Astronomy

28 ASTRONOMY • JANUARY 2018

One of humanity’s greatest discov-

eries will be the confirmation that

life exists elsewhere in our uni-

verse. We haven’t made this dis-

covery yet, but each year, astrono-

mers move closer to that goal. This

past year was no exception.

In February, a team of planetary

scientists announced — both at a

press conference and in a Nature

article — that it had found seven

Earth-sized planets orbiting a

nearby star. All seven were within

the so-called habitable zone.

Inside this region, the amount of

starlight the exoplanets receive

could lead to ideal temperatures

for liquid water on those worlds’

surfaces. (Though just because

a planet is in the habitable zone

doesn’t mean there’s water.) The

researchers also ran computer

models and estimated that three

of those exoplanets could harbor

water oceans on their surfaces,

provided those worlds also have

Earth-like atmospheres.

To find these seven worlds,

astronomers used several

ground-based telescopes in

addition to the infrared Spitzer

Space Telescope to focus on the

star known as TRAPPIST-1. They

watched as its starlight flickered.

From their hours of data, the

researchers determined the flick-

ering was due to multiple planets

crossing in front of the star, block-

ing a tiny amount of starlight at

each crossing. As of February,

they could confirm six exoplan-

ets in orbit around TRAPPIST-1.

Confirmation of the seventh

world, which orbits farthest from

the star, came in May. And there

could even be more exoplanets in

the system.

From the amount of light each

exoplanet blocked, the astrono-

mers could calculate its diameter.

By analyzing the amount of time

that passed between subsequent

transits of the same exoplanet,

and comparing the six different

inner worlds’ orbits, the scien-

tists could also estimate those

exoplanets’ masses. With diam-

eter and mass, you can calculate

density. The calculations suggest

the six inner worlds are rocky like

Earth, Mars, Venus, and Mercury,

although the numbers are still

highly uncertain. (The seventh

exoplanet’s density has even

greater uncertainty.)

While the exoplanets might be

rocky and Earth-sized, their star is

nothing like the Sun. TRAPPIST-1

holds only 8 percent of the Sun’s

mass and is about 12 percent of

the width of our star. It is also

much cooler, giving off a red glow

instead of our Sun’s yellow-white

light. This planetary system is

also far more condensed than

our solar system. The innermost

planet completes an orbit around

TRAPPIST-1 in 1.5 days, whereas

the most distant one takes 18.8

days. Even though astronomers

haven’t found an exact solar

system analogue, the discovery

of seven likely terrestrial planets

orbiting a star only 40 light-years

away is a good indication that an

Earth twin may be out there.

• BepiColombo, the European-
  Japanese collaborative mis-
  sion to Mercury, is set to
  launch in October.


• SpaceX is still hoping for
  crewed missions to start up
  in 2018.


• NASA’s next space-based
  exoplanet observatory, the
  Transiting Exoplanet Survey
  Satellite (TESS), launches in
  early 2018.


• The agency’s InSight
  (Interior Exploration using
  Seismic Investigations,
  Geodesy and Heat
  Transp or t) mission to Mar s
  has a launch window begin-
  ning May 5, and a landing
  date of November 26.


• The X-ray satellite
  Spectrum-Roentgen-
  Gamma (SRG) will launch
  in 2018. This Russian and
  German collaboration proj-
  ect will scan the entire sky
  and is expected to discover
  millions of supermassive
  black holes. — L.K.

Te r r e s t r i a l p l a n e t

plethora

The TRAPPIST-1 system

contains at least

seven planets circling

an M-dwarf star. All

seven appear Earth-

sized and terrestrial

in nature, prompting

astronomers to

wonder whether they

might also host life.

NASA/JPL-CALTECH