2019-08-03_The_Economist

TheEconomistAugust 3rd 2019 65

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I

n the delugeof planets found beyond

the solar system over the past decade,

those of system toi-270 might not seem

special. There are three of them, orbiting a

star 73 light-years away. This is neither the

closest system known, nor does it contain

the most Earthlike exoplanet. It has,

though, sent a buzz through astronomy.

toi-270 was discovered earlier this year

by the Transiting Exoplanet Survey Satel-

lite, tess, an American instrument

launched in 2018. (toistands for tessob-

ject of interest.) Its innermost world is a su-

per-Earth, a rocky planet a little bigger than

Earth that scurries around its parent star

once every three days. Farther out, at orbits

of around six and 11 days, are a pair of larger

objects known as mini-Neptunes. Having

representatives of these two types of planet

in a single system is a valuable find. It

should help astronomers understand a bit

better how different sorts of planet form.

The proximity of toi-270 to Earth

means that it is within range of instru-

ments such as the Hubble Space Telescope,

making detailed follow-up studies possi-

ble. These will improve estimates of the

planets’ sizes and better calculate their

masses—information that will show what

they are made of. toi-270 is thus, in the

words of Maximilian Günther, an astrono-

mer at the Massachusetts Institute of Tech-

nology (mit), an “exceptional laboratory”

that will help answer some of the biggest

questions in the rapidly growing science of

exoplanetology. These include: why do

planets form in the sizes they do? How does

the solar system fit into a wider galactic

context? And, is there life beyond Earth?

Whole new worlds

Dr Günther does, admittedly, have skin in

the game. He is the lead author of the study

that describes the new star system, pub-

lished this week in Nature Astronomy. That

publication was timed to coincide with a

gathering, in Cambridge, Massachusetts,

of astronomers, chemists, biologists, geol-

ogists and others to discuss the results of

tess’s first year of operation.

tessis a pathfinder. It discovers nearby

planetary systems so that other instru-

ments may study them. It locates its quarry

by looking for characteristic dips in a target

star’s light caused by planets passing in

front of that star. So far, it has found 24

planets the existence of which has been

confirmed by other means, and a further

1,000 possibles that are waiting to be

checked. It is expected to find thousands

more, perhaps tens of thousands, over the

course of its four-year mission.

As astronomers have catalogued plan-

ets found by tessand other instruments,

they have spotted both patterns and puz-

zles. There are, for instance, lots of super-

Earths and lots of mini-Neptunes. But

there is a gap between them. Few planets

are known that have between one-and-a-

half times and twice the diameter of Earth.

This so-called “Fulton gap”—named

after Benjamin Fulton, a phdstudent who

noticed it in 2017—could have several ex-

planations. Possibly, planets on either side

of the gap are different sorts of object. Su-

per-Earths might be born from dust and

rocks, and be kept small by a lack of suit-

able material from which to grow, while

mini-Neptunes, constructed of commoner

materials such as ice, can grow much larg-

er. Alternatively, all planets may form in

the same way, as mini-Neptunes with

Exoplanets

A measured approach

As astronomers probe faraway planets with ever greater sophistication, a new

science is emerging

Science & technology

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