Mar 162016
 

By Jason Tumlinson, Astronomer at the Space Telescope Science Institute

What are the most amazing astronomical discoveries in our lifetime? The realization that the Universe is dominated by dark matter? The finding that Hubble’s expanding Universe is actually accelerating? That planets orbiting normal stars are common? To me, the most amazing discovery is one that has yet to be made, but which many astronomers are spending their careers to pursue: whether or not life as we know it has arisen beyond the Earth, even beyond our own Solar System. This question was asked by the ancients of many cultures, and has preoccupied some of the deepest thinkers up to the present day. We astronomers working now are privileged to live at a time when we can foresee, and personally work toward, the day when this question may be answered.

Talk to the right kind of biologist, and you’ll find that “origins of life” research has become a respectable branch of their field, in a multidisciplinary brew of molecular and cell biology, biochemistry, genomics, and even quantum physics. Researchers in their labs have created simple genomes from scratch, synthesized self-organizing membranes to hold them, and replicated many possible variants on the primordial chemical conditions where life on Earth may have originated. Yet there is one ultimate experiment that no Earth-bound lab can ever hope to perform: has Nature replicated her experiment on Earth by giving rise to life elsewhere? This is a problem for the astronomers.

How will we do it? In short, by finding Earth-like planets around nearby stars and remotely sniffing their air. Since the discovery of exoplanets 20 years ago, and the first direct measurement of an exoplanet atmosphere in 2002, it has become routine to measure the composition of planetary atmospheres. But detecting direct signs of life on other Earths will be much more challenging than anything we can do today, chiefly because each Earth is lost in the glare of its parent star, shining 10 billion times brighter than the planet itself. If we can achieve suppression of the starlight so that the planet can be seen, we can look for oxygen, ozone, water, and methane – the signs of life.

Astronomers have now started serious efforts to find and look for signs of life with the next generation of space telescopes. The James Webb Space Telescope, launching in 2018, will excel at studying the atmospheres of “SuperEarth” planets (about 1.5-2 times Earth mass) around stars smaller than the Sun. The WFIRST mission that NASA has just begun will improve starlight suppression to within about a factor 10 from that needed to study true Earth analogs around Sun-like stars.

To truly answer the origins of life question, we need to reach statistically significant samples of Earth-like planets around nearby stars. This is a problem for a large space telescope, something still larger than JWST. One such concept was dubbed the “High Definition Space Telescope” (HDST) in a report issued by AURA last year. Another name is LUVOIR, the Large Ultraviolet/Optical/Infrared Surveyor, just now under study by NASA. In either case, a telescope of 10 meters or more in aperture will be necessary to characterize dozens of Earth-like planets and look for signs of life there.

Such an observatory also promises to revolutionize virtually every other area of astrophysics with its high resolution imaging and multiplexed spectroscopy. It should be to the astronomical community in two decades what Hubble is now – the all-purpose eagle eye on the cosmos.

In future posts I’ll expand on these themes and describe the incredible potential of such a telescope, the science behind testing “origins of life” theories with astronomical measurements, and the energizing possibilities of a 10 meter telescope in space. Please come back and see how cool the future can be!

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Figure: Notional design of a High Definition Space Telescope (HDST).

Jan 122015
 

By Massimo Robberto, Observatory Scientist at STScI

In 1994, when I was a young Assistant Astronomer in Italy, I started regularly visiting STScI for a collaboration on stellar coronagraphy with Mark Clampin and Francesco Paresce. Those were the months immediately following the first Hubble Servicing Mission and excitement was in the air at the Institute. New pictures comparing the “pre” and “post’ performance of the telescope were posted daily, as testimony for NASA’s spectacular achievement and the bright future ahead. Still, traces of the shock caused by the initial failure (spherical aberration) were evident. In particular, in the control room of the first floor an entire wall had been covered with hundreds of cartoons from all over the world mocking NASA for the Hubble primary mirror disaster. STScI staff had diligently collected and posted all of them, regardless on their quality or even language. A bit of irony helps keeping things in the right perspective when a crisis strikes, and staying focused, to work on a solution.

One of those cartoons attracted my attention. It showed a group of perplexed experts, the “Hubble Design Group”, wondering if “outer space really is blurry and out of focus” (Figure 1). I liked it immediately and discretely made my copy. The cartoon was different from the others because, I thought, it was a cartoon about us, about how our intelligence works. We always look for an explanation, for a reason, and when all good possibilities fail, we start considering the bizarre and the improbable. We feel that saying “I don’t know” and stop wondering is even worse than opening the door to what may look absurd. And our secret hope, as scientists, is to discover that something apparently absurd is actually real.

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Figure 1: Ralph Dunagin’s strip, Dunagin’s People, ran from 1960 to 2001. The Hubble Servicing Mission occurred in 1993.

Fast-forward 20 year: today, the Hubble Space Telescope is still fully operational at its best, acquiring data that are transforming our knowledge of the Universe.  In particular, a Hubble Legacy Program is underway to study one of the most spectacular phenomena gloriously unveiled by the Hubble images: gravitational lensing (Figure 2). As predicted by Einstein, mass warps space-time. When a large mass is present, as in the case of the dark matter halos surrounding clusters of galaxies, the light of more remote objects along our line of sight gets distorted. The signal is so strong (“strong gravitational lensing”) that we can use it to map the distribution of dark matter in these halos. This is at present the only direct method we have to probe the effects of what may be the most enigmatic particles in physics.

