Astronomy - USA (2022-06)

26 ASTRONOMY • JUNE 2022

The hot Jupiters

In 1995, astronomers studying the nearby

star 51 Pegasi found that it appeared to

be wobbling back and forth, movement

that revealed itself through a regular pat-

tern of Doppler shifts in its spectrum.

The observations suggest that the star is

in orbit around a position just slightly

offset from its center. By measuring the

size of the wobble, it was possible to get

an estimate of the mass of the perturbing

object, which turned out to be less than

half the mass of Jupiter, too small to be

a star. They had found an exoplanet!

The discovery was puzzling. The new

planet completed an orbit every 4.23 days.

That placed this giant world, which we

would have expected to find in the icy

reaches of its outer solar system, seven

times closer to its

star than Mercury

is to the Sun. To

find a giant planet

so close to its star

that its year was

measured in just a

few days was com-

pletely unexpected,

calling into question

everything we’d assumed

about planetary systems.

One such planet might have been

an exception, the result of some gloriously

unlikely freak accident of planet forma-

tion. But “hot Jupiters” like the planet

perturbing 51 Peg were found to be com-

mon. More than 400 are known to date,

and they account for around 10 percent of

known exoplanets. This overstates their

actual abundance — it is easier to find a

large planet close to its star, where it will

induce significant wobbles, than to pick

out the signal of a puny Earth-sized world.

Correcting for these biases, hot Jupiters

seem to account for about 1 percent of all

worlds. That doesn’t sound like

much, but it could easily

mean there are a billion

hot Jupiters in the

Milky Way alone!

The method used

to find the planet

around 51 Pegasi,

known as the radial

velocity method,

requires enough

telescope time to pay

close attention to a

star of interest over

many nights. Most exoplan-

ets known today have been

found instead by searching for transits, a

method which allows us to survey many

stars at once. This technique relies on the

fact that, if a planet’s orbit happens to be

aligned such that — as seen from Earth

— it passes in front of the star, the star

will appear to fade just slightly. A similar

effect is seen from Earth when Mercury

or Venus transit in front of the Sun.

Thanks to the complexities of planetary

geometry, such events are rare, but

throughout the galaxy there must be

worlds which happen to be positioned

such that the planets of our solar system

cross in front of the Sun with every orbit,

causing a regular series of blinks. Indeed,

the nearby dwarf Teegarden’s Star seems

to have just such a system. If there are

astronomers on these planets, what could

they learn about our solar system?

They could measure the radius of

each planet relative to the size of the Sun;

the amount of light blocked by the planet

corresponds to the ratio of the two areas.

By timing the period between successive

transits, they would get the orbital period

and — thanks to Kepler’s laws — the dis-

tance of the planet from the star. (You

can see why large, close-in planets would

be easier to detect via this method than

smaller planets which are further from

their stars.) Applying these techniques to

the known hot Jupiters tells us that they

span a range in size that starts from

100 Earth masses (about a third of the

ALMA made the first large-scale survey of protoplanetary disks around nearby stars. This image shows 20
nearby systems where dust and gas are forming new planets. It is possible to make out gaps in the disks,
which seem to demarcate their inner and outer portions. ALMA (ESO/NAOJ/NRAO), S. ANDREWS ET AL.

WHILE ASTRONOMERS HAVE BEEN EXPLAINING the

origin and composition of our Sun’s family of planets for hundreds

of years, this story has only come together in the last 30 years or so.

Before then, astronomers assumed that planets were born in the

location and configuration in which we see them today. The idea of

planets moving about while they are forming was only seriously

considered once planets in other systems — exoplanets —

had actually been found.

“Hot Jupiters”

like the planet

perturbing 51 Peg were

found to be common.

More than 400 are

known to date, and they

account for around 10

percent of known

exoplanets.