Astronomy

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An exomoon alone could be a challenge
to detect. While Beta Pic b’s entire Hill
sphere will take from early April 2017 to
late January 2018 to transit, a massive
moon detectable from Earth would zip by
in two days, says Wang. And it may make
only a single transit. To confirm an exo-
planet, scientists typically have to view at
least three transits to rule out other possi-
bilities. Since Beta Pic b takes more than
20 years to orbit its star — and the moon
may not be visible during some transits,
according to Heller — it could take a
century to confirm a moon this way.
Massive rings could be quite a bit easier
to spot. Saturn boasts the solar system’s
most massive ring system, but all the giant
planets have rings. Saturn’s set remains an
enigma, however, because its expected

lifetime is much shorter than the age of

the solar system, says Heller. Fragments

from colliding moons are one possible

source, and exoplanets presumably could

host similar systems. And a young planet

like Beta Pic b may still possess a dazzling,

pristine ring system that could put

Saturn’s to shame.

Matthew Kenworthy of Leiden

Observatory in the Netherlands knows

about rings. In 2007, he and his colleague,

Eric Mamajek of the University of

Rochester, spotted a massive ring system

around a planet circling another star,

J1407, only a few million years younger

than Beta Pic. The enormous rings stretch

nearly 200 times farther than Saturn’s, and

they have gaps that the researchers tenta-

tively identify with the gravitational pull

of exomoons in much the same way that

Saturn’s moons sculpt its rings.

Ironically, although the rings are visible,

the planet has yet to be discovered. The

moons also have evaded direct detection;

the evidence for them remains circumstan-

tial. Nor have the strange rings eclipsed

J1407 again, although Kenworthy and his

colleagues are keeping their eyes on the

star. But without seeing a planet, there’s no

way to determine an orbit to know when

the geometry might repeat.

“We think that the same thing may

happen with Beta Pic and its planet,” says

Kenworthy. And this world might provide

even better signs of its rings than J1407 did.

“The difference is that we know when the

planet is moving between us and its star.”

There’s another reason Kenworthy is

mildly optimistic about finding rings.

Because Beta Pic is reasonably bright —

at 4th magnitude, it’s easily visible to the

naked eye from a dark site — astronomers

have long used it as a standard star to help

calibrate observations of other objects. In

1981, a strange f luctuation changed the

star’s usually steady light over a two-week

period, creating a pattern that puzzled

astronomers. According to Wang’s calcula-

tions, there’s a good chance that Beta Pic b’s

Hill sphere transited the star at that time,

suggesting that something associated with

the planet blocked the star’s light. So when

Kenworthy heard that the planet itself

wasn’t going to transit, he began wonder-

ing if perhaps a set of massive rings could

produce a signal.

A ring system that spans some 200 times

the diameter of Saturn’s rings circles the

exoplanet J1407 b. Astronomers suspect

that undetected moons may create the

observed gaps. This artist’s depiction

shows the rings blocking light from the

host star, J1407. RON MILLER

Rings around an

exoplanet

Astronomers deduced the presence of

J1407 b’s ring system from the rapid brightness variations it caused as the

rings passed in front of the Sun-like star J1407 in 2007. In this plot, the

intensity of the red color corresponds to how much light each ring blocked.

ASTRONOMY: ROEN KELLY, AFTER KENWORTHY AND MAMAJEK

Track of J1407

J1407 b