Can a planet survive the death of its sun?
Scientists find two that did.
Natalie Batalha has had plenty of experience fielding1
questions from both layfolk and other scientists over the past
couple of years — and with good reason. Batalha is the deputy
principal investigator for the spectacularly successful Kepler4
space telescope, which has found evidence of more than 2,000
planets orbiting distant stars so far — including, just last week,
a world almost exactly the size of Earth.7
But Kepler is giving astronomers all sorts of new
information about stars as well, and that’s what an European
TV correspondent wanted to know about during an interview10
last year. Was it true, she asked, that stars like the sun will
eventually swell up and destroy their planets? It’s a common
question, and Batalha recited the familiar answer, one that’s13
been in astronomy textbooks for at least half a century: Yes,
it’s true. Five or six billion years from now, Earth will be burnt
to a cinder. This old news was apparently quite new to the16
European correspondent, because when she reported her
terrifying scoop, she added a soupçon of conspiracy theory to
it: NASA, she suggested, was trying to downplay the story.19
It was not a proud moment for science journalism, but
unexpectedly, at about the same time the European
correspondent was reporting her nonbulletin, Kepler scientists22
did discover a whole new wrinkle to the planet-eating-star
scenario: it’s apparently possible for planets to be swallowed
up by their suns and live to tell the tale. According to a paper25
just published in Nature, the Kepler probe has taken a closer
look at a star called KOI 55 and identified it as a “B
subdwarf”, the red-hot corpse of a sunlike star, one that already28
went through its deadly expansion. Around it are two planets,
both a bit smaller than Earth — and both so close to their home
star that even the tiniest solar expansion ought to have31
consumed them whole. And yet they seem, writes astronomer
Eliza Kempton in a Nature commentary, “to be alive and well.
Which begs the question, how did they survive?”34
How indeed? A star like the sun takes about 10 billion
years to use up the hydrogen supply. Once the hydrogen is
gone, the star cools from white hot to red hot and swells37
dramatically: in the case of our solar system, the sun’s outer
layers will reach all the way to Earth. Eventually, those outer
layers will waft away to form what’s called a planetary nebula40
while the core shrinks back into an object just like KOI 55.
If a planet like Earth spent a billion years simmering
in the outer layers of a star it would, says astronomer Betsy43
Green, “just evaporate. Only planets with masses very much
larger than the Earth, like Jupiter or Saturn, could possibly survive.”
And yet these two worlds, known as KOI 55.01 and46
KOI 55.02, lived through the ordeal anyway. The key to this
seeming impossibility, suggest the astronomers, is that the
planets may have begun life as gas giants like Jupiter or Saturn,49
with rocky cores surrounded by vast, crushing atmospheres. As
the star expanded, the gas giants would have spiraled inward
until they dipped into the stellar surface itself. The plunge52
would have been enough to strip off their atmospheres, but
their rocky interiors could have survived — leaving,
eventually, the bleak tableau of the naked cores of two planets55
orbiting the naked core of an elderly star.
Internet: <www.time.com> (adapted).