Astronomy

ASTRONEWS

NOTABLE

METEORITES

DATE

DIAMETER

ENERGY

FREQUENCY

Southeast Michigan

January 16, 2018

6 feet (2 meters)*

100 tons of TNT*

Every 5 years

Chelyabinsk, Russia

February 15, 2013

60 feet (20 meters)*

1 megaton*

Every 100 years

Tunguska, Russia

June 30, 1908

120 feet (40 meters)*

15 megatons*

Every 200 to 1,

years

Chicxulub,

Yucatán Peninsula

66 million years ago

6 miles (10 kilometers)*

100,000,000 megatons*

Every 50 to 100

million years

Meteorites not to scale with world map * Numbers based on estimates

WWW.ASTRONOMY.COM 17

Free-floating

extragalactic

planets found

Microlensing occurs when light from a back-

ground source is distorted by a foreground

object, such as a star or planet. The amount of

distortion reveals clues about the intervening

(or “lensing”) object, including information

about its mass.

Microlensing allows astronomers to detect

small, dim objects, such as planets that are

smaller and more distant than those accessible

via other methods. Two astronomers at the

University of Oklahoma recently used this

method to serendipitously discover possibly

thousands of extragalactic planets.

The discovery, published February 2 in The

Astrophysical Journal Letters, focuses on the

lensed quasar RXJ 1131–1231 and the elliptical

galaxy between it and Earth. The intervening

lens galaxy is 3.8 billion light-years away, too

far for astronomers to probe for planets via

other methods. But while attempting to explain

a shift in the light coming from the background

quasar, the team’s models showed that the best

explanation is a group of up to 2,000 rogue

exoplanets with masses ranging between the

Moon and Jupiter within the lensing galaxy.

Rogue planets are not bound to a star, and

instead freely float through space. Little is

known about these objects, even in the Milky

Way, because they are difficult to detect. If

they can be characterized through microlens-

ing, astronomers might be better able to quan-

tify this population in our own galaxy.

“This is an example of how powerful the

techniques of analysis of extragalactic micro-

lensing can be,” said Eduardo Guerras of the

University of Oklahoma and the second author

on the paper, in a press release.

“There is not the slightest chance of observ-

ing these planets directly, not even with the

best telescope one can imagine,” he added.

“However, we are able to study them, unveil

their presence, and even have an idea of their

masses. This is very cool science.” — A.K.

DELICATE BLOOM.

The elegant Rosette

Nebula’s central hole is

caused by stellar winds

blasting from massive

stars in the heart of the

cloud. But given the age

of the stars, the nebula’s

cavity should be much

larger. A study led by

the University of Leeds

simulated different

nebula formation

scenarios, and found the

nebula formed as a thin

disk with the strongest

stellar winds focused

away from its center,

resulting in the cavity’s

small size. — A.J.

Inside

the heart

of the

Rosette

How big was the Michigan meteorite?

SMALL POTATOES. A meteor exploded over southeastern Michigan on

January 16, 2018. Although NASA estimates the object was only 6 feet (2 meters)

in diameter, the U.S. Geological Survey said that its sonic boom was so powerful,

it registered as a magnitude 2.

earthquake. But how does this

compare to other well-known

events? — J.P.

FAST

FAC T

The Tunguska meteor (roughly 15 times the diameter

of the Michigan meteorite) exploded above the sparsely

populated region of Tunguska in eastern Russia, flattening

500,000 acres (2,000 square kilometers) of forest.

ASTRONOMY

: ROEN KELLY

NICK WRIGHT, KEELE UNIVERSITY

X-RAY: NASA/CXC/

UNIVERSITY OF MICHIGA

N/R.C.

REIS ET AL.;

OPTICAL: NASA/STS

CI

DISTORTED LIGHT. RXJ 1131–1231 is a distant

quasar that appears as four bright spots when viewed

in X-ray emission (pink). It is lensed by an intervening

galaxy 3.8 billion light-years away (yellow, optical),

in which astronomers have discovered a population

of up to 2,000 free-floating planets.