planet in our Solar System is Jupiter and why there are none more massive is not understood. The lifetime of our gaseous proto-planetary disk may have been short 28 or the disk may have been tidally truncated by the growing planet itself 29 • Good statistics on the population of EGPs will better constrain protoplanetary disk processes. The absence of a Jupiter-class planet in a planetary system would imply a very different population of cometary-sized planetesimals than exists in our own Solar System, and this may have important implications for the origin, evolution and survival of life on rocky 'terrestrial' planets 30 .Only sensitive and systematic searches, such as those we anticipate over the next decade, will directly address these important issues of planet formation. D RECENT work 1 ' 2 has shown that superconducting copper oxyhalides can be synthesized, with the La 2 Cu0 4 structure but with no oxygen in the 'apical' positions outside the Cu0 2 planes. For these materials to be rendered superconducting, charge carriers must be introduced into the Cu0 2 planes; in previous work, positive carriers (holes) have been introduced by the incorporation of interstitial fluorine in Sr 2 Cu0 2 F z+O (transition temperature Tc = 46 K) 1 , or by the substitution of sodium for calcium in (Ca,Na)zCuOzCiz (Tc = 26 K) 2 . Here we present an alternative doping approach for this copper oxyhalide family: holes are introduced by partially replacing the (monovalent) halogen that occupies the apical sites with (divalent) oxygen. We have obtained bulk superconductivity with Tc above 77 K in a new double-layer compound (Sr,Ca) 3 Cuz0 4 +0Ciz-y, which is isostructural with (La,Sr)z-CaCu206 (ref.3), with (Sr,Ca) at the (La,Sr) orCa sites, and Cl at the apical sites outside the Cu02 planes.We synthesized Sr 3 -xCaxCu 2 0 4 +5Cl 2 -y compounds using a high-pressure, high-temperature technique, which has proven effective in searching for new homologous series of high-Tc superconducting copper oxides 4 -6 . The precursors Sr 2 Cu0 3 , SrCu0 2 and Ca 2 Cu0 3 were prepared by solid-state reaction between CuO and SrC0 3 or CaC0 3 (each 99.9% pure). Precursor Sr 2 Cu0 2 CI 2 (CI source of the sample) was obtained by the reaction between CuO and fully dehydrated SrCI 2 (99% pure)

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Halooxocuprates with the so-called 02(n − 1)n (n = 1−5) and the 0222 structures, containing F, Cl, and Br, are reviewed. These compounds are prepared by conventional solid-state reaction: low-temperature fluorination or high-pressure synthesis. Their structural properties and superconductivity are focused on in this review. Bulk superconductivity has been observed in various samples prepared using high-pressure techniques. On the other hand, the reported halooxocuprate superconductors prepared at ambient pressure need more data for verification of superconducting behavior. The carrier-doping routes for the halooxocuprates are discussed. It is confirmed that p-type superconductivity occurs in oxychlorides in which the apical sites are completely occupied by chlorine. This indicates that the existence of the apical oxygen is not a must for high-T c superconductivity with p-type carriers.

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