Astronomers celebrating Hubble’s past focus on its future

BALTIMORE — A technique for sniffing around in the atmospheres of supersized planets near super bright stars could also help the Hubble Space Telescope unravel the history of the universe.

Hubble scientists developed a way to slowly scan the sky while collecting starlight so the telescope could explore the neighborhoods of stars that would otherwise be too bright for its sensitive detectors. It turns out that this trick also enables astronomers to measure distances to nearby stars with unprecedented precision, helping to zero in on the expansion rate of the universe and why it’s picking up speed.

This surprising connection is just one revelation to come out of a four-day symposium at Hubble’s headquarters, the Space Telescope Science Institute in Baltimore. Astronomers gathered there April 20–23 to celebrate the 25th anniversary of the orbiting observatory, which rocketed into space April 24, 1990, and to look ahead to what the telescope can accomplish in its remaining years (SN: 4/18/15, p. 18).

Hubble is now the premier instrument for dissecting distant skies, says Heather Knutson, an astronomer at Caltech. Knutson uses Hubble to examine the atmospheres of super-Earths, planets with masses somewhere between Earth’s and Neptune’s. The atmosphere maintains a record of how a planet forms, she says.

Unfortunately, many of the planets she would like to examine orbit stars that are too bright for Hubble’s sensitive detectors. Knutson gets around this by directing Hubble to slowly move while collecting light, which causes the star to drift across the telescope’s field of view. Rather than blasting a small cluster of pixels, the light trickles across the detector, preventing any one area from being overwhelmed.

Knutson and colleagues have requested time with Hubble to examine the atmosphere of a super-Earth recently discovered by the revived Kepler space telescope. The planet, designated EPIC 201912552b, will be the first super-Earth in a habitable zone, the region around a star where liquid water can exist on the planet’s surface, to have its atmosphere measured. Researchers hadn’t planned on making such measurements until at least 2018 after the launch of Hubble’s successor, the James Webb Space Telescope. “Hubble could scoop James Webb,” Knutson says.

Knutson hopes to measure the ratio of carbon to oxygen in the atmosphere of this potentially habitable world. The relative abundance of those elements can help researchers pinpoint where the planet formed. The planet is also cool enough, she says, that they might be able to detect methane or ammonia if there’s any around.

Adam Riess has set his sights a bit farther out — more than halfway across the known universe. The Johns Hopkins University astrophysicist is using a class of exploding stars known as type 1a supernovas to track the expansion rate of the universe, a number known as the Hubble constant.

One of the biggest obstacles to getting better precision, says Riess, is measuring distances to stars within the Milky Way that he and his colleagues can use to calibrate the brightness of type 1a supernovas. Knutson’s scanning trick, it turns out, is perfect for clearing this hurdle.

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When triangulating the distance to a nearby star, astronomers must precisely measure the position of the star relative to other stars. By scanning the telescope instead of holding it steady, researchers can measure the positions of lines of starlight instead of points, thereby averaging out many errors that creep in. It’s like getting thousands of observations in one snapshot, Riess says.

Based on earlier tests, Riess says that this trick will reduce uncertainties in the Hubble constant by half within the next year. Such improvements have a whopping effect on how precisely astronomers can measure the effect of dark energy, the repulsive force that is pushing the universe apart at an ever increasing rate.

The symposium has really driven home Hubble’s diversity and flexibility, says Johns Hopkins astronomer Steve Rodney. From alien worlds to the evolution of the universe, Hubble is pushing at frontiers undreamt of 25 years ago. And despite no hope for repairs should any critical parts break, astronomers in Baltimore are optimistic that the “people’s telescope” has got at least a few more good years left. The evidence is right there in the symposium’s title: “Hubble 2020: Building on 25 years of discovery.”