- Press Release
- Sep 29, 2022
An orb by any other name: Debate over what constitutes a planet is far from settled
BERKELEY — Ask any kid how many planets are in our solar
system, and you’ll get a firm answer: nine.
But knock on a few doors in Berkeley’s astronomy
department, and you’ll hear, amid the hemming and hawing,
a whole range of numbers.
Professor Gibor Basri, who plans soon to propose a
formal definition of a planet to the international body
that names astronomical objects, argues that there are
at least 14 planets, and perhaps as many as 20. To the
well-known list of nine he adds several large asteroids
and more distant objects from the rocky swarm called the
Kuiper Belt circling beyond the orbit of Neptune.
Professor Imke de Pater and Assistant Professor Eugene
Chiang, on the other hand, toss out Pluto without a
backward glance. It’s just a big rock, they say, a
former member of the Kuiper Belt, puppy-dogging Neptune
around the solar system.
Not so fast, says Professor Alex Filippenko. The
International Astronomical Union (IAU), which rules on
names for astronomical bodies, has officially said that
Pluto remains a planet, at least for the time being.
Thus, officially, there are nine. He cavils a bit,
however, making it clear to his students that Pluto is
"more fundamentally a Kuiper Belt Object (KBO), though
an unusually large one."
Professor Geoffrey Marcy and research astronomer Debra
Fischer, both "planet hunters" within the department,
also prefer to keep the number at nine, noting that the
sun, though it probably had 12 or 14 planets in the
past, will in five billion years probably lose Mercury
and Pluto, bringing the count down to seven.
Moons, fusors, brown dwarfs
This difference of opinion within the astronomy department
is part of a larger debate in the astronomical community
over what constitutes a planet. It provides endless hours
of beer-hall debate and Friday-afternoon tea-time chat,
with little hope for resolution in the near future.
"It’s something of an embarrassment that we currently
have no definition of what a planet is," Basri said.
"People like to classify things. We live on a planet;
it would be nice to know what that was."
The IAU has sidestepped any formal definition, largely,
Basri says, because a good definition would eject
Pluto from the list and relegate it to a "minor planet"
or, even worse, a comet. Basri has come up with a
definition that keeps Pluto in the fold, but necessarily
brings in other objects that until now have not been
considered planets — objects with names such as Vesta,
Pallas and Ceres, now considered asteroids, or KBOs
such as Varuna.
He’s now preparing a formal definition to put before
the IAU Working Group on Extra-Solar Planets, and has
posted an article on his Web site that lays out his
definition and arguments as to why it should be adopted.
"By 10 years from now, I’d be a little surprised if the
IAU had not adopted something along the lines I’m
proposing," Basri said. "It’s reasonable."
Most astronomers and the IAU agree that planets should
be orbiting a star — or more precisely, an object that
is big enough to ignite hydrogen fusion in its core
(what Basri calls a fusor). The IAU Working Group also
excludes anything, like a star, that is big enough to
manage core fusion itself. The consensus thus excludes
moons, even those such as Ganymede, which is almost as
large as Mars but which happens to be orbiting the
planet Jupiter rather than a star.
The definition also excludes failed stars called brown
dwarfs, which are too small to be stars but too big to
be planets. These are the subjects of Basri’s research.
In 1995, he was the first to obtain a spectrum
confirming that brown dwarfs exist, and he has
concentrated on tests that can distinguish brown
dwarfs from low-mass stars.
This work naturally led him to focus on mass as a way
to distinguish between planets and non-planets. He
proposes a natural upper limit for a "planetary mass
object" of about 13 times the mass of Jupiter, or
about 4,000 Earths. At this size, gravity will cause
an object to give off heat, as happens with Jupiter,
but the pressure at the core is a bit too cool to
fuse the element easiest to fuse, deuterium or heavy
hydrogen. Because anything bigger, including stars
and brown dwarfs, is able to fuse deuterium, Basri
argues that it makes sense to define a "planetary
mass object" — or planemo, as he has dubbed them —
as an object too small to achieve any fusion.
A natural lower limit to the mass of a planemo, Basri
says, would be a body large enough for self-gravity to
squash it into a round shape. On average, that would
be about 700 kilometers in diameter, though that number
is squishy — an iron wrecking ball like Mercury could
be smaller and round, while icy planets like Pluto
would need to be larger to achieve roundness. This
limit excludes all but a few asteroids and KBOs, most
of which bear a resemblance to potatoes.