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Figure 2. This HST/ACS image, obtained in 2009 immediately after the last Hubble Servicing Mission, shows a gravitationally lensed background galaxy in the field of the Arp 370 galaxy cluster.

If the alignment conditions are favorable, the brightness of some remote galaxy may be magnified: the gravitational lens effect can make visible objects that would have otherwise remained beyond Hubble’s reach. One can say that the so-called Frontier Fields program is using two telescopes in a series, one made by us (Hubble) and one provided by Nature (gravitational lensing), to search for the most distant galaxies and supernovae. With this “trick” Hubble can give us a glimpse of the type of science that will be routinely carried out by the James Webb Space Telescope.

As one moves further away from the center of a galaxy cluster, the gravitational lens effect becomes less pronounced. No matter where we look in the sky, the shapes of thousands of galaxies are all slightly distorted in some way (“weak gravitational lensing”). It is a vanishingly small signal, but it is correlated for all the galaxies that are at similar distances, so it is possible to detect it by applying statistical methods to the most exquisite wide-field images. Future space missions such as Euclid and WFIRST are designed so as to carry out this type of study, which is critical to understand the build-up of giant cosmic structures over time and the process of galaxy formation.

Another fascinating aspect is that amplification can be caused by the random and temporary alignment of stars in crowded fields. In this case the brief light amplification (micro-lensing) can be used to unveil the presence of planets like Earth. WFIRST has the capability of monitoring billions of stars in the Galactic Bulge, where lensed planets could flash like lights on a Christmas tree. Crafting a suitable cadence of observations to exploit this capability is one of the main challenges faced by WFIRST.

A 10m-class space telescope like the proposed ATLAST will eventually produce images of incredible sensitivity and spatial resolution. At that point the ubiquitous gravitational lensing will become more obvious and, perhaps, just another ordinary aspect of our perception of the Universe.

Twenty-five years after launch, the Hubble Space Telescope is showing us that because of gravitational lensing the Universe is really somewhat blurry and out of focus. The intuition of a cartoonist has anticipated one of the most spectacular discoveries of all time. Let’s keep an open mind to more surprises. What today looks like an absurd concept, imagined only in the mind a creative artist, may become tomorrow part of our understanding of this beautiful, and very extravagant, Universe.

Sep 242014
 

By Elodie Choquet, Postdoctoral Fellow at STScI

Debris disks are cold dust belts hosted by some main-sequence stars, composed of micrometer-size grains to kilometer-size planetesimals. As left-overs from planet formation, the study of these young cousins of our own solar system’s Kuiper belt can help us to better understand how planets are formed.

To do so, we need to image these disks in the visible or near-infrared, to deduce their composition and physical properties from the starlight scattered by the dust, and maybe also detect signposts of planets in their geometry. However, despite numerous surveys with the Hubble Space Telescope (HST) and the largest ground-based telescopes, only 19 debris disks had been imaged in scattered light so far. As far as planets are concerned, only 24 have been directly imaged, in significant contrast with the large numbers of discoveries by the radial velocity and the transit methods (with hundreds and thousands of planet detections respectively).

This is due to the very high contrast between the host star and the light reflected by a disk or emitted by a planet (more than a million times fainter!). To achieve such challenging detections, the instruments need to be equipped with carefully optimized coronagraphs, and with efficient adaptive optics systems for ground-based telescopes, and furthermore, the observer has to apply post-processing techniques on the resulting images to detect the dim circumstellar material.

The classical post-processing method consists of subtracting the image of a reference star from the science image to reveal material in its vicinity. However, such a subtraction is never perfect due to telescope instabilities and/or residual wavefront errors, and residual starlight still impedes the detection of cold material within 2’’ of the star (Fig. 1). New algorithms have been recently developed to solve this issue, by using large libraries of reference star images to generate a synthetic image of the star optimized to the actual science image. These new techniques improve the starlight subtraction by a factor of 10 to 100 over the classical method (Fig. 2).

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Figure 1: The Principle of the classical post-processing technique: the image of a reference star is subtracted from the science image to remove the starlight. Although this method improves the contrast by a factor of 5 to 10 compared to the raw image, the telescope instabilities prevent the detection of any material within 2’’ from the star.

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Figure 2: Images of the debris disk around HD181327 reduced with the classical technique (left, from [1]) and with the KLIP algorithm [2] (right, from [3]). This advanced post-processing algorithm typically improves the contrast by a factor of 10 to 100 over the classical method.

Our team has thus started the project of reprocessing the entire HST-NICMOS coronagraphic archive with such advanced algorithms to reveal new disks and planet candidates [4]. The archive is composed of images of 400 stars observed in the near-infrared between 1997 and 2008 and have been underexploited by the use of mainly old post-processing techniques. Among our recent discoveries from this project is the detection of five debris disks seen for the first time in scattered light (Fig. 3). These detections increase the total number by more than 20%. The on-going analysis and modeling of these disks should tell us more about their composition and properties and maybe present hints of possible planets.

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Figure 3: Five debris disks newly revealed in scattered light from the HST-NICMOS archive, discovered by reprocessing the images with advanced post-processing algorithm (from Soummer et al. 2014).

 

References:

  1. Schneider et al. 2006, ApJ 650, 414
  2. Soummer, Pueyo & Larkin, 2012, ApJ 755, 28
  3. Soummer et al. 2014, ApJ 786, 23
  4. Choquet et al 2014, Proc. SPIE, 9143-199