Planets are often younger than the stars around which they revolve.
Take the Sun, for example, it was formed 4.6 billion years ago, and Earth formed not long
after.
However, astronomers at KU Leuven have discovered that a completely different scenario is also
possible.
Some types of stars can still form planets even if they are near death.
If this is proven, theories on the origin of planets will have to be adjusted.
The Earth, like all the other planets in our Solar System, formed not long after the Sun,
and when our star began to burn, a million years later the matter around it clumped into
protoplanets.
The birth of planets in that protoplanetary disc, which is a massive pancake of dust and
gas with the Sun at the center, explains why they all orbit on the same plane.
However, such discs of dust and gas need not necessarily exist around newborn stars.
They can also form independently from star formation, for instance, around binary stars
whereby one star is dying.
When a medium-sized star like the Sun reaches the end of its lifetime, it catapults the
outer part of its atmosphere into space, following which it slowly dies to form a white dwarf.
However, when it comes to binary stars, the gravitational pull of the second star causes
the matter released by the dying star to form a flat, spinning disc.
Furthermore, this disc closely resembles the protoplanetary discs seen around young stars
elsewhere in the Milky Way.
Well, this is already known.
But what’s new is that the discs surrounding the so-called evolved binary stars frequently
show signals of planet formation, as discovered by an international team of astronomers led
by KU Leuven researchers.
Furthermore, their observations reveal that this is a rare incidence; it occurs in only
one in ten of these binary stars.
According to KU Leuven astronomer Jacques Kluska, in 10% of the evolved binary stars
with discs they studied, they saw a large cavity in the disc, like a void or an opening.
This indicates that something is floating around there that has collected all matter
in the cavity area.
The clean-up of the matter might be the work of a planet.
That planet might have formed at the very end of one of the binary stars’ life, rather
than at the very beginning.
Moreover, the scientists discovered further compelling evidence for the existence of such
planets.
In evolved binary stars with a huge cavity in the disc, the researchers saw that heavy
elements such as iron were very scarce on the surface of the dying star.
This observation leads one to suspect that a planet trapped dust particles rich in these
elements.
By the way, Jacques Kluska does not rule out the possibility of several planets forming
around these binary stars in this way.
The discovery of evolved binary stars giving birth to a planet was made when the astronomers
were writing down an inventory of evolved binary stars in our Milky Way.
They did so based on data that was already publicly available.
Kluska and his colleagues counted a total of 85 such binary star pairs.
The researchers came across only ten pairs that had a disc with a large cavity on the
infrared images.
If new findings confirm the existence of planets orbiting evolved binary stars, and it turns
out that the planets formed only after one of the stars had reached the end of its life,
then theories of planet formation will need to be revised.
The confirmation or refutation of this extraordinary way of planet formation will be an unprecedented
test for the current theories.
The astronomers at KU Leuven are eager to verify their hypothesis themselves.
To achieve this, they will use the massive telescopes of the European Southern Observatory
in Chile to study the ten pairs of binary stars whose discs show a
huge cavity.