"The upper limit of a planetary mass is the fusion
boundary, and the lower limit is roundness," he said.
"This definition does not depend on either circumstance
Basri then throws in the other traditional property
of planets to reach a final definition: a planet is a
planemo orbiting a fusor.
"If you take this definition," he says, "you don’t have
any trouble what to call these objects," including many
of the new extrasolar planets that Geoff Marcy and
Debra Fischer are discovering.
Marcy disagrees. In his search for planets around other
stars — he and his colleagues have found about
two-thirds of all known extrasolar planets — he has
come across planet systems that aren’t so neat. Two
years ago, his team discovered two bodies orbiting the
star HD168443 — one with a mass about 7.6 times that
of Jupiter, and one 17 times Jupiter. Basri would call
this a planetary system with one large gas planet and
one brown dwarf companion — sort of a failed binary
star system, where one "star" wasn’t big enough to
make the grade.
Talk show host David Letterman, an astronomy buff,
quizzed Marcy about these two objects when he was a
guest in April 2001. Marcy admitted that the larger
of the objects is "so large it doesn’t even seem like
a planet. We don’t know what to call it. Is it a
planet? Is it a star? Is it something in between?
"Well, what the hell are we going to do?" asked Letterman.
"We’re screwed," Marcy admitted.
"Run for your life, everybody," Letterman quipped.
Marcy and Fischer believe that consideration should be
given to how an object formed, with the name planet
reserved for objects forming in accretion disks around
a star. In the early dust and gas cloud from which
stars form, fluffy dust bunnies coalesce into bigger
dust bunnies, until they’re big enough for their own
gravity to actively sweep in even more stuff. Anything
that forms this way around a star should be called a
planet, they argue. Stars and brown dwarfs form
differently, in the middle of a swirling nebula, thus
providing a way to differentiate planets from the rest.
But, Basri counters, "I don’t think we should define
what an object is based on how it formed, because I
don’t think we know enough about formation mechanisms,
and you can’t easily observe how things form."
No one now knows how brown dwarfs form, and to throw
a wrench into things, there’s some doubt that Jupiter
formed the way the other planets did. Asks Basri, not
entirely rhetorically: "Is Geoff going to stop calling
Jupiter a planet if he discovers it was formed the way
a brown dwarf is?"
A taxonomy of planets
Marcy and Fischer believe that assigning a firm
definition to planet may also lock astronomers into a
taxonomy that will quickly become obsolete as we learn
more about the varieties of planets in the galaxy.
"I think any time you try to draw sharp lines you get
into trouble," said Fischer. "We should be a lot humbler
and say we are calling these things planets because we
have this historical precedent, this historical inertia.
Let’s admit that at either end, the high-mass end and
low-mass end, this has been completely arbitrary, and
that some things don’t fit with our classification scheme."
"It’s way too early to define a planet," Marcy said. "No
one would have predicted 10 years ago that we’d have any
extrasolar planets. Even though we have now found more
than 100 of them, these are still the early days in
He anticipates that 70-80 percent of all stars will be
found to have planets, most of these in multiple planet
systems. And even though no Earth-sized planets have
yet been discovered, the Milky Way galaxy could well
harbor hundreds of millions of Earths.
"It’s a little arrogant, I think, for us to imagine that
we understand what the full spectrum is going to shake
out to be. Are we really in the ultimate position right
now where we should redefine things, because it freezes
it in again? In a decade or two it may look incomplete
again," Fischer said.
Basri scoffs at these objections. "It’s like saying we
shouldn’t define what a star is until we understand
all about star formation and weird binary stars, and
so on. If we define a planet based on the basic
observable properties of these objects, people can
later apply all sorts of adjectives to them as they
are understood better, without changing what they are
basically talking about."
When Neptune dominates
Imke de Pater, who uses both radio telescopes and
optical telescopes to study planets such as Jupiter
and Neptune and volcanic activity on Jupiter’s moon
Io, also thinks that how a body forms should not
make a difference in deciding whether a body is a
"I would say a planet is a body in orbit about a star,
but not forming part of a larger swarm, like the
asteroids in the asteroid belt or the Kuiper Belt
Objects," she proposes. "A planet also would have to
be in a stable orbit for a few billion years — it
shouldn’t be a KBO in transit to becoming a comet."
