Astronomy - February 2014

ASTRONEWS

0 h

6 h

12 h

18 h

0.5°

1°

NCP

Polaris

(2013)

Polaris

(2102)

2h48m

5h59m

16 ASTRONOMY t FEBRUARY 2014

Astronomers have discovered more than

1,000 planets orbiting stars other than the

Sun, but uncovering those worlds’ charac-

teristics is much more difficult. The main

method for finding exoplanets is to watch a

star’s light dim, which happens if a body

passes between it and Earth. NASA’s Kepler

mission used this transit technique to tally

some 3,500 candidate planets; scientists

have confirmed about 160 of those. But a

planetary transit can tell astronomers only

how wide the planet is — it says nothing

about mass and thus density. And without

a mass measurement, researchers have no

way of knowing the planet’s composition.

Two groups report online October 30 in

Nature their studies of an exoplanet orbit-

ing extremely close to a Sun-like star. That

world has a size and density nearly identi-

cal to Earth’s.

Andrew Howard of the University of

Hawaii at Manoa and Francesco Pepe of

the University of Geneva lead separate

teams that studied the planet now known

as Kepler-78b. From Kepler observations,

the scientists knew the world is about 1.

times as wide as Earth and completes an

orbit in just 8.5 hours.

Both teams used ground-based tele-

scopes to observe how the light from

Kepler-78b’s host star shifts. This shift is a

result of the planet’s gravity pulling slightly

on the star toward and away from Earth.

(When moving toward, the light appears

bluer, when away, redder.) They could

determine how massive the planet is from

how much the light shifts. The answer:

Howard’s team measured 1.69 Earth masses

while Pepe’s team calculated 1.86.

By combining Kepler-78b’s volume (cal-

culated from its radius) and mass, the two

teams found slightly different density val-

ues, but both remarkably similar to Earth’s:

Howard’s team calculated 5.3 grams/cm^3

and Pepe’s team 5.6 g/cm^3 (our planet’s is

5.5 g/cm^3 ).

The astronomers also tried to determine

Kepler-78b’s composition by comparing

their calculations to different computa-

tional models. Pepe’s team wrote that “the

planet has a rocky interior and most prob-

ably a relatively large iron core (perhaps

comprising 40 percent of the planet by

mass).” Howard’s team calculated that iron

makes up anywhere from 0 to 53 percent of

Kepler-78b’s mass. Earth’s composition by

mass is roughly 67 percent silicate rock and

33 percent iron.

While this newly analyzed world might

seem like an Earth twin at first glance, the

reality is much different. Kepler-78b orbits

its star extraordinarily close-in — one-

hundredth of our planet’s distance from the

Sun. That proximity means its surface is a

searing 2300 to 3100 kelvins (3700 to 5100°

Fahrenheit). — L. K.

SPACE SCIENCE UPDATE

Planet with

earth-like

comPosition

found

MARS ROCKS. The Curiosity rover’s recent measurements of two kinds of argon gas in the martian atmosphere confirm that

some meteorites found on Earth are in fact from Mars, reported scientists online November 6 in Geophysical Research Letters.

how the

north star moves

Supernova’s

original star

confirmed

Scientists observe the sky

looking for objects that

change over short inter-

vals, like the rapid bright-

ening of a supernova.

Such explosions mark the

death of stars, but learn-

ing the original sun’s

characteristics is difficult.

Astronomers using the

intermediate Palomar

Transient Factory (iPTF)

search have identified the

initial star of Supernova

iPTF13bvn, which

appeared in the nearby

galaxy NGC 5806 in June

2013. This supernova was
      a rare type Ib, and the
      study published online
      October 4 in Astronomy & Astrophysics marked the
      first time scientists confirmed the original star of this
      kind of stellar blast.
      After comparing archived Hubble Space Tele-
      scope images to those taken after the explosion had
      faded away, Jose Groh of Geneva Observatory and
      colleagues could confirm the star that exploded. It
      was a “Wolf-Rayet,” which is a hot massive star that
      sheds its outer gaseous layer in a violent stellar
      wind. The star, says Groh’s team, had about 11 times
      the Sun’s mass right before it exploded. — L. K.

EARTH TWIN? Kepler-78b’s density is similar

to our planet’s, and this world likely has an

Earth-like composition of rock and iron. CFA

FAST

FACT

ZOOM GOES POLARIS. Polaris (Alpha [α] Ursae

Minoris), our North Star, does not sit at the exact posi-

tion of the North Celestial Pole (NCP). Currently, Polaris

lies 0.68° from the NCP and is moving closer. It will

reach its nearest point in 2102, when it will lie at a dis-

tance of 0.47°. ASTRONOMY: MICHAEl E. BAKICH ANd ROEN KElly

Polaris ranks as the sky’s

48th-brightest star. It shines

at magnitude 2.0.

SUPERNOVA SUN.

Astronomers compared

the location of a June

2013 supernova (top) with

archived Hubble Space

Telescope images (below)

to determine what star

(circled) likely exploded

in the supernova. IAIR ARCAvI

(WEIzMANN INSTITUTE OF SCIENCE)

After supernova blast

Before supernova