Spørgsmål og svar:At finde jordlignende exoplaneter kræver nye rumteleskoper

En ny type rumteleskop kan hjælpe med at finde liv på andre planeter eller opdage andre solsystemer som vores, ifølge en rapport for nylig udført af National Academies of Sciences, Teknik &Medicin.

Stanford astrofysiker Bruce Macintosh, der var medlem af rapportudvalget, sagde, at det foreslåede teleskop direkte ville afbilde jordlignende exoplaneter, der kredser om stjerner, der ligner vores sol, og kunne arbejde sammen med jordbaserede observatorier for at indsamle kemiske data om exoplanetatmosfærer.

Stanford Report talte med Macintosh om, hvordan det foreslåede teleskop ville fungere, hvordan det er at finde fjerne solsystemer, og hvad vi kan lære ved at lede efter beboelige planeter.

Hvorfor bad Kongressen om denne rapport?

Denne rapport fremhævede to nøglespørgsmål, der vil drive fremtiden for exoplanetforskning. Den store er, er der andre liv-bærende planeter derude? Det andet store spørgsmål er, hvordan dannes og udvikles planetsystemer, og er vores solsystem sjældent eller en fælles del af den proces?

Vi ved nu, at planeter omkring andre stjerner er ret almindelige, men vi forstår ikke helt, hvordan de andre solsystemer dannes. Faktisk, vi forstår mindre om, hvordan planeter dannes nu, end vi forstår om, hvordan sorte huller eller neutronstjerner dannes.

Hvordan opdages og studeres exoplaneter nu?

Der er flere måder at studere exoplaneter på. De dominerende er, hvad vi kalder indirekte teknikker, hvor du ikke rigtig ser planeten. Den mest almindelige af disse indirekte metoder er transitteknikken. Det er der planeten, som den kredser, passerer foran stjernen, blokerer den lidt set fra Jorden og får den til at dæmpe en lille smule i et par timer.

Men der er begrænsninger for transitteknikken. For nu, det lader dig kun studere de atmosfæriske detaljer for gigantiske planeter, planeter, der er flere gange større end Jorden, fordi de har store atmosfærer, der absorberer meget lys. Også, oddsene for at det virker stiger meget, hvis planeten er tæt på stjernen, og hvis stjernen er lille, så det er en fantastisk måde at studere planeter tæt på små stjerner. Det er interessant, men det virker ikke for jordlignende planeter omkring store stjerner, og det virker slet ikke for planeter langt væk fra deres stjerner.

Den anden tilgang, hvilket er hvad vores gruppe her gør, er direkte billeddannelse. Det er der, du faktisk ser en planet adskilt fra en stjerne. Det er virkelig, virkelig svært, fordi planeter er millioner og milliarder af gange svagere end deres stjerner. Lige nu, direkte billeddannelse virker kun for planeter, der er større end Jupiter og langt væk fra deres stjerne.

I øjeblikket, kun jordteleskoper er i stand til direkte at afbilde eksoplaneter. De teleskoper, vi har i rummet, er ikke rigtig designet til at gøre dette lige nu. Et klart budskab fra denne rapport er, hvis vi kommer til at se planeter som Jorden kredse om stjerner som vores sol, vi har brug for et rumteleskop, der er designet til at gøre dette.

Hvordan ville disse teleskoper fungere?

Der er to hovedtilgange, der overvejes. Det første er det, der kaldes et "koronagraf"-teleskop, som bruger spejle og masker inde i selve teleskopet til at skabe en kunstig formørkelse, der blokerer stjernelys, så den lille, svag planet ved siden af ​​den er sporbar.

Den anden tilgang, kaldet en stjerneskærm, repræsenterer en anderledes måde at skabe en kunstig formørkelse på. For at forstå, hvordan dette fungerer, forestil dig, at du vil se en fugl flyve tæt på solen. Hvad laver du? Du holder hånden op og blokerer for solen. En starshade fungerer efter det samme princip, bortset fra at det er plads, så du har en kæmpe rumhånd, der er omkring 50 meter på tværs og omkring 30, 000 til 50, 000 miles væk fra dit teleskop.

Denne kæmpe hånd, eller stjerneskygge, flies lined up between your telescope and the star so that the star's light is blocked and the planet can peep out around its edge. Every time you want to look at a new star, you move the pair of them around to point in a different direction.

When they're operating, they have to hold their alignment to about a meter or so relative to each other. That's hard, but it's engineering hard. The physics is really easy. We can show that the shape of the starshade is crucial for making the shadow dark enough so that it really, really blocks the star. And some of us at Stanford are working on a microsatellite to test the concept.

What would an exoplanet that has been imaged by one of these telescopes look like?

We're not making pictures like the Apollo 8 picture, where you see the continents and so on. Lige nu, and for the foreseeable future, exoplanets imaged this way will still look like a dot – but it's a dot that we can use to measure a planet's chemistry and understand what it's made of.

What can you learn about an exoplanet through direct imaging that you can't with indirect methods?

Because you've blocked out the star, you're actually seeing reflected light from the planet itself, not just inferring it's there. And if you see light from objects, you can do what we call spectroscopy, where you look for the light signatures of particular atoms or molecules that are present in the planet's atmosphere.

The hope is you'd see the signature of oxygen because we think the only way you can get a lot of oxygen in a planet together with other substances like methane is if something changes the chemistry of that planet and kicks it out of equilibrium.

The reason we have oxygen on Earth is life. If you kill everything on Earth, then the oxygen will go away in a few million years. It's not impossible other planets could make oxygen on their own, but by far the best explanation we know of is the presence of life, so that's really what you're looking for is that signature of oxygen.

When could the first of these planet-imaging telescopes launch?

That's the less good news. We already have the next big space telescope, James Webb rumteleskopet, which is currently scheduled to launch in 2021.

The next project beyond that is a telescope called WFIRST. The proposed planet imager would have to start after WFIRST. That probably translates into a launch in something like 2035 or even a little bit later.

I 2015 your group discovered a Jupiter-like exoplanet using the Gemini South Telescope in Chile, and before that, you helped discover a four-planet system. What does it feel like to discover a new world?

It's pretty awesome. We've had Kepler's laws for 400 years, but when we discovered the HR8799 planets, we were witnessing Kepler's laws in action – in a system with four giant planets that's light-years away. It's just spectacularly awesome.

Why is studying exoplanets important?

That's a legitimate question to ask. This is not knowledge that leads to concrete results on Earth, and we're not going to visit these planets for hundreds of years at least, but it's important to our perspective on the universe.

Der var engang, humans were the center of the universe, and then astronomers proved that we were not the center of the universe. That shifted, fundamentally, our view of how important we are, and how the universe doesn't really revolve around us, but we're still the only life we know of in the universe.

If we discover that life exists elsewhere in the universe, that's a similarly epochal shift in our perception of how we fit in it. Or it's possible that life is really rare, and the exact circumstances that made Earth such a beautiful planet haven't happened in all these other thousands of solar systems, and we're the only one that got it right.

That's almost as important to know. If we're the only habitable planet within 1, 000 lysår, we really should do a good job of looking after this one habitable planet because it's even more precious and special than we knew.

We could build the equipment that's needed to answer that fundamental question, but it's going to take us 20 years to build it, so we better get started now.