4 Earth-size planets may orbit Tau Ceti, the nearest sun-like star


exoplanets extrasolar planets earth like artist illustration shutterstock_600502655
An
illustration of Earth-like exoplanets


Shutterstock


  • Astronomers have detected what may be four roughly
    Earth-size planets orbiting Tau Ceti, the nearest sun-like
    star.
  • Two of the worlds appear to orbit within Tau Ceti’s
    habitable zone, though a cloud of debris and asteroids may pose
    a threat to any life on them.
  • If the result is confirmed, independent astronomers say
    it would be “astonishing.

Astronomers may have just hit a crucial milestone in the search
for other Earth-like planets. Scientists from the University of
Hertfordshire and University of California, Santa Cruz announced
they’ve discovered four planets orbiting a nearby, sun-like
star — two of which may be habitable.

Researchers have turned up thousands of planet candidates in
recent years,
about 10
of which may be small, rocky, and habitable like the
Earth. You’d think most of these worlds would orbit suns like
ours, but that’s not the case, since such stars are so big and
bright that they easily drown out the faint signals of tiny
planets.

That’s why the new discovery of four roughly Earth-size
worlds  — some 12 light-years away from our solar system —
is all the more exciting.

“If true this discovery is absolutely phenomenal — that one of
our nearest neighboring sun-like stars might have rocky worlds,”
Sara Seager, a planetary scientist at
MIT who wasn’t involved in the research, told Business Insider in
an email.

Worlds in the closest sun-like solar system

The suspected planets all orbit Tau Ceti, a star located 11.9
light-years away from us, according to a forthcoming study in The
Astrophysical Journal. (You can read a pre-print version of the
paper on arXiv.)

The star is about three-quarters the mass of the sun, but its
brightness and color are very sun-like. It’s the closest
sun-like star to Earth.

The planets’ sizes aren’t known yet, but they’re estimated to
have about 1.7 times the Earth’s mass. That would make
them the smallest planets ever detected around a distant
sun-like star, according to a press release emailed by the
University of Hertfordshire.

Two of the four worlds orbit in a searing-hot zone close to Tau
Ceti. The other two “super-Earths” seem to orbit within a
“Goldilocks” habitable zone, where water on the surface can be
liquid (rather than frozen solid or boiled away), according to
a
press release
by the University of California Santa Cruz.


tau ceti solar system habitable zone planets university hertfordshireFabo Feng/University of
Hertfordshire

Two things make this particular discovery stunning to astronomers
like Seager.

First, our solar system is the only place we know where life
exists, which means that sun-like stars may be the best places to
look for life (though there is some debate about
whether smaller,
cooler red dwarf stars
could be better to explore).

Second, it’s incredibly difficult to spy a relatively tiny planet
in the figurative shadow of a sun-like star.

“Earth is so small in mass compared to its host sun-like star
that finding the signal amidst the noise is really like finding
the proverbial needle in a haystack,” Seager added. “The authors
have come up with a special technique to get rid of the noise to
find the signal. It’s always a tricky situation to look for very
weak signals.”

Scouting for tiny wobbles

Most planets are detected by a “wobble method,”
where a planet slightly jerks around its star.

Such wobbles or wiggles (technically called Doppler shifts) come
from planets sharing a center of gravity that doesn’t reside
precisely in the middle of a star. Jupiter, for example, actually

orbits a spot
some 30,000 miles above the sun’s surface — and
not in a perfect circle.


jupiter sun barycenter center of mass distance nasa business insider labeled
A
to-scale illustration of where the sun-Jupiter barycenter is
located.


NASA/SDO;
Business Insider



This wobbling causes slight speeding up and slowing down of a
star as a planet orbits it, leaving an imprint on the light the
star emits.

In their new study, the team detected variations on the order of
30 centimeters per second — a gap of about 2/3 of a mile per
hour. Astronomers think they’ll need a sensitivity three times
better (about 0.2 mph) to reliably detect Earth analogs around
sun-like stars.

“We’re getting tantalizingly close to observing the correct
limits required for detecting Earth-like planets,” Fabo Feng, an
astrophysicist at the University of Hertfordshire who led the
research, said in the press release. “Our detection of such weak
wobbles is a milestone in the search for Earth analogs and the
understanding of the Earth’s habitability through comparison with
these [new planets].”


telescopes hawaii w m keck observatory
The Keck I and Keck II
telescopes on Mauna Kea in Hawaii.


W.M. Keck
Observatory



To find the planets, it took Feng and his colleagues four years
to sift through, analyze, and model the data gathered by two
telescope instruments: the European Southern Observatory’s
HARPS spectrograph
in Chile and the W.M. Keck Observatory’s HiRes instrument in Hawaii.

Seager emphasized that “it would take a long time” for other
astronomers to replicate and verify the discovery.

“There is no sugar-coating how hard it is to find an Earth-mass
planet signal in a sun-like star, due to star noise and
instrument noise,” Seager said.

Two hits to habitability

Small, rocky worlds in a habitable zone
aren’t necessarily cozy environments for life
.

One possible problem with Tau Ceti’s two
habitable-zone-worlds is their unknown size.

“They surely don’t conclude that the four planets are
Earth-sized, because the method they used to detect them provides
no information on their sizes,” Michaël Gillon, an astronomer at
the Université de Liège who wasn’t involved in the study, told
Business Insider in an email. (Gillon helped with the
discovery of the seven Earth-size planets
orbiting the
TRAPPIST-1 star system.)

“All they can say is that each planet is more massive than X
Earth-masses,” he said. In the case of Tau Ceti, this lower limit
seems to be about 1.7 times as hefty as Earth, and it’s uncertain
if complex life could thrive on a planet like this.

Another problem is the debris.


dust asteroids disk planets star solar system nasa PIA06939 1920x1200
NASA/JPL-Caltech

In fact, estimates suggests that the Tau Ceti system
has roughly 10
times the mass of asteroids (and dust)
 than our solar
system.

That’s not a great sign, because asteroid impacts are known to

wipe out vast numbers of species
.

Still, Seager characterized the result as “astonishing.” Once the
planets are confirmed by other researchers, she added, it could
be “a tremendous milestone en route to finding our Earth twin.”

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