Eugene Chiang, a new member of Marcy’s Center for
Integrative Planetary Studies, knows these swarms well.
He’s part of a national team called the Deep Ecliptic
Survey that is scanning the plane of the solar system
in search of as many Kuiper Belt Objects as it can
find. They’ve discovered some 250 since 1998, bringing
the total known KBOs to about 600, all swarming beyond
Neptune’s orbit, 30 times farther from the sun than
Pluto, Chiang notes, is the largest of the Kuiper Belt
Objects, and its orbit, like that of all the KBOs, is
dominated by Neptune. In fact, it orbits in lock-step
with Neptune: Pluto goes around the sun twice for every
three Neptune orbits. A large class of such objects in
the Kuiper Belt has been dubbed Plutinos because they
also inhabit this so-called 3:2 resonance. Of the 100
KBOs that Chiang has tracked well, 25 percent are in
resonant orbits with Neptune.
"The asteroid belt is dominated by Jupiter, and the
Kuiper Belt is dominated by Neptune," he says, and
objects in neither of these belts should be called
planets. In fact, because the Kuiper Belt is the source
of many short-period comets that plunge through the
interior solar system, Pluto could even be called a comet.
Chiang’s interest in the KBOs with resonant orbits comes
from his theory that planets migrate inward or outward
after their initial formation. The many objects in
resonant orbits with Neptune argue that it has migrated
outward, he says, shepherding the KBOs with it and
locking many into resonances. The theory could explain
some of the bizarre planetary systems that Marcy,
Fischer, Paul Butler and others have found, in which
large gas planets seem to be sitting awfully close to
their star, in contrast to our own solar system, where
the gas giants are far out. Early in a system’s history,
gravitational interactions between large gas planets
and the gaseous disk or small objects called
planetesimals can drive planets in or out, he said.
The case for Pluto
None the less, Basri feels that Pluto needs to remain
a planet, partly for historical reasons, but primarily
because it fits a consistent and reasonable definition
of a planetary mass object orbiting a fusor. And if we
include Pluto, how can we exclude other Kuiper Belt
Objects and asteroids that look almost identical?
There’s really no difference between Mercury and Ceres,
he says, so any consistent definition of a planet would
have to include both. He suggests calling the eight
undisputed planets "major planets" and the others,
including Pluto, "minor planets" — a usage once
applied to the asteroids before their numbers
skyrocketed. But they’d all still be planets.
"I’ve thought about this for two years now, and I think
I’ve seen all the arguments, I’ve chewed on them for a
long time, I’ve played with them. So I’m ready," he
said. "That doesn’t mean anyone else is."
Basri’s proposed definition means that the number of
planets in the solar system will continue to grow as
more large objects are discovered in the Kuiper Belt.
The Caltech team that discovered the largest known KBO
last year — a body half the diameter of Pluto that
they named Quaoar (kwah-o-wahr), after a creation
force in California Indian mythology — estimates that
they "should be able to find 5 to 10 more of these
really big Kuiper Belt Objects over the next couple
of years, including perhaps a couple [of]
‘super-Plutos,’" according to their Web site. That
means an eventual 25 planets.
Someday kids may be stumping their parents with planet
names such as Vesta, Quaoar and Varuna, if not Ixion
or Radamanthus. They’ll be around for a while — at
least a few billion years — so you might as well get
used to them.
What’s in a name? Help us remember
For ages, teachers have been creating mnemonics to help
students remember the order of the planets. One well-
known version is "My Very Educated Mother Just Sent Us
Nine Pizzas." Another variant, mentioned in Robert
Heinlein’s book "Have Space Suit, Will Travel," goes:
"Mother very thoughtfully made a jam sandwich under no
protest." (Thoughtfully stands for Terra, Earth’s other
name.) But with another five (or more) potential planets,
it’s back to the drawing board. Please send us your
mnemonics for the latest solar
system lineup: Mercury, Venus, Earth (or Terra), Mars,
Vesta, Ceres, Pallas, Jupiter, Saturn, Uranus, Neptune,
Pluto, Quaoar and Varuna. The best submissions will be
featured in a future issue of the NewsCenter,
* Gibor Basri’s thoughts on defining planets
* Geoff Marcy’s Web site for extrasolar planets
[NOTE: Images supporting this release are available at