Astronomy

32 ASTRONOMY • JANUARY 2018

Among the Sun’s family, moons are
abundant. Only two planets — Mercury
and Venus — lack orbiting companions.
It’s not unreasonable to think that moons
might be plentiful around exoplanets. So
fa r, however, t hey’ve dodged detect ion. Pa r t
of the trouble is technological. Small rocky
worlds are barely discernible, and tinier
moons present even greater challenges.
Luck also plays a role. Although scientists
are hunting for exomoons using a variety of
techniques, each requires a specific align-
ment between the candidate object and
Earth. And even then, the largest search
has targeted hot Jupiters, massive gas giants
that circle their host stars in days or even
hours. If these giants traveled inward from
the outskirts of their system, as many sci-
entists suspect, they may have shed their
moons along the way or lost them in a
gravitational tug-of-war with their star.
For Beta Pic, however, the stars — and
planets — seem to be aligned. The system
boasts a gas giant orbiting at about Saturn’s
distance from the Sun, far enough away
that it should be able to hold on to any
satellites. Also, the star is only about
20 million years old, so even if the violent
interactions common to adolescent systems
wind up stripping its moons, there’s a good
chance they’re still orbiting today.

Even more important, the region

around the planet where any rings or

moons could exist is passing between

Beta Pic and Earth over the course of

10 months, with its closest brush in late

August 2017. Any moons or rings hiding

around the gas giant may well be revealed.

A limited opportunity

When Jason Wang turned his eye toward

Beta Pictoris in 2016, he was far from the

first astronomer to find it intriguing. Since

scientists discovered a disk of gas and dust

orbiting the young star in the 1980s, it has

garnered plenty of attention. In the 1990s,

astronomers detected a warp in this disk

that suggested an unseen planet. But it was

not until 2009 that Anne-Marie Lagrange

of Grenoble Observatory and her col-

leagues confirmed Beta Pic b through

direct imaging. The gas giant holds about

10 times the mass of Jupiter.

Later photos revealed the world drawing

closer to our line of sight to Beta Pic. If the

planet should cross in front of its sun, the

light that it blocked would reveal more

information about the world. NASA’s

Kepler space telescope used this technique,

known as the transit method, to discover

thousands of exoplanets. With the hope

that Beta Pic b would make a similar

transit, Wang, a graduate student at the

University of California, Berkeley, calcu-

lated the planet’s path.

To his disappointment, Wang learned

that Beta Pic b dances just outside our line

of sight to the star. But he quickly realized

that its Hill sphere — the region where a

planet’s gravity dominates that of its host

star and where any rings or moons could

orbit — would cross. If the planet carries

a massive ring system tilted with respect

to Earth, it could be visible, as could a

massive moon.

But it can be spotted only if astrono-

mers are looking.

Moons near and far

The solar system’s moons play an impor-

tant role in helping scientists understand

how planets form and evolve. For example,

Jupiter’s four big moons — Io, Europa,

Ganymede, and Callisto — help track

how water surrounded the gas giant when

it was young.

“These four moons, they serve as tracers

— or records — of the water or tempera-

ture distribution in the circumplanetary

accretion disk, which has long gone today,”

says René Heller of the Max Planck

Institute for Solar System Research in

Germany. To Heller, these objects could

reveal similar characteristics about other

planetary systems. Scientists still aren’t sure

just how planets form; exomoons could

help clarify the process. “We can take these

moons as yardsticks to calibrate our models

of giant planet formation,” he says.

The young star Beta Pictoris boasts a huge, flat disk of dust and gas. The disk glows brightly

thanks to its edge-on orientation and vast amounts of starlight-scattering dust. The Hubble Space

Telescope captured this view by blocking Beta Pic’s light. NASA/ESA/D. APAI AND G. SCHNEIDER (UNIVERSITY OF ARIZONA)

The gas giant planet

Beta Pictoris b

appears as a faint

blip to the upper

left of center in this

2003 image, though

confirmation did

not come until

2009. Astronomers
      carefully subtracted
      the light of Beta
      Pic itself to reveal
      the planet’s glow.
      ESO/A.-M. LAGRANGE ET AL.

Astronomers confirmed the presence of Beta

Pictoris b with this 2009 image. The massive

planet’s faint glow appears on the opposite

side of Beta Pic (indicated by the star symbol

at center) than it did in observations taken

six years earlier. ESO/A.-M. LAGRANGE