Astronomy

34 ASTRONOMY • APRIL 2018

A: This is a question that

comes up again and again, but

as it turns out, there isn’t a

clear definition for binary

planet, at least as far as the

International Astronomical

Union is concerned.

Take Pluto and Charon. Is

Charon a moon, or do Pluto

and Charon constitute a

binary? What about Earth and

our Moon? In both cases the

mass of the moon is exception-

ally large compared to the mass

of the planet. And if the differ-

ence between the two masses is

small enough, the center of

gravity can lie outside the

planet, in the space between the

planet and the moon, and the

two objects both actually orbit

this point in space. Such is the

case for Pluto and Charon, so

you might call it a binary. But

many of us continue to call

Charon a moon. The dividing

line is quite blurry, and evi-

dently a matter of preference.

What is clear, though, is that

Charon and our Moon must

have formed differently from

the moons around, say, Jupiter,

which are thought to have

formed in a disk of material

swirling around the planet in

the early days of the solar sys-

tem. By contrast, both Charon

and our Moon are thought to

have resulted from catastrophic

collisions in the deep past.

The exomoon candidate we

identified, if it exists, is also

much too large to have formed

in a disk of material around

Kepler-1625b. If we’re correct,

it’s about the size of Neptune.

That’s something hardly any-

one has anticipated and is rea-

son enough to be agnostic

about the moon’s existence

until we’ve confirmed it with

our recent Hubble observa-

tions. We tend to think theo-

rists shouldn’t worry too much

about how it formed until we

know it’s real, but recent work

has suggested that a system like

Kepler-1625b could be formed

through gravitational capture,

more akin to the way Neptune

acquired its large moon Triton.

The universe is really good

at surprising us, and some-

times the most exciting obser-

vations are the ones that aren’t

easily explained. If the moon

Astronomy’s experts from around the globe answer your cosmic questions.

PLANET

OR MOON?

around Kepler-1625b turns out

to be real, it should be a nice

puzzle for the theorists. But

we’ve got to confirm it first, so

stay tuned.

Alex Teachey

NSF Graduate Research Fellow,

Department of Astronomy,

Columbia University, New York

Q: IS JUPITER’S MOON

IO GRADUALLY LOSING

MASS BECAUSE OF ALL

THE VOLCANIC ACTIVITY?

AND IF SO, WHAT WILL BE

THE MOON’S FINAL FATE?

Jim Vaaler

Phoenix, Arizona

A: Yes, Io is losing mass

because of its volcanic activity,

but not as much as you’d think.

Several of Io’s volcanoes spray

gas and particles high above

the surface, but most of that

material falls back down to the

surface, creating colorful and

bright ring deposits. However,

some material does eventually

escape the inf luence of Io’s

gravity and enters Jupiter’s.

This material may stay in orbit

around Jupiter as part of the Io

Plasma Torus, a ring of elec-

trons and ions produced by Io

and heated by Jupiter’s mag-

netic field as it rotates.

Results from the Galileo

ASKASTR0

Q: I HAVE HEARD THAT EXOPLANET

KEPLER-1625B MIGHT HAVE AN EXOMOON.

IS IT POSSIBLE FOR TWO PLANETS TO BE IN

A BINARY SYSTEM AROUND A SINGLE STAR?

Robert Byerly, Windsor, California

Much of the material spewed from Io’s volcanoes never leaves the moon’s gravitational influence. Instead, it falls

back to the surface, creating rings of material that circle Io’s volcanoes, such as those that appear around Pele and

Tvashtar Catena in these Galileo spacecraft images. NASA/JPL/UNIVERSITY OF ARIZONA

Two moons orbit a gas giant in a distant star system. Determining whether

a pair is a binary planet or a planet and moon is difficult, given our current

astronomical definitions. If Kepler-1625b has a companion, it is much

larger than either exomoon pictured here. NASA/JPL-CALTECH

Tv as ht ar

Pele