery planets orbiting white dwarfs hold the key to finding alien life

In the vast cosmic ballet, where stars are born, live, and die, an intriguing chapter unfolds in the fading embers of white dwarfs. These stellar remnants, having exhausted their nuclear fuel, may hold a surprising secret: the potential for life-bearing planets.

But how could planets survive the tumultuous death throes of their parent star, and what makes them such promising targets in the search for extraterrestrial life?

White dwarfs

White dwarfs are the remnants of stars that have exhausted their nuclear fuel. When stars like our sun run out of hydrogen to fuse into helium, they undergo a series of transformations.

Initially, they expand into red giants, shedding their outer layers. The core that remains collapses into a very small, dense object about the size of Earth but with a mass comparable to the sun. This is a white dwarf.

White dwarfs no longer undergo fusion reactions and instead, slowly cool over billions of years. Despite their small size, they are incredibly dense, with one teaspoon of white dwarf material weighing tons. They emit a faint, steady light due to residual thermal energy but do not generate new energy.

These characteristics make white dwarfs intriguing objects for studying the potential for life on surrounding exoplanets, as their stable luminosity and small size provide a clear backdrop for observations.

Death and rebirth

As a star like our Sun nears the end of its life, it undergoes a dramatic transformation. It swells into a red giant, engulfing any close-orbiting planets. Then, it expels its outer layers, leaving behind a dense, white-hot core –- the white dwarf.

Juliette Becker, an astronomy professor at the University of Wisconsin-Madison, likens this process to a fiery trial by ordeal.

“There are two pulses,” she says, “during which the star grows to 100 times its normal radius. While it does that — we can call this part Destruction Phase No. 1 — it will engulf any planets that are within that radius,” notes Becker.

Even if a planet escapes this initial onslaught, it faces another danger: the star’s dramatic increase in brightness.

“The fact that the star gets so much brighter means that all planets in the system, even ones that used to be cold in the outer solar system, will suddenly have their surface temperatures increase drastically,” Becker explains.

This intense heat could vaporize any existing oceans, leaving the planet barren and lifeless.

Finding refuge in the outer reaches

So, how could a water-rich planet possibly survive this cosmic inferno? The key, according to Becker’s research, lies in distance.

A planet orbiting at least 5 to 6 astronomical units (AU) away from its dying star — about the distance of Jupiter from our Sun — might just stand a chance.

At this distance, the planet could potentially retain enough water to support life, even after enduring the star’s expansion and intense radiation.

But there’s a catch: as the white dwarf cools and shrinks, the habitable zone — the region where liquid water can exist — moves inward.

This means the planet, once safely distant, would now be too far from the star to maintain a life-friendly environment.

Tidal migration

Enter tidal migration, a phenomenon that could potentially save the day. “A planet’s orbit changing is pretty normal,” Becker says.

“In tidal migration, some dynamical instability between planets in the system puts one of them into a high-eccentricity orbit, like a comet, where it swings in really close to the central body in the system and then far out again.”

Over time, these wild orbits can settle down, bringing the planet closer to the white dwarf and potentially placing it within the new habitable zone.

“If you put all these models together,” Becker concludes, “what you see is that it is a perilous journey for the planet and difficult for oceans to survive this process, but it is possible.”

Search for life on white dwarfs

Why are white dwarfs such appealing targets in the search for extraterrestrial life? For one, their small size and dim light make it easier to detect and study the atmospheres of orbiting planets.

“White dwarfs are so small and so featureless, that if a terrestrial planet transited in front of them, you could actually do a much better job of characterizing its atmosphere,” Becker explains.

This means that if we were to find a planet orbiting a white dwarf, we could potentially determine its atmospheric composition with greater accuracy than ever before.

This, in turn, could reveal telltale signs of life, such as the presence of oxygen, methane, or other biosignatures.

The road ahead

While the prospect of finding life on planets orbiting white dwarfs is exciting, there’s still much work to be done.

Becker and her colleagues are developing theoretical models to better understand the complex dynamics of these systems and identify the most promising targets for future observations.

As telescopes become more powerful, we may soon be able to peer into the atmospheres of these distant worlds and answer one of humanity’s most profound questions: Are we alone in the universe? The ashes of dead stars may yet hold the spark of life, waiting to be discovered.


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