A Planet Transiting a Stellar Grave

News & views white dwarfs are so small, the planetary transit Astronomy is very ‘deep’: 56% of the white dwarf’s light is blocked, compared with the typical 1–2% that is blocked by gas-giant planets around normal stars. In the case of WD 1856+534, the A planet transiting transiting planet is similar in size to Jupiter, and therefore has a diameter about ten times a stellar grave that of the white dwarf (Fig. 1). In principle, such a deep transit should be Steven Parsons easy to detect, so it might seem odd that such systems have escaped discovery for so long. Evidence has been found of a planet circling the smouldering However, the small size of white dwarfs also remains of a dead star in a tight orbit. The discovery raises the means that the transits are brief, lasting just question of how the planet survived the star’s death throes — 8 minutes in this case (compared with several hours for normal stars). Therefore, finding and whether other planets also orbit the remains. See p.363 these planets requires white dwarfs to be both rapidly and constantly monitored — some- thing that has become possible only in the past In the past few decades, the number of and destroyed. However, Mars, the asteroid decade, thanks to missions such as TESS and planets discovered beyond our Solar System belt and all the gas-giant planets will probably NASA’s Kepler (see ref. 6, for example). has increased rapidly, and current estimates survive and stay in altered orbits around the The shape of the transit of WD 1856+534 are that around one-third of all Sun-like stars Sun’s remains. More broadly, we might expect gives us a good idea of the radius of the orbit- host planetary systems1. Given that the Milky many white dwarfs to host remnant planet- ing planet, but Vanderburg et al. were unable Way contains around ten billion Sun-like stars, ary systems. Indeed, there has been growing to place strong constraints on the planet’s there are likely to be billions of planets in our evidence of this in the form of asteroids that Galaxy. All of these planet-hosting stars will have wandered too close to white dwarfs and “Until now, no planet in orbit eventually die, leaving behind burnt-out rem- then been torn apart by intense gravitational nants known as white dwarfs. What becomes of forces4. Debris from these asteroids rains around a white dwarf had the stars’ planetary systems when this happens down onto the surfaces of many white dwarfs, been detected directly. ” is unclear, but in some cases it is thought that whereupon we can detect it5. However, until planets will survive and remain in orbit around now, no planet in orbit around a white dwarf the white dwarf2. On page 363, Vanderburg had been detected directly. mass. Using infrared data, they calculate et al.3 report the discovery of a planet that Enter Vanderburg et al., who used data col- an upper limit of 14 times the mass of Jupi- passes in front of (transits) the white dwarf lected by NASA’s Transiting Exoplanet Survey ter. This confirms that the orbiting object is WD 1856+534 every 1.4 days. Their work not Satellite (TESS) mission to detect the periodic indeed a planet (rather than a failed star), but only proves that planets can indeed survive the dimming of the white dwarf WD 1856+534. the unknown mass makes it impossible to tell death of their star, but might offer us a glimpse This dimming is caused by a planet passing whether the planet has been fundamentally of the far future of our own Solar System. between the white dwarf and Earth. Because altered by the death of its host star. A mass Sun-like stars fuse hydrogen into helium in their cores, producing copious amounts of energy that they use to support themselves a Orbital distances against gravitational collapse. Stars are born with huge reserves of hydrogen, but eventually Sun Mercury Venus Earth Mars this supply is exhausted. The Sun has burnt through roughly half of its hydrogen supply. Jupiter-sized planet When this runs out, in five billion years, the Maximum size of original WD 1856+534 star Sun — and, by extension, the rest of the Solar WD 1856+534 System — will undergo a fundamental change. When only a small amount of hydrogen remains, fusion will continue in a shell around 0 0.5 1 1.5 (AU) the Sun’s core. This will cause the outer envelope of the Sun to swell to an enormous size. b Relative sizes At its maximum extent, the surface of the Sun might reach all the way to Earth’s orbit, engulf- Sun Earth WD 1856+534 ing Mercury, Venus and, potentially, Earth itself. The Sun will then start to rapidly eject Jupiter-sized planet its outer envelope into interstellar space. The decreasing mass of the Sun will cause the other Figure 1 | Comparison between the inner Solar System and a