Mars-size ex­o­planet is light­est ever mea­sured

Sci­en­tists de­ter­mine its mass and den­sity, which may help ex­plain how plan­e­tary sys­tems evolve.

Los Angeles Times - - OPINION - By Amina Khan amina.khan @latimes.com Twit­ter: @am­i­nawrite

You’ve heard of hot Jupiters, mini-Nep­tunes and su­per-Earths — but how about an al­most-Mars?

Sci­en­tists us­ing data from NASA’s Ke­pler space­craft have pinned down the mass of an ex­o­planet that’s slightly lighter than the Red Planet, mak­ing it the small­est such world to have its den­sity mea­sured.

The planet Ke­pler-138b, de­scribed in the jour­nal Na­ture, could shed light on the kinds of small, rocky worlds that might be found or­bit­ing dis­tant stars — and whether these plan­e­tary sys­tems are sim­i­lar, or very dif­fer­ent, from our own in­ner so­lar sys­tem.

“The de­tec­tion of such a small ex­o­planet in a tight or­bit could help to clar­ify how we fit into the big pic­ture,” Gre­gory Laugh­lin of UC Santa Cruz, who was not in­volved in the study, wrote in a com­men­tary.

The red dwarf star Ke­pler-138 and its plan­ets lie about 200 light-years away in the di­rec­tion of the con­stel­la­tion Lyra.

Three plan­ets, Ke­pler-138b, Ke­pler-138c and Ke­pler-138d, cir­cle their star tightly, with re­spec­tive or­bits of about 10, 14 and 23 days. Be­cause they cir­cle so closely, these plan­ets, par­tic­u­larly Ke­pler-138b, are sear­ingly hot and un­suit­able for life.

Ke­pler-138b was first de­tected us­ing data from NASA’s Ke­pler space­craft, which from 2009 to 2013 stared at a patch of sky con­tain­ing more than 150,000 stars, wait­ing for the re­peated dips in starlight that sig­naled that a planet was regularly pass­ing in front of its star.

By mea­sur­ing the amount of blocked starlight, sci­en­tists can de­ter­mine the size of the planet — but not its mass, and thus not its den­sity.

In some cases, astronomers can de­ter­mine a planet’s mass by look­ing at the mo­tions of its home star — the planet’s tiny grav­i­ta­tional pull can cause the star to wob­ble, which squeezes and stretches the starlight in a pre­dictable way.

The prob­lem is, this only re­ally works for sig­nif­i­cantly mas­sive plan­ets, which have enough weight to give their star a de­tectable wob­ble.

But even if Ke­pler-138b can’t push its mas­sive star around, it can cer­tainly give its fel­low plan­ets a shove, and vice versa. Each planet’s grav­ity tugs the neigh­bor­ing plan­ets slightly off-kil­ter from their proper or­bit tim­ing.

Us­ing those slight de­vi­a­tions in each planet’s tran­sits, the sci­en­tists were able to de­ter­mine the mass and den­sity of each world. Ke­pler-138b is 0.066 of an Earth mass; its mass and den­sity are some­what lower than those of Mars, whose mass is about one-tenth that of Earth’s.

“Ke­pler-138b is by far the small­est ex­o­planet, both by ra­dius and mass, to have a den­sity mea­sure­ment,” the au­thors wrote. “Thus it opens up a new regime to phys­i­cal study. It is likely to be­come the pro­to­type for a class of small close-in plan­ets that could be com­mon.”

Ke­pler-138c and Ke­pler-138d are both slightly larger than Earth, but their den­si­ties are very dif­fer­ent: Ke­pler-138c’s is sim­i­lar to Earth’s, while Ke­pler-138d is less than half as dense.

It seems Ke­pler-138d could have a greater share of lighter ma­te­ri­als, in­clud­ing wa­ter and hy­dro­gen, the au­thors said. If that’s true, then it’s pos­si­ble that Ke­pler-138d didn’t form so close into its red dwarf star, but far­ther out where wa­ter could sur­vive with­out be­ing boiled off.

“A planet made of rock and wa­ter would be more sta­ble against mass loss, and would im­ply that the planet formed at a greater dis­tance from the star and mi­grated,” the study au­thors wrote.

Those sorts of mi­gra­tions re­veal much about the evo­lu­tion of a plan­e­tary sys­tem. For ex­am­ple, in our own backyard, Jupiter is thought to have made ma­jor mi­gra­tions early in the so­lar sys­tem’s history, leav­ing a trail of chaos in its wake.

The find­ings, Laugh­lin said, could help sci­en­tists bet­ter un­der­stand the kinds of com­plex dy­nam­ics that have formed other plan­e­tary sys­tems — and how they com­pare to our own.

“It is im­per­a­tive to im­prove our un­der­stand­ing of how our sys­tem’s ar­chi­tec­ture and evo­lu­tion fit into the over­all cen­sus — the au­thors’ study is a step to­wards that goal,” he wrote.

Danielle Futselaar

THIS il­lus­tra­tion shows part of the plan­e­tary sys­tem that con­tains Ke­pler-138b.

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