14 DAY TRIAL // Since 1992, when the first exoplanet discovery was confirmed, astronomers have identifies more than 530 such objects, with an additional 1,200 or so awaiting confirmation. Experts now believe the search should be focused on analyzing white dwarfs, which may be home to such objects.
White dwarfs are dim stars that have reached the end of their burning cycle, and became incapable of supporting nuclear fusion a long time ago. As such, they inflated into red giants, and then collapsed into a helium-burning core.
Such stars can burn for billions of years. Astronomers believe that such objects may be home to exoplanets that are very similar in mass, size and composition to Earth. Of the planets found thus far, none could be cataloged as an Earth-class one.
Sure, super-Earths were discovered, rocky exoplanets a few times more massive than our own corner of the Universe, but none of them was determined to meet the necessary requirements for supporting life.
According to a proposal set forth by University of Washington associate professor of astronomy Eric Agol, it would be a lot easier to find exoplanets orbiting white dwarfs than those around other classes of stars. His work is published in the March 29 online issue of The Astrophysical Journal Letters.
“If a planet is close enough to the star, it could have a stable temperature long enough to have liquid water at the surface – if it has water at all – and that's a big factor for habitability,” the investigator explains, quoted by SpaceRef.
Experts say that the habitable zone around white dwarfs – the area where temperatures are just right to allow for the existence of liquid water – are a lot closer to the stars' surfaces than the same region is around the Sun. Our star is a yellow dwarf.
Agol explains that, as a white dwarf precursor star passes through the last stages of its life, it expands its atmosphere until it completes the red giant phase. When this happens, planets that are withing the radius of expansion are gobbled up.
This will happen to Mercury, Venus and Earth (maybe even Mars) a few billion years from now, when the Sun will begin its own transformation into a white dwarf. But the consequences of such a process are multiple, specialists say.
Distant planets in a star system may have their orbits affected by the swelling of their parent star, and may therefore have to migrate further inwards. As an analogy, it could be that Jupiter and Saturn will become the first two planets from the Sun once the star turns into a white dwarf.
“From the planet, the star would appear slightly larger than our Sun, because it is so close, and slightly more orange, but it would look very, very similar to our Sun,” Agol explains.
The major difference between such a system and the one we have today is that planets orbiting too close to their parent stars are tidally locked, which means that they always keep the same face oriented towards the system's center of gravity.