white-dwarf system. Vanderburg et al.3 planets to move outwards, away from the Sun, report that a Jupiter-sized planet orbits the white dwarf WD 1856+534. a, The orbit is extremely small — to conserve angular momentum. When the last the planet is roughly 20 times closer to the white dwarf than is Mercury to the Sun. The white dwarf was of the envelope is ejected, the Sun’s core will previously a giant star, the outer envelope of which once extended well beyond the planet’s orbit. This raises be revealed: a smouldering, Earth-sized white the question of how the planet arrived in its current orbit. All distances are in astronomical units (au), and dwarf that will slowly cool for the rest of time. the size of the giant star is shown to scale; the sizes of the other stars and planets are not shown to scale. In this scenario, it is clear that the closest b, The relative sizes of the Sun and Earth, and of WD 1856+534 and its orbiting planet, are shown here for planets to the Sun are likely to be engulfed comparison. 354 | Nature | Vol 585 | 17 September 2020 | Corrected 25 September 2020 ©2020 Spri nger Nature Li mited. All ri ghts reserved. ©2020 Spri nger Nature Li mited. All ri ghts reserved. and radius measurement for this planet would enable us to compare it with similar planets Tumour biology orbiting Sun-like stars, possibly revealing any changes that the planet has undergone in the past. Unfortunately, it seems highly unlikely How cancer invasion that the mass will be determined precisely any time soon. This is because WD 1856+534 is too cold to produce any absorption features in its takes shape spectrum that could be analysed to determine the white dwarf’s radial velocity, a measure- Karolina Punovuori & Sara A. Wickström ment that is typically used to calculate the masses of orbiting planets. Skin cancers resulting from distinct mutations have One of the biggest questions to emerge from characteristic tissue forms and different disease outcomes. Vanderburg and colleagues’ study is how the Analysing the architecture of benign and aggressive tumours planet ended up so close to the white dwarf. reveals how mechanical forces drive these patterns. See p.433 The planet is located just 4 solar radii from the white dwarf (or roughly 20 times closer to the white dwarf than Mercury is to the Sun). Assuming that the inner planetary system was The interplay between form and function is a types of mutation caused cancer cells to prolif- swallowed by the expanding star, it seems cornerstone of biology, and the dismantling erate faster than did their surrounding normal extremely unlikely that the planet has always of normal tissue organization is a hallmark of cells, but the mechanical properties of the been this close to its star. many diseases. A long-standing question is tumour environment differed profoundly Vanderburg et al. suggest two possible whether changes in tissue architecture are between the two tumour types. explanations. The first is that the planet merely a by-product of destructive diseases Using an impressively broad selection avoided destruction by tearing off the outer such as cancer, or whether they actively of methods and combining theoretical layers of the expanding star when it was influence disease progression. Distinct types and experimental approaches, Fiore et al. engulfed. The second is that several distant of skin cancer are driven by specific genetic demonstrated that the two cancer-promoting planets survived the death of the star, but abnormalities and give rise to distinctive mutations had different effects on the produc- their altered orbits caused them to interact tumour shapes. However, how these struc- tion, turnover and stiffness of the basement with each other — whereupon the observed tures arise, and whether their specific forms membrane. This is a thin layer of specialized planet was thrown towards the white dwarf by affect the different outcomes of benign and extracellular matrix material that separates another planet. This latter explanation seems malignant cancers, has been unclear. On the epidermal cells from the rest of the skin, the most likely, and offers the tantalizing pros- page 433, Fiore et al.1 report an analysis of skin such as the adjacent compartment below pect of detecting additional planets in this sys- cancer in mice that uncovers some of the key called the dermis. The authors report that tem in the future. Given that WD 1856+534 is principles involved. the BCC-like tumours actively produced and only 25 parsecs (82 light years) from Earth, the The skin’s outer region, called the epidermis, remodelled the basement membrane, and the gravitational effects of any further planets on is made of layers of epithelial cells.

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