Sky & Telescope - USA (2019-09)

NEWS NOTES

10 SEPTEMBER 2019 • SKY & TELESCOPE

EXOPLANETS

Images Reveal Growing

Newborn Planets

SOLAR SYSTEM

Was ‘Oumuamua

a Fragment from a

Disintegrated Comet?

ASTRONOMERS HAVE CAPTURED

the growth of infant planets forming

around 5-million-year-old star PDS 70.

Sebastiaan Haffert (Leiden University,

The Netherlands) and his colleagues

report the new observations June 3rd in

Nature Astronomy.

Last year, another team directly

imaged the massive planet PDS 70b

orbiting within a gap in the protoplan-

etary disk surrounding the star (S&T:

Nov. 2018, p. 9). Now, Haffert’s team

used a different instrument on the

Very Large Telescope in Chile to detect

hydrogen gas fl owing onto the planet.

What’s more, they’ve discovered a

similar gas fl ow onto another putative

planet: PDS 70c.

“PDS 70b is certainly the most

defi nitive case for a protoplanet,” says

1I/2017 U1 ‘OUMUAMUA, the interstel-

lar mystery object that briefl y visited the

inner solar system in 2018, experienced

a mysterious “non-gravitational accel-

eration” away from the Sun on its way

back out. New calculations by Zdenek

Sekanina (Jet Propulsion Laboratory)

rule out comet-like behavior as the

cause. Instead, Sekanina suggests radia-

tion pressure could have pushed the

object — if its density were ultra-low.

While observations had already lim-

ited comet-like activity from ‘Oumua-

mua, Darryl Seligman and Greg Laugh-

lin (both at Yale) suggested in a recent

study that a small jet of sublimating

water ice might have escaped detection

and could have explained the object’s

acceleration (S&T: July 2019, p. 11).

However, in a paper posted on the

astronomy preprint arXiv in May,

Sekanina objects to the explanation of

Thayne Currie (Subaru Telescope), who

was not involved in the study. “The

PDS 70c object looks very encouraging.

Hopefully, future studies will clearly

show evidence for orbital motion.”

outgassing altogether. He puts forth

multiple arguments, but the nail in the

coffi n is his calculation that water ice —

even if present — couldn’t have subli-

mated quickly enough to spur ‘Oumua-

mua on its way.

Seligman accepts Sekanina’s asser-

tion, agreeing that ‘Oumuamua

wouldn’t have received enough energy

from the Sun to power the jet.

Without comet-like activity, the only

force that remains under consideration

is solar radiation pressure. But for the

Sun’s photons to push ‘Oumuamua

hard enough to explain its acceleration,

Sekanina calculates, the object itself

Both protoplanets have mass

estimates many times Jupiter’s, and

spectral measurements show that these

gas giants are both still growing. But

at current rates, it would take them

50 million to 100 million years to add

a Jupiter’s worth of mass. Given that

protoplanetary disks only stick around

for about 10 million years, these planets

have probably experienced growth

spurts in the past.

Based on the current positions of

PDS 70b and c, they seem to be in or

near a 2:1 resonance, in which PDS 70c

completes a single orbit for every two

orbits by PDS 70b. This arrangement

is similar to the 3:2 orbital resonance

that Jupiter and Saturn were thought to

have had billions of years ago. If future

observations confi rm the resonant

orbits around PDS 70, the system may

provide a glimpse into the dynamics

that shaped the early solar system.

■ MONICA YOUNG

would have had to have a density less

than about 0.001 gram per cubic centi-

meter — making it effectively as light as

air. This scenario is possible, Sekanina

writes, if ‘Oumuamua were a surviving

fragment from a comet that disinte-

grated while passing near the Sun.

Seligman adds that he and Laughlin

are also exploring alternative scenarios,

including the possibility that ‘Oumua-

mua was a porous pile of icy dust

known as a fractal aggregate.

“Either way,” Seligman notes, “the

ramifi cations for planet formation are

extremely interesting.”

■ MONICA YOUNG

p By canceling out most of the light from the

central star, PDS 70 (whose location is marked

by a white asterisk), astronomers revealed the

signature of hydrogen gas fl owing onto PDS

70b and another possible planet, PDS 70c.

p One artist’s impression shows ‘Oumuamua as a cigar-shaped object.

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